Coal Preparation Course Content

TEST PAPERS
AND
MODEL ANSWERS
Insert here in numerical order each marked Test Paper with Model Answers. They will provide you with a useful check on your lesson work, for revision or reference.

LIST OF TEST PAPERS
Test Paper 1 (on Lesson 1)
Test Paper 2 (on Lessons 2 and 3)
Test Paper 3 (on Lesson 4)
Test Paper 4 (on Lesson 5 and 6)
Test Paper 5 (on Lesson 7 and Appendix 1)
Test Paper 6 (on Lessons 8 and 9)
Test Paper 7 (on Lesson 10)
Test Paper 8 (on Lessons 11 and 12)
Test Paper 9 (on Lesson 13)

TEST PAPER 1 (on Lesson 1)

Please read Lesson 1 Sections 1 and 2(a) to (c), also the introduction to the “Test Papers for Completion” which is printed just in front of Test Paper 1 questions 1 and 2 below.
(Encircle the letter for the correct answer in this table. Is it a, b, or c?)

The coal seams of today were mainly made from:
Decayed animal remains
Decomposed mineral matter
Partly delayed plant life

The coal-forming deposits developed into peat due to the action of:
Hot sunshine
Pressure of sediments
Deep Water

Turn back to lesson 1 and read section 2(d), (e) and (f). then answer question 3

What layers, besides coal are to be found in the coal measure strata? What were they made from?
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Read lesson 1 section 3, then answer Questions 4, 5, and 6

What was “squeezed out” of the peat to transform it into coal? What other actions assisted the coalification process? What was the source of these actions?
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How would you distinguish by sight between pieces of peat, lignite, bituminous coal and anthracite? How does the gas content vary through these stages?
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Why is anthracite a very hard coal?
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Turn back to lesson 1 and read section 4 Before answering question 7

What are the four main constituents which make up the bands in Bituminous coal?
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Read lesson 1 Section 5 and 6 before answering questions 8, 9, 10, and 11.
What parts of coal are incombustible?
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Why does the inert matter reduce the heat value of the coal? ___________________________________________________________________________________________
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Explain briefly why it is impossible to remove the inherent moisture by dewatering screens or centrifuges
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Which two elements associated with the coal substance produce undesirable corrosion and pollution effects during combustion?
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Read lesson 1 sections 7and 8, then answer question 12 .

What is meant by the term “true middlings”? If you have a piece of coal and a piece of middlings of the same size, which is the heavier?
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Lesson 1 section 9 should be read before You answer questions 13 and 14

What is meant by the term “run-of-mine coal”? _________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
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How have modern mining methods affected the moisture content of run-of-the coal?
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COAL PREPARATION COURSE

Model Answers to Test Paper 1 (on Lesson 1)

Model answer papers give correct answers to the a, b, c, type questions, as well as examples to questions requiring longer answers. Do not think that your answers were necessarily wrong because you did not express them in the same way shown here. These answers are to help you and other students who may be a little uncertain as to what is required. They can also be useful for revision at any time.
The coal seam of today were mainly made from:

The coal-forming deposit developed into past due to the action of:
(b) Pressure of sediments

What layers, besides coal are to be found in the coal measure strata? What were they made from?

ANSWER:
Seat earth made from soil.
Sandstone made from sand.
Shale made from mud.

What was “squeezed out” of the peat to transform it into coal? What other actions assisted the

coalification process? What was the source of these actions?
ANSWER:
Pressure from the deposits above the peat “squeezed out” gases and moisture. Heat also assisted the coalification process: it was caused by the pressure, the bending of the strata and by igneous intrusions.
How would you distinguish by sight between pieces of peat, lignite, bituminous coal and anthracite? How does the gas content vary through these stages?
ANSWER:
Peat varies in color from light brown to nearly black, does not appear consolidated, and the remains of the plant substance from which it was formed can be seen. Lignite is of similar color to peat, but looks more consolidated than peat. Bituminous coals are always black in color but have dull and bright bands, whereas anthracite has a shiny appearance and bands are not normally visible. The gas content decreases through the stages of formation.
Why is anthracite a very hard coal?
ANSWER:
Because it has suffered the maximum effects of pressure and temperature.
What are the frour4 main constituents which make up the bands in Bituminous coal?
ANSWER:
Durian, Vitrain, Clarain, Fusain
What parts of coal are incombustible?
ANSWER:
Incombustible material in coal consists of water and mineral matter.
Why does the inert matter reduce the heat value of the coal?
ANSWER:
The inert matter reduces the heat value of the coal in two ways. Firstly, the inert matter does not burn; therefore, the more inert matter in say 1kg of coal, the less will be the amount of burnable material. Secondly the water in th4e coal uses up some of the heat provided by the b unable material, in order to convert it to steam.
Explain briefly why it is impossible to remove the inherent moisture by dewatering screens or centrifuges.
ANSWER:
The inherent moisture cannot be removed by dewatering screens or centrifuges because it is moisture left in the coal substance by the coalification process and is thus p[art of the coal substance, so much so, that it can only be removed by heating.
Which two elements associated with the coal substance produce undesirable corrosion and pollution effects during combustion?
ANSWER:
Sulphur and Chlorine
What is meant by the term “true middlings”? If you have a piece of coal and a piece of middlings of the same size, which is the heavier?
ANSWER:
“Middlings” is the term given to pieces of material which are a mixture of coal and shale. If the coal and shale are so intimately mixed that crushing cannot liberate he coal and shale portions, then it is called a “true middling.”
What is meant by the term “run-of-mine coal”?
ANSWER:
Run of mine coal is he coal as mined, that is, in the untreated state.
How have modern mining methods affected the moisture cont3ent of run-of-the coal?
ANSWER:
Modern mining techniques produce more shall coal, and have increased the dust produced at the face. This dust is suppressed both at the face and at transfer points by the use of water sprays, which increases the moisture content of the run of mine coal. Pulsed infusion shotfiring using water is also practiced in some collieries.
SPECIMEN
TEST PAPER QUESTION AND MODEL ANSWERS (ESSAY TYPE)
The later test papers ask you to provide longer, essay type answers to certain questions. This type of question is often best answered by laying out the information as shown in the following specimen question and essay type answer.
QUESTION:
What is meant by the proximate analysis of coal?
Explain briefly how this analysis is carried out.
ANSWER:
By proximate analysis, we mean the analysis of coal to show the proportions of moisture, ash, volatile matter and fixed carbon ion the coal.
Moisture: The total moisture can be determined by either (i) taking a special moisture sample, which is dried in an air-oven at 105 degree Celsius, and the percentage loss in weight is recorded as the total moisture content: or (ii) the gross sample is air-dried at 30 to 45 degree Celsius and the loss of weight noted. The analysis sample is then taken from the gross sample, and a known weight of this sample is dried at 105 degree Celsius in a nitrogen filled minimum free space oven until it reaches constant weight. The percentage loss in weight represents the inherent moisture of the coal, and is added to the loss on air drying to give the total moisture content.
Volatile Matter: one gram of coal is heated out of contact with air for seven minutes at 900 degree Celsius. The loss in weight is then expressed as “the percentage of volatile matter”. NOTE: If the sample has not been pre-dried, the moisture content must be deducted.
Ash: One gram of the analysis is weighed into a silicon dish, and heated in a ventilated muffle furnace, gently at first to allow the emission of moisture and volatile matter without spitting, and finally at 815 degree Celsius until constant in weight. The residual weight is recorded as “per cent ash”.
The fixed carbon is calculated by subtracting the sum of the percentage of moisture, volatile matter and ash from 100.

TEST PAPER 2 (on Lessons 2 and 3)
Please read Lesson 2 Section 1 before stating the test.

What are the three main operations carried out in coal preparation?
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What affects the handle ability or a 12.5mm x 0 washed coal?
Turn back to lesson 2 and read section 2 before answering question 3
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3 Explain the basic reason why it is necessary to use the use the correct sampling techniques
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Now read Lesson 2 Section 3 and answer 4 and 6

4 What constitutes of coal do we measure directly in a proximate analysis? How do we complete the analysis.
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5 What factor in the ultimate analysis is used to differentiate between different coals?
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6. What is the meaning and importance of the term ‘calorific value’ of coal.
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Turn back to Lesson 2 and read Section 4, before answering questions 7 and 8

7. What is the object of preliminary preparation?
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8. Name three important techniques which can be used in preliminary preparation.
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Please read Lesson 3 Sections 1 and 2 before answering questions 9 – 13.

Which of the following are units of volume? Liter, grams, cubic meters, tons, centimeters. What do you understand

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Is 2.3g/mm3 a density or a relative density? Give the reason for your answer.
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You have two pieces of material:
Has a volume of 2 cubic meters and weighs 2400 kilograms
Has a volume of 10 cubic centimeters and weighs 9 grams
What are their densities? __________________________________________________________________________________________
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Which of the pieces of material mentioned in question 11 will float in water? Why does it float?
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Why are there no units of relative density?
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Read Lesson 3 Sections 3 and 4 before answering questions 14 – 15.

How is the relative density of coal related to its ash content? __________________________________________________________________________________________
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A cleaned coal with an average density of 1460 kg/m3. How large a bunker will be needed for 30 tons of the cleaned coal?
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COAL PREPARATION COURSE
Model Answers to Test Paper 2 (for Lessons 2 & 3)
LESSON 2

QUESTION:
What are the three main operations carried out in coal preparation?
ANSWER:
The main operations in coal preparation are sizing cleaning and blending.
QUESTION:
What affects the handle ability of a 12.5mm washed coal?
ANSWER:
The handle ability is affected by the fines content and the free moisture content.
QUESTION:
Explain the basic reason why it is necessary to use the correct sampling techniques.
ANSWER:
The correct sampling techniques have been designed to ensure that a representative sample is obtained, ie a sample which contains all the types of particles present in the whole, in their correct proportions.
QUESTION:
What constituents of coal do we measure directly in a proximate analysis? How do we complete the analysis?
ANSWER:
In a proximate analysis we determine directly the percentage of moisture, volatile matter and ash present in the coal. We complete the analysis by subtracting the sum of these percentages from 100 to give the percentage of fixed carbon.
QUESTION:
What factor in the ultimate analysis is used to differentiate between different coals?
ANSWER:
The carbon content which is an indication of rank.
QUESTION:
What is the meaning and importance of the term “calorific value” of coal?
ANSWER:
The “calorific value” is a measure of the heat content of the coal. It is important because it tells how much heat is available from a given weight of coal.
QUESTION:
What is the object of preliminary preparation?
ANSWER:
To ensure that the feed to a cleaning plant is acceptable in (i) rate of feed, (ii) size of feed, and (iii) consistency of feed quality.
QUESTION:
Name three important techniques which can be used in preliminary preparation?
ANSWER:
Any three of the following:
Screening
Removal of Tramp
Breaking
Blending
Bunkering
Weighing and Control of Feed Rate
Homogenizing
LESSON 3
QUESTION:
Which of the following are units of volume? Liters, grams, cubic meters, tons, centimeters. What do you understand by the term unit volume?
ANSWER:
Liters and cubic meters are units of volume. Unit volume is one cubic measurement eg one cubic centimeter.
QUESTION:
Is 2.3 g/m3 a density or a relative density? Give the reason for your answer.
ANSWER:
g/m3 is a density, because the units of measurement are given.
QUESTION:
You have two pieces of material:
Has a volume of 2 cubic meters and weight 2400 kilograms
Has a volume of 10 cubic centimeters and weighs 9 grams. What are their densities?
ANSWER:
The density of A is 2400/2 = 1200 Kg/m3, or 1.2 g/m3.
The density of B is 9/10 = 0.9 g/m3.
QUESTION:
Which of the pieces of material mentioned in Question 12 will float in water? Why does it float?
ANSWER:
The piece B (0.9 g/m3) will float in water (1.0 g/m3). Since the density of B is less than that of water, B is lighter than the volume of water it would have to displace in order to sink. The piece A has a density greater than water, and therefore would sink.

QUESTION:
Why are there no units of relative density?
ANSWER:
Relative density is a ratio of two densities, and so is just a number.
QUESTION:
How is the relative density of coal related to its ash content?
ANSWER:
Increase in relative density is accompanied by increase of ash content.
QUESTION:
A cleaned coal with an average density of 1460 kg.m3 has a bulk density of 750 kg.m3. How large a bunker will be needed for 30 tons of the cleaned coal? Give your answer in cubic meters.
ANSWER:
Bulk density is the weight of unit volume of material in the loose state, and is therefore used for estimating bunker series.
Volume required = 30 x 1000/750 = 40 cubic meters.

TEST PAPER 3 (ON LESSON 4)

QUESTION: What is meant by the relative density fraction 1.5 – 1.6?
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QUESTION: What safety precautions must be taken when using perchlorethylene for float and sink tests?
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QUESTION: What substances may be used to carry out a float and sink test at 1.8 relative density.
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QUESTION: What test is usually done on each relative density fraction to complete the float and sink analysis?
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Before answering questions 5-8, read Lesson 4 Sections 2a and 2b.

You should try to draw, on a large scale, the washability and M curves (see Lesson 4 Fig. 2 to 7, described in Lesson 4 Sections 2 and 3), in order to understand them more fully and to read the results more accurately.

QUESTION: Why do we plot in graph form the information from the float and sink tests?
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QUESTION: What is the densimetric curve? What is another name for it?
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Now study the graph at lesson 4 fig. 2 referred to in Lesson 4 section 2b and answer question 7 and 8
QUESTION: What would be the percentages of float and sinks produced by washing at the following relative densities?
Answer (from graph)
% Floats % Sinks
1.60
1.44
1.33
1.67
QUESTION: At what relative density would we need to separate the coal in order to obtain the following yields or products?
Answer (from graph)
71% of cleaned coal
34% of reject
58% of cleaned coal
29% of reject
Before answering questions 9-11, read Lesson 4 Section 2c.

QUESTION: What is the cumulative float curve?
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Study the graph at Lesson 4 FIG. 3, referred to in Lesson 4 Section 2c, then answer questions 10 and 11.

QUESTION: What will be the approximate ash percent of the cleaned coal for the following percentage yields?
Answer (from graph)
36% of floats
72% of floats
85% of floats
33% of sinks
QUESTION: What will be the yield of cleaned coal if its ash content has the following values?
Answer (from graph)
Ash Yield of cleaned coal
3%
8%
17%
7%
Please read Lesson 4 Section 2d before answering questions 12 to 14.

QUESTION: Which graph shows the relationship between the ash content of the reject and the percentage of reject produced?
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Now study the graph at Lesson 4 Fig. 4, referred to in Lesson 4 Section 2d, and answer questions 13 and 14.

QUESTION: What will be the ash percentage of the reject for the following yields of products?
Answer (from graph)
Yields of products Ash content of reject
61% of reject
70% of cleaned coal
50% of reject
QUESTION: If the ash content of the reject has the following values, what yield of reject will be obtained?
Answer (from graph)
Ash Content of Reject Yield of Reject
66%
80%
43%

Read Lesson 4 Section 2e, studying the graph at Lesson 4 Fig. 5, then answer question 15.

QUESTION:
From the following floats yields, what will be the highest possible ash content of an individual particle present in the cleaned coal?
Answer (from graph)
Floats Yields Highest Possible Ash Content
43%
60%
80%
Read Lesson 4 Section 2f and 2g, and using the combined curves graph at Lesson 4 Fig. 6 answer questions 16 through 19.

QUESTION: Using the4 combined curves (Lesson 4 Fig. 6), for each of the following average ash contents of sink yields, what will be the lowest possible ash content of an individual particle in the sinks?
Answer (from graph)
Average Ash Content Lowest Possible
of sinks yields ash content
78%
57%
62%
QUESTION: Assuming a separation at relative density of 1.50, what will be:
ANSWER:
The yield of cleaned coal?
The ash content of cleaned coal?
The yield of reject?
The ash content of the reject?
The maximum ash content of an individual particle in the float yield?
QUESTION:
If we wish to obtain a cleaned coal product of 10% ash:
ANSWER:
What will be the yield of the cleaned coal?
At what relative density should the separation be made?
What will be the ash content of the rejects?
What will be the highest ash content among the particles present in the floats yield?
QUESTION:
If we make a primary separation at 1.50 relative density and a secondary separation at 1.87 relative density:
What yield of middlings will be obtained?
What will be the lowest ash content of an individual particle in the middlings?
What will be the highest ash content of an individual particle in the middlings?
What will be the3 ash content of the middlings?
What is the total percentage recovery of combustible material?
Read Lesson 4 section 3, then using the combined Mayer and Desimetric curves (Lesson 4 fig. 7) answer questions 20 – 22.
QUESTION:
Find the cleaned coal ash percentage and the relative density of separation for each of the following cleaned coal yields:
ANSWER: Answer (from graph)
Cleaned Cleaned RD of
Coal Yield Coal Ash % Separation
30%
58%
70%
QUESTION:
For each of the following cleaned coal ash percentages, find the yield of cleaned coal:
ANSWER: Answer (from graph)
Cleaned Coal Ash % Cleaned Coal Yield
5.0
9.0
14.0
QUESTION:
If a primary separation at 1.42 RD and a secondary separation at 1.75 RD are made, find the yields and ash percentages of each of:
Answer (from graph)
Yield Ash %
Cleaned Coal
Middlings 65 – 49 =
Reject 100 – 65 =
REVISION QUESTION:
The following revision question is covered by subject matter in an earlier lesson. Your answer should be from about 100 up to 400 words.
Use a separate sheet of paper for your answer. You are invited to include a sketch.
Name and describe briefly the two main theories of how the coal-forming material was deposited. Describe a typical section of coal measure strata

MODEL ANSWERS TO TEST PAPER 3
(ON LESSON 4)
QUESTION:
What is meant by the relative density fraction 1.5 – 1.6?
ANSWER:
Material which is all heavier than 1/5 relative density but all lighter than 1.6 relative density.
QUESTION:
What safety precautions must be taken when using perchlorethylene for float and sink tests?
ANSWER:
A means must be provided for drawing the fumes away from the operator and out of the room in which the test is being carried out. Smoking must be prohibited. Skin and eye contact must be avoided.
QUESTION:
What substances may be used to carry out a float and sink test at 1.8 relative density.
ANSWER:
A mixture, in the correct proportions, or perchlorethylene and bromoform.
QUESTION:
What test is usually done on each relative density fraction to complete the float and sink analysis?
ANSWER:
An ash determination.
QUESTION:
Why do we plot in graph form the information from the float and sink tests?
ANSWER:
So that we can read off values of percent float and relative density other than those used in the test.
QUESTION:
What is the densimetric curve? What is another name for it?
ANSWER:
A graph showing the relationship between the relative density of separation and the yield of clean coal. The relative density curve, or the specific gravity yield curve.

QUESTION:
What would be the percentages of float and sinks produced by washing at the following relative densities?
ANSWER: Answer (from graph)
% Floats % Sinks
1.60 59 41
1.44 51 49
1.33 36 64
1.67 62 38

QUESTION:
At what relative density would we need to separate the coal in order to obtain the following yields or products?
ANSWER: Answer (from graph)
71% of cleaned coal 2.00
34% of reject 1.80
58% of cleaned coal 1.57
29% of reject 2.00
QUESTION:
What is the cumulative float curve?
ANSWER:
A graph showing the relationship between the ash content of the clean coal and the yield of clean coal.
QUESTION:
What will be the approximate ash percent of the cleaned coal for the following percentage yields?
ANSWER: Answer (from graph)
36% of floats 3.4
72% of floats 18.0
85% of floats 26.0
33% of sinks 13.5
QUESTION:
What will be the yield of cleaned coal if its ash content has the following values?
ANSWER: Answer (from graph)
Ash Yield of cleaned coal
3% 28%
8% 57%
17% 71.5%
7% 54.5%

QUESTION:
Which graph shows the relationship between the ash content of the reject and the percentage of reject produced?
ANSWER:
The cumulative sink curve.
QUESTION:
What will be the ash percentage of the reject for the following yields of products?
ANSWER: Answer (from graph)
Yields of products Ash content of reject
61% of reject 56%
70% of cleaned coal 81.5%
50% of reject 65%
QUESTION:
If the ash content of the reject has the following values, what yield of reject will be obtained?
ANSWER: Answer (from graph)
Ash Content of Reject Yield of Reject
66% 50.5%
80% 33.5%
43% 80.5%
QUESTION:
From the following floats yields, what will be the highest possible ash content of an individual particle present in the cleaned coal?
ANSWER: Answer (from graph)
Floats Yields Highest Possible Ash Content
43% 9.5%
60% 40%
80% 79%
QUESTION:
Using the4 combined curves (Lesson 4 Fig. 6), for each of the following average ash contents of sink yields, what will be the lowest possible ash content of an individual particle in the sinks?
ANSWER: Answer (from graph)
Average Ash Content Lowest Possible
of sinks yields ash content
78% 48%
57% 8%
62% 13%

QUESTION:
Assuming a separation at relative density of 1.50, what will be:
ANSWER:
The yield of cleaned coal? 54%
The ash content of cleaned coal? 6.6%
The yield of reject? 46%
The ash content of the reject? 69.3%
The maximum ash content of an individual particle in the float yield? 27%

QUESTION:
If we wish to obtain a cleaned coal product of 10% ash: ANSWER:
What will be the yield of the cleaned coal? 61%
At what relative density should the separation be made? 1.65%
What will be the ash content of the rejects? 76%
What will be the highest ash content among the particles present in the floats yield? 42%
QUESTION:
If we make a primary separation at 1.50 relative density and a secondary separation at 1.87 relative density:
What yield of middlings will be obtained?
What will be the lowest ash content of an individual particle in the middlings?
What will be the highest ash content of an individual particle in the middlings?
What will be the3 ash content of the middlings?
What is the total percentage recovery of combustible material?
ANSWER:
Separating at 1.87 RD, floats yield = 66.5% at 13.2% ash content.
Separating at 1.50 RD, floats yield = 54.0% at 6.6% ash content.
Yield of middlings = 66.5 – 54.0 = 12.5%.
Lowest ash content of an individual particle in the middlings is the same as that of the highest in the cleaned coal – 28%
Highest ash content of an individual particle in the middlings is similarly, the same as that of the lowest is the reject = 62%
Yield % Ash% Mass of Ash
Middlings and
Cleaned coal 66.5 13.2 (66.5 x 13.2)/( 100) = 8.87
Cleaned Coal 54.0 6.6 (54.0 x 6.6)/100 = 3.56
Middlings 12.5 5.22
Middlings ash = (5.22)/(12.5) x 100 = 41.8%
Raw coal ash = 35.4%.
Combustible material in raw coal = 64.55%
% Combustible material in cleaned coal and
Middlings = 100 – 13.2 = 88.6%
Mass of combustible material in combined yield of cleaned coal and middlings (66%) =
(86.8/100) x 100 = 57.7%
Recovery of combustible material = (57.7/64.55) x 100 = 89.4%
QUESTION:
Find the cleaned coal ash percentage and the relative density of separation for each of the following cleaned coal yields:
ANSWER: Answer (from graph)
Cleaned Cleaned RD of
Coal Yield Coal Ash % Separation
30% 3.2 1.31
58% 8.0 1.57
70% 15.8 1.94
QUESTION:
For each of the following cleaned coal ash
percentages, find the yield of cleaned coal:
ANSWER: Answer (from graph)
Cleaned Coal Ash % Cleaned Coal Yield
5.0 49.0%
9.0 60.0%
14.0 68.0%
QUESTION:
If a primary separation at 1.42 RD and a secondary separation at 1.75 RD are made, find the yields and ash percentages of each of:
ANSWER: Answer (from graph)
Yield Ash %
Cleaned Coal 49% 4.8
Middlings 65 – 49 = 16% 34.5
Reject 100 – 65 = 35% 79.0

REVISION QUESTION:
The following revision question on subject matter in an earlier lesson required an essay type answer and preferable a sketch.
QUESTION:
Name and describe briefly the two main theories of how the coal-forming material was deposited. Describe a typical section of coal measure strata.
(No model answer is provided, but the information is contained in Lesson 1, Section 2 (d), with Lesson 1, Fig.

TEST PAPER 4 (ON LESSONS 5 AND 6)

Before answering question 1, 2 and 3 in this Test Paper, please read Lesson 5 Section 1, 2 and 3.

List the main uses of screens in coal preparation.

Give examples of the uses of fixed or static screens.

List the more modern types of screen, in order of increasing speed

A run-of-mine coal is screened at 25mm on a screen which has an efficiency of 90%. If there are 120 tons of 25mm – o in the run-of-mine feed, how many tons of 25mm – o will pass with the screen overflow

Explain the influence of the size analysis of the feed when considering the efficiency and/or capacity of screening. What other properties of the feed influence the efficiency and/or capacity of screening?

Turn to Lesson 5 and read Section 6, 7 and 8

Describe the advantages that modern material such as stainless steel, rubber and polyurethane have conventional materials when used as screening media.

7. How may fines which are smaller than the screen aperture blind on the screen mesh? If both the screen overflow and underflow are to be subsequently washed, how may the blinding be simply overcome?

7A. Describe the various types of screen deck used in coal preparation for sizing at less than 12.5mm.

8. What factors do we have to take into account when considering how fast a particle will fall through a liquid?

9. Explain briefly the difference between free and hindered settling.

10. What basic practical step is taken to ensure that the separation in a launder or upward current washer is according to relative density?

11. Several pieces of raw coal just pass through a 25mm screen plate. In what way do other properties of the particles affect their stratification in a trough washer?

12. Describe briefly how the removal of products is achieved in a Blackett barrel washer.

13. On what general principle is the design of the upward current washer based?

14. Name two types of combined trough and upward current washers which were used in coal preparation

REVISION QUESTIONS
15. What do the following abbreviations stand for, and what are they units of?
t/m3
l
kg

Describe in your own words, the type of information which may be derived from:
The cumulative sink curve
The characteristic ash curve

COAL PREPARATION
MODEL ANSWERS TO TEST PAPER 4
(ON LESSONS 5 AND 6)
LESSON 5

Question:
List the main uses of screens in coal preparation.
Answer:
Primary screening
Product sizing
Dewatering
Medium recovery – Draining and Rinsing
Question:
Give examples of the uses of fixed or static screens.
Answers:
Rough separation of large sizes over bar screens.
Fines or breakage extraction in fixed chutes.
Dewatering on fixed wedgewires.
Dewatering and de-sliming on sieve-blends.
Question:
List the more modern types of screen, in order of increasing speed.
Answers:
Fixed or static screens
Shaking screens
Low speed vibrators
High speed vibrators and resonance screens
Question:
A run-of-mine coal is screened at 25mm on a screen which has an efficiency of 90%. If there are 120 tons of 25mm-0 in the run-of-mine feed, how many tons of 25mm-0 will pass with the screen overflow?
Answers:
Let x = tonnage passing through screen
x/120 x 100 = 90 x = 108
amount passing with overflow = 120 – 108 = 12 tons
Question:
Explain the influence of the size analysis of the feed when considering the efficiency and/or capacity of screening. What other properties of the feed influence the efficiency and/or capacity of screening?
Answers:
The size analysis of the feed affects the capacity and/or efficiency of screening because it is the particle near to the size of separation which are difficult to screen. Thus, as the proportion of these nearsize particles increases the capacity and/or efficiency are reduced.
Other properties which effect the capacity and/or efficiency are:
Tonnage fed to the screen
Moisture of the feed
Whether the feed causes obstruction by blinding or pegging of the screen deck.
Question:
Describe the advantages that modern materials such as stainless steels, rubber and polyurethane have over conventional materials when used as screening media.
Answers:
Stainless Steel: abrasion resistance; extended life in service; less tendency to blind; less maintenance cost.
Rubber: abrasion resistance; extended life in service; reduction in impact noise; “live” surface reduces pegging and blinding; less maintenance costs.
Polyurethane: abrasion resistance; extended life in service; easy maintenance with modular construction; reduction in impact noise; less maintenance costs.
Question:
How may fines which are smaller than the screen aperture blind on the screen mesh? If both the screen overflow and underflow are to be subsequently washed, how may the blinding be simply overcome?
Answers:
If the fines are damp the moisture may cause the fines to bind together so that they form a bridge over the screen mesh. Blinding may be overcome by wet screening.
7a. Question:
Describe the various types of screen deck used in coal preparation for sizing at less than 12.5mm.
Answers:
Loose rod deck: small diameter steel rods held in polyurethane bearers, and able to rotate with screen vibrations.
Heated deck: woven wire deck, electricity heated.
Piano wire deck: tensioned wires held in frame.
Rotational probably screen: rotating radial spokes mounted in vertical shaft, fed from center.
Woven wire deck: efficient only with very dry feeds.
Perforated plate deck: or in wet screening applications.

LESSON 6

Question:
What factors do we have to take into account when considering how fast a particle will fall through a liquid?
Answers:
Acceleration due to gravity.
Volume of the particle.
Relative density of the particle.
Relative density of the liquid.
Viscosity of the liquid.
Shape of the particle.
Question:
Explain briefly the difference between free and hindered settling.
Answers:
Free settling means settling with no interference from neighboring particles. Hindered settling means that the particles are close enough to affect each other’k settling movement, and that the water current set up as a result of the movement of the particles affects their settling.
Question:
What basic practical step is taken to ensure that the separation in a launder or upward current washer is according to relative density?
Answers:
Primary screening to close size ranges so that he effective of size and shape are reduced.
Question:
Several pieces of raw coal just pass through a 25mm screen plate. In what way do other properties of the particles affect their stratification in a trough washer?
Answers:
Relative Density: the higher relative density
Particles will settle more rapidly (hindered settling conditions) to the bottom of the trough, whilst the lower relative density particles will be more readily carried along by the water stream.
Shape: flat particles tend to lie parallel to the stream, whereas cubes roll downstream. The former are usually shale, the latter coal.
Question:
Describe briefly how the removal of products is achieved in a Blackett barrel washer.
Answers:
The waterflow down the barrel carries the coal forward and out of the barrel, over the screen attached at the end, which removes the water and undersize. The shale particles sink in the waterflow and are trapped by the spiral screw conveyor, which with the rotation of the barrel, conveys the shale to discharge at the upper end of the barrel.
Question:
On what general principle is the design of the upward current washer based?
Answers:
If the particles do not have very different sizes, upward water currents lift particles of low relative density more readily than they lift particles of high relative density.
Question:
Name two types of combined trough and upward current washers which were used in coal preparation.
Answers:
Rheo-Laveur
Hoyois
REVISION QUESTIONS
The following are revision questions on subject matter in earlier lessons.
Question:
What do the following abbreviations stand for, and what are they units of?
t/m3
l
kg
Answers:
Tons per cubic meter: density
Liters: volume
Kilograms: weight
Question:
Describe in your own words, the type of information which may be derived from:
The cumulative sink curve
The characteristic ash curve
(no model answer provided)

TEST PAPER 5
(ON LESSON 7 AND APPENDIX

Please read Lesson 7 Sections 1 and 2 before starting the test paper.

Which of the following statements is correct:
Only on a pulsation stroke
When the bed is at rest
On both pulsation and suction strokes
Which of the following statements is correct:
Weakens the pulsation stroke
Strengthens the suction stroke
Weakens the suction stroke
What adjustments should be made to a Baum jig to increase the suction without changing the pulsation?

What conditions are important to achieve the most efficient separation possible in a Baum jig? Why is it difficult to achieve these conditions with run-of-mine coal? How may these problems be overcome

Explain the fundamental difference between a Baum and a Creaves type jig. Illustrate your answer with simple sketches of each type, indicating the main constructional differences.

Please read Section 3 of the Lesson, and answer questions 6, 7 and 8.

How is refuse extracted in a Baum jig washing say 125mm – 0 raw coal? What system may be employed to wash 10mm – 0 raw coal?

(a) Name the three features common to all auto refuse extraction mechanisms.
(b) Name these three features in the Hirst automatic refuse discharge system.

In the SIMONACCO refuse control, what would be the effect in the wash box if the diaphragm operated air control valve unit was not properly bolted down to the air still?

Draw a solids flow diagram for a Baum plant treating 175mm – 0 raw coal. The plant consists of a three elevator wash-box, with middlings extraction at the third elevator, and produces cobbles, trebles, doubles, singles and 12.5mm – 0 smalls. Middlings are screened at 12.5mm and the – 12.5mm undersize discarded with the refuse; the +12.5mm middlings are crushed and returned to the wash-box feed. Your flow diagram should show major items of plant, ie wash-box, screens, crusher, main sumps and settling tower.
(Use a separate sheet of paper for your drawing.)

APPENDIX 1: SAFETY
The following questions are covered by subject matter in Appendix 1: Safety. Your answer to each should be from about 100 to 400 words. Use a separate sheet of paper for each answer.
Float and sink analysis to determine the performance of a separating unit is commonly carried out using perchlorethylene, benzene, toluene, paraffin or mixtures of these liquids. What hazards do these present, and what precauti9ons should be taken in their use?

Outline the safety precautions which apply to walkways and means of access to plants

Notices are an important factor in helping to avoid accidents. Write a short list of notices which are commonly used in coal preparation plants, and comment on their positioning and clarity

(a) What electrical apparatus may NOT be generally operated on a coal preparation plant without special authorization?
(b) What persons are allowed to operate this class of electrical apparatus?
(c) Under what conditions may any person operate any electrical apparatus?

COAL PREPARATION
MODEL ANSWERS TO TEST PAPER 5
(ON LESSON 7 AND APPENDIX 1)

LESSON 7

QUESTION:
Which of the following statements is correct?
ANSWERS:
Separation in a Baum jig occurs:
On both pulsation and suction strokes
QUESTION:
Which of the following statements is correct?
ANSWERS:
In a Baum jig, an increase in the amount of backwater: (c) weakens the suction stroke.
QUESTION:
What adjustments should be made to a Baum jig to increase the suction without changing the pulsation?
ANSWERS:
Increase the air, and decrease the backwater until the original strength of pulsation is achieved.
QUESTION:
What conditions are important to achieve the most efficient separation possible in a Baum jig? Why is it difficult to achieve these conditions with the run-of-mine coal? How may these problems be overcome?
ANSWERS:
Uniformity of feed rate
Uniformity of feed composition
Uniformity of feed distribution
Mobility of bed
Run-of-mine coal has variations in the size consist and refuse content, and these make it difficult to achieve a uniform feed composition to a washbox.
By use of devices such as the CAROL feeder it is possible to ensure that the washbox is not overloaded with coal or shale. By blending or homogenizing, by a suitable arrangement of washery feed bunkers or ground stockpile, the feed to the washbox may be made more uniform both in terms of mine and refuse content.
The feed should then be fed evenly over the width of the box to ensure an even mobility of the bed without size segregation.
QUESTION:
Explain the fundamental difference between a Baum and a Greaves type jig. Illustrate your answer with simple sketches of each type, indicating the main constructional differences.
ANSWERS:
The pulsations in the Baum jig are obtained by using compressed air to move the water through the fixed bedplate, whereas in the Greaves type of washbox the pulsations are obtained by moving the basket up and down in a static tank of water.
Greaves (2) Baum

See Lesson 7, Fig. 1 See Lesson 7, Fig. 1(a)

QUESTION:
How is refuse extracted in a Baum jig washing say 125mm – 0 raw coal? What system may be employed to wash 10mm – 0 raw coal?
ANSWERS:
When washing a wide size range in a Baum jig the larger dirt is carried over the bedplate and discharged to the dirt elevator through the refuse gates. At the same time the fine dirt is extracted through the bedplate perforations on the suction stroke.
When washing fine coal a feldspar bed jig may be used, which extracts all of the dirt through the bedplate.
QUESTION:
Name the three features common to all auto refuse extraction mechanisms.
Name these three features in the Hirst automatic refuse discharge system.
ANSWERS:
The indicator
The extracting method
The link mechanism
Air pressure in stand pipe
Diaphragm valve unit
Monitor unit
QUESTION:
In the SIMONACCO refuse control, what would be the effect in the washbox if the diaphragm operated air control valve unit was not properly bolted down to the air still?
ANSWERS:
Coal would be lost with the dirt discharged to the elevator. This would occur since, even with the diaphragm valve shut, there would be an air leak and pulsations could occur between the refuse gates and thus the dirt bed and eventually coal would be lost.

QUESTION:
Draw a solids flow diagram for a Baum plant treating 175mm – 0 raw coal. The plant consists of a three elevator washbox, with middlings extraction at the third elevator, and produces cobbles, trebles, doubles, singles and 12.5mm and the – 12.5mm undersize discarded with the refuse; the + 12.5mm middlings are crushed and returned to the washbox feed. Your flow diagram should show major items of plant, ie washbox, screens, crusher, main sumps and settling tower.
ANSWERS:
(Flow diagrams will vary slightly but the principles will be as shown in Lesson 7, Fig. 7.)
APPENDIX 1: SAFETY
The questions required essay type answers.
QUESTION:
Float and sink analysis to determine the performance of a separating unit is commonly carried out using perchlorethylene, benzene, toluene, paraffin or mixtures of these liquids. What hazards do these present, and what precautions should be taken in their use?
ANSWERS:
Type of liquid Hazard
Perchlorethylene Toxic vapor, produces
phosgene when heated
Toluene, Benzene Toxic vapors, highly
inflammable
Paraffin Less toxic and inflammable
than Toluene and Benzene,
but care in use is still required.
Precautions
Proper float and sink bench with extractor fan, and drainage and drying facilities.
No smoking in the vicinity (large, clear notices).
Wear rubber gloves and goggles.
Fire extinguishers nearby (foam or dry powder).
Seek medical attention if dizzy or sick. Persons overcome by fumes should be removed to fresh air and given artificial respiration.
Only authorized personnel should use these liquids.
Large volumes of benzene, toluene and paraffin should be kept in special store because of fire hazard.
Main containers should be sealed, immediately liquid has been drawn off.
The liquids used should be covered, or placed in sealed containers, after use.

QUESTION:
Outline the safety precautions which apply to walkways and means of access to plants.
(No model answer provided.)
QUESTION:
Notices are an important factor in helping to avoid accidents. Write a short list of notices which are commonly used in coal preparation plants and comment on their positioning and clarity.
(No model answer provided.)
QUESTION:
What electrical apparatus may NOT be generally operated on a coal preparation plant without special authorization?
What persons are allowed to operate this class of electrical apparatus?
Under what conditions may any person operate any electrical apparatus?
ANSWERS:
Special authorization to operate is need for: any machinery which must be operated in accordance with signals;
Any machinery capable of developing more than 10 hp;
Any electrical apparatus operating at more than 25 volts.
Persons allowed to operate such machinery
Must be: Electricians of the Mine.
In emergencies anyone may cut off the power.

COAL PREPARATION
MODEL ANSWERS TO TEST PAPER 6
(ON LESSONS 8 AND 9)

LESSON 8

Question:
Why is the separation in a well-designed dense medium bath much more efficient than that in a Baum washbox?
Answer:
The size and shape of particles play little part in the separation in a dense medium bath because there is very little movement of the medium itself, whereas the stratification in a Baum jig is affected by the size and shape of particles. Moreover, in a dense medium bath the float and sink products are widely separated, and their removal is easy.
Question:
What would be the effect of diluting a suspension of 1.8 relative density with an equal volume of water?
Answer:
Mass of 1 volume of suspension @ 1.8 RD = 1.8 units
Mass of 1 volume of water @ 1.0 RD = 1.0 units
Mass of 2 volumes of dilute suspension = 2.8 units
Density = 2.8/2 untis = 1.40
Relative Density = 1.40/(Density of water = 1) = 1.40
The Suspension would be reduced in relative density from 1.8 to 1.4
Question:
Why is it necessary to clean a dense medium continuously?
Answer:
In order to control the degree of contamination of the medium.
Question:
What do you understand by an unstable medium?
Answer:
A medium from which the solids settle out rapidly.
Question:
Contamination of a medium by clay slimes normally make it:
Answer:
More stable
More viscous
Question:
How may the viscosity of a medium be reduced?
Answer:
The viscosity of a medium may be reduced:
By reducing the proportion of slimes in the medium
By using the medium solids in a coarser state.
Question:
List the following materials, which are each used as a dense medium, in increasing order or relative density (ie the lowest relative density material first). Write alongside their relative densities.
Answer:
Shale relative density 2.4
Sand relative density 2.6
Barytes relative density 4.5
Magnetite relative density 5.0
Question:
Draw a general products flow diagram for a plant treating run-of-mine coal, incorporating:
Cracker for +200mm raw coal (state type of cracker)
Dense medium washing for the 200mm – 1.25mm raw coal
Outloading of 12.5mm – O untreated smalls
Answer:
The diagram should be similar to a combined version of the following:
Lesson 2 Fig. 5 Preliminary preparation
Lesson 9 Fig. 1 Dense medium
Question:
Name three deep type three product dense medium baths. Describe with the aid of a simple sketch how the middlings are exgtracted from any one of these.
Answer:
Chance cone
Barvoys
Tromp (deep type)
The description should include details of:
The middlings tube employed in the Chance cone or Barvoys baths.
The flows of medium necessary to move the middlings to the appropriate extraction point.
Question:
Describe with the aid of a sketch how Ridley-Scholes dense medium baths may be used to provide a three product separation.
Answer:
See Lesson 9 Section 3(c) (i) final paragraph, and Lesson 9 Fig. 6.
Question:
Describe very briefly the principles adopted for cleaning and concentrating two of the following media:
Sand
Barytes
Magnetite
Answer:
Sand
Sand settles more quickly in water than coal and shale slimes, and hence forms the lower layer in a settling tank. The coal and shale are taken off in the overflow. In settling the sand suspension becomes concentrated.
Barytes
Barytes is cleaned by froth flotation, and re-concentrated by settlement in a thickener.
Magnetite
Magnetite is cleaned and re-connected by a magnetite separator, which lifts the magnetite out of the dilute contaminated suspension.
Question:
Describe very briefly with the aid of a simple sketch the medium cleaning and recovery circuit used for a shale medium.
Answer:
The sketch should be similar to that of Lesson 9 Fig. 12.

COAL PREPARATION
MODEL ANSWERS TO TEST PAPER 6
(ON LESSONS 8 AND 9)

LESSON 8

Question:
Why is the separation in a well-designed dense medium bath much more efficient than that in a Baum washbox?
Answer:
The size and shape of particles play little part in the separation in a dense medium bath because there is very little movement of the medium itself, whereas the stratification in a Baum jig is affected by the size and shape of particles. Moreover, in a dense medium bath the float and sink products are widely separated, and their removal is easy.
Question:
What would be the effect of diluting a suspension of 1.8 relative density with an equal volume of water?
Answer:
Mass of 1 volume of suspension @ 1.8 RD = 1.8 units
Mass of 1 volume of water @ 1.0 RD = 1.0 units
Mass of 2 volumes of dilute suspension = 2.8 units
Density = 2.8/2 units = 1.40
Relative Density = 1.40/(Density of water = 1) = 1.40
The Suspension would be reduced in relative density from 1.8 to 1.4
Question:
Why is it necessary to clean a dense medium continuously?
Answer:
In order to control the degree of contamination of the medium.
Question:
What do you understand by an unstable medium?
Answer:
A medium from which the solids settle out rapidly.
Question:
Contamination of a medium by clay slimes normally make it:
Answer:
More stable
More viscous
Question:
How may the viscosity of a medium be reduced?
Answer:
The viscosity of a medium may be reduced:
By reducing the proportion of slimes in the medium
By using the medium solids in a coarser state.
Question:
List the following materials, which are each used as a dense medium, in increasing order or relative density (ie the lowest relative density material first). Write alongside their relative densities.
Answer:
Shale relative density 2.4
Sand relative density 2.6
Barytes relative density 4.5
Magnetite relative density 5.0
Question:
Draw a general products flow diagram for a plant treating run-of-mine coal, incorporating:
Cracker for +200mm raw coal (state type of cracker)
Dense medium washing for the 200mm – 1.25mm raw coal
Outloading of 12.5mm – O untreated smalls
Answer:
The diagram should be similar to a combined version of the following:
Lesson 2 Fig. 5 Preliminary preparation
Lesson 9 Fig. 1 Dense medium
Question:
Name three deep type three product dense medium baths. Describe with the aid of a simple sketch how the middlings are exgtracted from any one of these.
Answer:
Chance cone
Barvoys
Tromp (deep type)
The description should include details of:
The middlings tube employed in the Chance cone or Barvoys baths.
The flows of medium necessary to move the middlings to the appropriate extraction point.
Question:
Describe with the aid of a sketch how Ridley-Scholes dense medium baths may be used to provide a three product separation.
Answer:
See Lesson 9 Section 3(c) (i) final paragraph, and Lesson 9 Fig. 6.
Question:
Describe very briefly the principles adopted for cleaning and concentrating two of the following media:
Sand
Barytes
Magnetite
Answer:
Sand
Sand settles more quickly in water than coal and shale slimes, and hence forms the lower layer in a settling tank. The coal and shale are taken off in the overflow. In settling the sand suspension becomes concentrated.
Barytes
Barytes is cleaned by froth flotation, and re-concentrated by settlement in a thickener.
Magnetite
Magnetite is cleaned and re-connected by a magnetite separator, which lifts the magnetite out of the dilute contaminated suspension.
Question:
Describe very briefly with the aid of a simple sketch the medium cleaning and recovery circuit used for a shale medium.
Answer:

The sketch should be similar to that of Lesson 9 Fig. 12.

TEST PAPER 6 (on Lessons 8 and 9)

LESSON 8

Please read Lesson 8 Sections 1 and 2 before starting the test paper.
Why is the separation in a well-designed dense medium bath much more efficient than in a Baum washbox?

Now read Lesson 8 Section 3
What would be the effect of diluting a suspension of 1.8 relative density with an equal volume of water

Read Lesson 8 Section 4 and questions 3-6.
Why is it necessary to clean a dense medium continuously?

What do you understand by an unstable medium?

Contamination of a medium by clay slimes normally makes it:
More stable
Less stable
More viscous
Higher in relative density
How may the viscosity of a medium be reduced?
Turn back to Lesson 8 and read Section 5

List the following materials, which are each used as a dense medium, in increasing order of relative density (ie the lowest relative density material first) Barytes, Magnetite, Sand, Shale
Write alongside their relative densities.
Use a separate sheet of paper to answer each of the following questions. Remember to use pencil rather than pen when drawing.
Now read Lesson 9 Sections 1-2.

Draw a general products flow diagram for a plant treating run-of-mine coal, incorporating:
Cracker for +200mm raw coal (state type of cracker)
Dense medium washing for the 200 – 12.5mm raw ciak
Outloading of 12.5mm – 0 untreated smalls
Read Lesson 9 Section 3, and answer question 9 and 10.
Name three deep type three-product dense medium baths. Describe with the aid of a simple sketch how the middlings are extracted from any one of these.
Describe with the aid of a sketch how Ridley-Scholes dense medium baths may be used to provide a three product separation.
Read Lesson 9 Section 4, and answer questions 11 – 14.
Describe very briefly the principles adopted for cleaning and concentrating two of the following media:
Sand
Barytes
Magnetite
Describe very briefly with the aid of a simple sketch the medium cleaning and recovery circuit used for a shale medium.
REVISION QUESTIONS
The following revision questions are covered by subject matter in earlier lessons.
If a raw coal feed containing 48% of – 12.5mm material is screened at 12.5mm, and the amount of screen overflow produced represents 68% of the raw coal feed, what is the screening efficiency at 12.5mm? List the possible reasons for the value of efficiency obtained.
List the qualities of coal which are produced from your plant, and the purposes for which they are sold. What tests are performed on the plant to ensure that the quality of these products is maintained?

MODEL ANSWERS TO TEST PAPER 6 (on Lessons 8 and 9)
LESSON 8

Lesson 2 Fig. 5 Preliminary preparation

Lesson 9 Fig, 1 Dense medium

Question:
Name three deep type three product dense medium baths.
Describe with the aid of a simple sketch how the middlings are extracted from any one of these.
Answer:
Chance cone
Barvoys
Tromp (deep type)
The description should include details of :
The middlings tube employed in the Chance cone or Barvoys baths.
The flows of medium necessary to move the middlings to the appropriate extraction point.
Question:
Describe with the aid of a sketch how Ridley-Scholes dense medium baths may be used to provide a three product separation.
Answer: See Lesson 9 Section 3(c) (i) final paragraph, and Lesson 9 Fig. 6.
Question:
Describe very briefly the principles adopted for cleaning and concentrating two of the following media:
Sand
Barytes
Magnetite
Answer:
Sand Sand settles more quickly in water than coal and shale slimes, and hence forms the lower layer in a settling tank. The coal and shale are taken off in the overflow. In settling, the sand suspension becomes concentrated.
Barytes Barytes is cleaned by froth flotation, and re-concentrated by settlement in a thickener.
Magnetite Magnetite is cleaned and re-concentrated by a magnetite separator, which lifts the magnetite out of the dilute sontaminated suspension.
Question:
Describe very briefly with the aid of a simple sketch the medium cleaning and recovery circuit used for a shale medium.
Answer:
The sketch should be similar to that of Lesson 9 Fig. 12.
Question:
If a raw coal feed containing 48% of – 12.5mm material is screened at 12.5mm and the amount of screen overflow produced represents 68% of the raw coal feed, what is the screening efficiency obtained.
Answer:
Amount of undersize in feed = 48%
Amount of underside passed = 100-68=32%
Therefore in every 100 tons
Weight of undersize in feed = 48 tons
Weight of undersize passed = 32 tons
Efficiency = (Weight of underside passed)/(Weight of underside in feed) x 100/1
= 32/48 x 100/1 = 66.7%
This represents rather inefficient screening, and may be due to:
Excessive amount of near-sized material
Too short a length of screen
Too small amount of open area
Inclination of screen too great
Speed of screen too slow
Amplitude of vibration too low
Blinding or puffing of screen
Excessive moisture in feed
Exceeding nominal capacity of screen

TEST PAPER 7 (on Lesson 10)

Please read Lesson 10 Sections 1 to 7 before beginning this test paper.
1. Name three uses of cyclones on coal preparation plants.

2. Draw simple sketches indicating the movement of both solids and liquids in a dense medium washing cyclone. Label the parts of the cyclone.
(Answer this question of a separate sheet of paper.)
3. Use the formula
Settling velocity oc gv (d-D) – R

4. Draw a simple flow diagram showing how cyclones can be used in a Baum Jig circuit.
(Answer this question of a separate sheet of paper.)
5. Say which of the following statements are true and which are false:
T F The medium that comes out of the cyclone with the clean coal is lower in relative density than the feed medium.
T F Thickening cyclones can be used to produce a “gin clear” overflow.
T F A thickening cyclone is working most efficiently when the solids underflow is “roping”.
T F A vorsyl is a type of cyclone used for cleaning small coal.
T F Thickening cyclones operate most efficiently at an inlet pressure of 50 kPa.
T F 1 kg of magnetite is used for every ton of raw coal washed in a well-run cyclone plant.
Read Lesson 10 Sections 8-11, and then answer questions 6-8.

6. Why is it necessary for a coal washing plant to have efficient water clarification facilities

7. Draw a simple flowsheet showing the stages in a water clarification plant which is treating raw slurry. Label the important features carefully.
(Answer this question of a separate sheet of paper.)
8. (a) Describe what takes place in a suspension of froth flotation tailings, when a very dilute solution of polyelectrolyte flocculant is added.
(b) What would be the adverse effects of adding too much polyelectrolyte?
(c) What is the purpose of the rakes in a conventional thickener?

Read Lesson 10 Sections 12-22. Answer questions 9-15 each on a separate sheet, and in the same order of question number.

9. Explain the actions of frothers and collectors in the froth flotation process.
10. Make a cross-sectional sketch of a sub-aeration type froth flotation cell showing the position of entry for the feed, and of exit for the products.
11. Outline the principles involved in froth flotation. Why is froth flotation therefore restricted, in principle, as well as in practice, to the cleaning of fine coal?
12. What practical observations should be made to ensure that a froth flotation plant is operating satisfactorily?
Revision Questions
The following revision questions are covered by subject matter in earlier lessons. Your answers to each should be from about 100 to 400 words.
13. Describe, with the aid of a flow diagram, how the medium is recovered from the products and reconcentrated for use in the Chance Washing System.
14. You find one of your workmates, apparently unconscious, laying on the floor. He appears to have been handling electrical equipment. Describe in detail how you would deal with the emergency.
15. How do the length and width of a screen effect its performance?

MODEL ANSWERS TO TEST PAPER 7
(ON LESSON 10)

Question:
Name three uses of cyclones on coal preparation plants.
Answer:
(i) The cleaning of small coal.
(ii) The thickening and recovery of fine coal from washery circuits (and hence partial clarification of the water).
(iii) The recovery of dense medium (sometimes in conjunction with other equipment such as magnetic separators).
Question:
Draw simple sketches indicating the movement of both solids and liquids in a dense medium washing cyclone. Label the parts of the cyclone.
Answer:
See Lesson 10 Fig. 3-7.
Question:
Use the formula
Settling velocity oc gv (d-D) – R
To explain why cyclones can be used to clean small coal.
Answer:
In cyclones the force due to gravity g is replaced by the much greater centrifugal force created by the flow of liquid in a vortex. The rapid rotation of medium means that the layers of medium move relative to one another. This “shearing” action considerable reduced the viscosity of the medium, thus decreasing the resistance R.
Question:
Draw a simple flow diagram showing how cyclones can be used in a Baum Jig circuit.
Answer:
See Lesson 10 Fig. 11.
Question:
a) The medium that comes out of the cyclone with the clean coal is lower in relative density than the feed medium. TRUE
Answer:
TRUE
b) Thickening cyclones can be used to produce a “gin clear” overflow.
Answer:
FALSE
c) A thickening cyclone is working most efficiently when the solids underflow is “roping”.
Answer:
FALSE
d) A vorsyl is a type of cyclone used for cleaning small coal.
Answer:
TRUE
e) Thickening cyclones operate most efficiently at an inlet pressure of 50 kPa.
Answer:
FALSE (100 dpa)
f) 1 kg of magnetite is used for every ton of raw coal washed in a well-run cyclone plant.
Answer:
FALSE (The loss of magnetite on an efficiently run plant should be 0.23-0.5 kg/ton.)
Question:
Why is it necessary for a coal washing plant to have efficient water clarification facilities?
Answer:
By the use of a water clarification plant it is possible to control the solids in the wash water and thereby eliminate one major cause of product inconsistency. It is possible to ensure that any effluent will be clear, thus meeting the requirements of the water pollution legislation.
By careful control of water input to the plant, closed circuit operation is possible. The maximum recovery of fine coal particles can be achieved.
Question:
Draw a simple flowsheet showing the stages in a water clarification plant.
Answer:
See Lesson 10 Fig. 12.
Question:
a) Describe what takes place in a suspension of froth flotation tailings, when a very dilute solution of polyelectrolyte flocculant is added.
b) What would be the adverse effects of adding too much polyelectrolyte?
c) What is the purpose of the rakes in a conventional thickener?
Answer:
a) The electrostatic charges on the solid particles in suspension are neutralized by the action of the polyelectrolyte, so causing them to join together for flocculate, but the reagent has so many active charges on each molecule, that the flocs formed can be large, so aiding rapid settlement. All the solids are settled out in this fashion leaving a clear water over the settled solids.
b) The addition of too much polyelectrolyte is wasteful and adds to the costs of the operation due to the high price of the reagent. Some polyelectrolyte reagents can give the particles in suspension another charge, so that the suspension tends to become stable again. Over-dosage therefore both wastes flocculant and hinders flocculation.
c) The rakes in conventional thickener compress the flocs and squeeze out much of the water, and also push the thickened compressed sludge to the center of the thickener for discharge.
Question:
Explain the actions of frothers and collectors in the froth flotation process.
Answer:
A frother works by reducing the tension at the surface of the water. This ensures that the air bubbles which have been formed do not readily break up on reaching the surface, so that a stable froth is formed.
A collector forms a film or skin on the coal particles to strengthen the air/coal bond. An air bubble clings to the collector and the collector clings to the coal particle.
Question:
Make a cross-sectional sketch of a sub-aeration type froth flotation cell showing the positions of entry for the feed, and of exit for the products.
Answer:
See Lesson 10 Fig. 17.
Question:
Outline the principles involved in froth flotation.
Why is froth flotation therefore restricted in principle as well as in practice, to the cleaning of fine coal?
Answer:
No model answer is provided to this question.
Question:
What practical observations should be made to ensure that a froth flotation plant is operating satisfactorily?
Answer:
No model answer is provided to this question.
REVISION QUESTIONS
The following revision questions covering subject matter in earlier lessons required essay type answers.
Question:
Describe, with the aid of a flow diagram, how the medium is recovered from the products and reconcentrated for use in the Chance washing system.
Answer:
No model answer is provided to this question.
14. Question:
You find one of your workmates, apparently unconscious, lying on the floor. He appears to have been handling electrical equipment. Describe in detail how you would deal with the emergency.
Answer:
a) Isolate the supply if possible and/or remove the patient from the apparatus, taking suitable precautions against self-electrocution.
b) Examine th3e patient for signs of breathing and signs of injury. If breathing has stopped apply artificial respiration, preferably mouth-to-mouth. If there are no signs of recovery after 6-8 “inflations”, examine for heart activity. If absent, apply external heart message.
c) Keep the patient warm. Treat for shock and if necessary give first aid for any external injuries.
d) At the earliest opportunity after discovering the patient send for medical aid, but if you are alone do not leave the patient until he is breathing normally and the skin color is returning to normal.
e) The answer should make it plain that speed in giving artificial respiration is important above all else after the patient has been removed from the electrical supply.
f) Ensure that the apparatus is checked by an electrician for safety, before any attempt is made to re-start.
NOTE: The wording of the question implies that the casualty is no longer in contact with the supply.
15. Question:
How do the length and width of a screen affect its performance?
Answer:
See Lesson 5 Section 5.

TEST PAPER 8
(ON LESSON 11 AND 12)

Lesson 11

Please read Lesson 11 Section 1 before starting the test paper.

1. Explain clearly the difference between free and inherent moisture. Why is the moisture content of washed smalls several per cent higher than that of washed cobbles produced form the same raw feed?
(Answer on a separate sheet of paper).
Before answering the next question, read Lesson 11 Sections 2 and 3.

2. What free moisture content would you expect in the following products from the dewatering screens of the washery?
(a) Washed cobbles (150-100mm) ______________
(b) Washed smalls (12.5mm 1/2mm) ____________
(c) Slurry (-1/2mm) __________________________
3. Why is it often necessary to centrifuge a washed smalls product after dewatering on screens?

4. Describe, with the aid of a sketch, the basic principle of dewatering coal by the use of centrifuges. (Use a separate sheet of paper.)
5. How is the coal moved through the Birtley Humboldt centrifuge? State two advantages over this arrangement compared with that in a Reineveld centrifuge.

6. Describe with the aid of sketches the basic difference between the solid bowl centrifuge and th3e screen-bowl centrifuge. What are their separate uses? (Use separate sheet of paper for your answer.)

Please read Lesson 11 Section 5.
Answer questions 7 – 10 each on a separate sheet.

7. Briefly describe, with the aid of simple sketches, two methods of discharging the cake from a rotary vacuum drum filter.
8. Sketch a pressure filter, showing the flow of feed and filtrate.
9. Wat is the difference between a cascade dryer and a flash dryer? (Your answer should be illustrated by simple sketches.)
10. Describe, with the aid of a sketch, a fluidized bed dryer.

Please read Lesson 11 Section 7 to 11 before answering questions 11 and 12. Use a separate sheet of paper for your answers.

11. Name two different types of pump used in coal preparation plants for liquid pumping duties, and briefly describe how they work.
12. List the different uses for pressurized air which may occur on coal preparation plants, with a brief description of the type of machine used to supply the air in each case.

LESSON 12

Now read Lesson 12 Sections 1 to 4 before answering questions 13 to 15.

13. What types of comminution process are encountered in coal preparation plants? Name one machine of each type.

14. What practical advantages may be obtained by using a Bradford Breaker rather than a jaw breaker for reducing +100mm raw coal to -100mm?

15. Sketch a swing hammer crusher. Describe how this type of crusher is designed to avoid damage to the machine by uncrushable foreign material.
(Answer on a separate sheet of paper.)
Now read Lesson 12 Sections 5 – 9, and then answer question 16, using a separate sheet of paper.
16. Describe two problems which might be encountered in the operation of belt conveyors. What measures could be taken to overcome these problems?
Now read Lesson 12 Sections 10 to 15 before answering questions 17 and 18, using a separate sheet of paper.
17. Describe with the aid of sketches the different ways in which coal flows through a bin.
18. Describe the various ways in which size degradation within a bin can be reduced.
Now read Lesson 12 Sections 16 and 19, and answer question 19 using a separate sheet of paper.
19. For what purpose is a boom loader used? Describe it with a sketch.

MODEL ANSWERS TO TEST PAPER 8
(ON LESSONS 11 AND 12)

1. Question:
Explain clearly the difference between free and inherent moisture. Why is the moisture content of washed smalls several per cent higher than that of washed cobbles produced from the same raw feed?
Answer:
Free moisture is that retained on the surface of the coal, especially at the points of contact between particles in a mass. Inherent moisture is that contained in the innermost pores of the coal particles themselves.
The washed smalls product will have a much greater surface area of coal and many more points of contact between particles than the same weight of washed cobbles, and therefore the amount of water retained as free moisture will be higher.
2. Question:
What free moisture content would you expect in the following products from the dewatering screens of the washery?
Answer:
(a) Washed cobbles (150 x 100mm) 1% – 2%
(b) Washed smalls (12.5 x 1/2mm) 12% – 15%
(c) Slurry ( -1/2mm) 20% – 25%
3. Question:
Why is it often necessary to centrifuge washed smalls product after dewatering on screens?
Answer:
The washed smalls product with 12% – 15% free moisture usually contains more water than is acceptable to the customer for use, and thus centrifuging is necessary if the product is to attain a moisture content suitable for marketing purpose.
4. Question:
Describe, with the aid of a sketch, the basic principle of dewatering coal by the use of centrifuges.
Answer:
(See sketch of basic centrifuge design – Lesson 11 Fig. 4)
The basic principle is the replacement of the gravity by a high centrifugal force. The wet coal is introduced on to the walls of rotating perforated container or basket. The coal is retained by the wall but the water flows through the perforations. The dewatered coal from the inside, and the water which flows through the basket, are collected separately.

5. Question:
How is the coal moved through the Birtley-Humboldt centrifuge? State two advantages over this arrangement compared with that in Reineveld centrifuge.
Answer:
In the Birtley-Humboldt centrifuge, the revolving container is vibrated along the axes of rotation, and the coal moves through the basket in the same way as coal moves along a vibrating screen.
(1) Less degradation of coal; the vibration does not grind the particles together as much as the scraper arrangement in a Reineveld centrifuge.
(2) The wear of the basket mesh is not as great.
6. Question:
Describe with the aid of sketches the basic difference between the solid bowl centrifuge and the screen-bowl centrifuge. What are their separate uses?
Answer:
(See sketches, Lesson 12 Fig. 8 and 9.)
The difference between two machines is that the screen-bowl centrifuge has an additional wedge-wire screen section at the discharge end of the solid bowl, thus allowing two-stage dewatering.
The solid bowl centrifuge has limited use in dewatering raw slurries and tailings; the screen-bowl centrifuge is used to dewater frothed fine coal.
7. Question:
Briefly describe, with the aid of simple sketches, two methods of discharging the cake from a rotary vacuum drum filter.
Answer:
(Two of the following 🙂
Scraper knife and blow-off; a knife is set near to the filter drum in the discharging position. A blow-off device applies compressed air to the filter segment, and aids discharge of cake by the knife.
String discharge: See Lesson 11 Fig. 13 and Lesson 11 para. 5(c).
Belt discharge: See Lesson 11 Fig. 14 and Lesson 11 para. 5(d).
8. Question:
Sketch a pressure filter, showing the flow of feed and filtrate.
Answer:
No model answer provided. See Lesson 11 Fig. 5.
9. Question:
What is the difference between a cascade dryer and a flash dryer? (Your answer should be illustrated by simple sketches.)
Answer:
(See sketches, Lesson 11 Fig. 17 and 18.)
In the cascade dryer, coal flow downwards by gravity and is met by the rising current of drying air. In the flash dryer the coal is fed into the stream of drying air and carried along with it.
10. Question:
Describe, with the aid of a sketch, a fluidized bed dryer.
Answer:
See Lesson 11, Fig. 19 and Lesson 11, para. 6c.
11. Question:
Name two different types of pump used in coal preparation plants for liquid pumping duties, and briefly describe how they work.
Answer:
No model answer provided. Answers should generally follow the description given in Lesson 11 para. 8 and 9.
12. Question:
List the different uses for pressurized air which may occur on coal preparation plants with a brief description of the type of machine used to supply the air in each case.
Answer:
(i) Operating Baum Jig. Supplied by a turbo-blower with a high volume, low pressure output.
(ii) Blow-off discharge filter cake from vacuum filters. Supplied by a rotary positive displacement blower of the Rootes or sliding vane type.
(iii) Instrument air. Supplied by small air-cooled reciprocating compressor delivering clean, dry and oil-free air up to 50m3/h at a maximum pressure of 275 kN/m2.
(iv) Agitation air (for tank and sump agitation), and
v) Power air, both supplied from the plant’s main air compressors which are normally reciprocating air compressors rated according to the plants requirements, but with a maximum size of 900m3/h at a pressure of 700 kN/m2.
Lesson 12
13. Question:
What types of comminution process are encountered in coal preparation plants? Name one machine of each type.
Answer:
Primary breaking: Pick breaker
Bradford breaker
Jaw breaker
Gyratory crusher
Secondary reduction: Single and double roll crushers
Swing hammer crusher
Ring crusher
Grinding: Ball mill Rod mill
Atritor
14. Question:
What practical advantages may be obtained by using a Bradford breaker rather than a jaw breaker for reducing +100mm raw coal to -100mm.
Answer:
With a jaw breaker, a large high capacity machine is required to deal with a low tonnage of large pieces. The feed requires to be pre-screened and manually inspected for tramp metal and wood etc.
With a Bradford breaker, the pretreatment of the ROM is done in one operation, as it takes a high tonnage capacity of raw coal, prescreens, sizes and also removes large dirt and tramp etc. to discard. It also has a low maintenance requirement and no manpower involvement.
15. Question:
Sketch a swing hammer crusher. Describe how this type of crusher is designed to avoid damage to the machine by uncrushable foreign material.
Answer:
See Lesson 12, Fig. 6(a).
The hammers are free to swing. Uncrushable object will push back the hammers from the crushing position, and the objects are delivered into the “tramp trap” at the back of the crusher. In addition the breaker plate is sprung, and the driving pulley may have a shear pin.
16. Question:
Describe two problems which might be encountered in the operation of belt conveyors. What measures could be taken to overcome these problems?
Answer:
No model answer provided. This question has been included to get students to draw on their own experience as well as using the course material.
17. Question:
Describe with the aid of sketches the different ways in which coal flows through a bin.
Answer:
Descriptions and sketches of core flow and mass flow should be given. See Lesson 12, Fig. 14 and 15 and Lesson 12 para. 11a and b.
18. Question:
Describe the various ways in which size degradation within a bunker can be reduced.
Answer:
No model answer provided. Students are expected to summarize the methods of reducing degradation when inloading, by spiral or cascade chutes, including rubber cascades; and when outloading by chimney discharge.
(See Lesson 12 para. 13(e)).
19. Question:
For what purpose is a boom loader used? Describe it with a sketch.
Answer:
A boom loader is used to reduce to the absolute minimum the fell of coal, when loading wagons. Used mainly for large and graded coal (which suffer most damage from being dropped), it enables loading to take place with the minimum amount of degradation.
Description and sketch as in Lesson 12 para. 16 and Lesson 12 Fig. 26 or similar.

TEST PAPER 9 (on Lesson 13)

Lesson 13

Please read Lesson 13 Section 1 – 10 before beginning this test paper.
1. What are the benefits of homogenization of raw coal prior to feeding to a coal preparation plant?
2. Describe briefly two types of automatic control which are associated with Baum Jigs
3. Draw a simple sketch showing the two main items of a nucleonic density gauge. Use a separate sheet of paper for your answer.
4. What is the ash content of a blessed coal containing 150 tons of washed smalls at 7% ash, mixed with 300 tons of untreated coal at 28% ash?
5. State briefly the principles of the AERE ash monitor. Use a separate sheet paper for your answer.
6. Explain the use of a clarometer. Use a separate sheet of paper for your answer.
7. Draw a simple flow diagram showing the equipment necessary to produce a consistent blend of washed and untreated small coal. Use a separate sheet of paper for your answer.
Now turn back to Lesson 13 and read sections 11 – 15 before answering questions 8 – 10.
8. Why is a liquids flowsheet necessary to a Coal Preparation Plant Manager?
9. List three factors you would recommend to a plant designer as being of primary importance when designing the layout of a coal preparation plant.
10. Suppose your own coal preparation plant is subject to periods when the amount of refuse in the feed is too high for efficient cleaning. To improve the situation what steps would you take
(a) immediately; and
(b) in the longer term
Use a separate sheet of paper for your answer.
Please turn back to Lesson 13 and read Sections16 – 18, the last in this book.
REVISION QUESTIONS
The following revision questions, which cover subjects in earlier lessons, require essay type answers, which should be written on a separate sheet of paper. Model answers are not provided.
11. Describe briefly what types of screen you would favor for the following duties, and give reasons for your choice:
a) sizing 150mm – 25mm washed coal into saleable grades.
b) dewatering a 25mm – 0 Baum washed coal
c) extracting 6mm – 0 dry fines from a damp raw coal feed
12. Describe briefly what steps should be taken to provide a reasonable working environment in a coal preparation plant.
13. If samples of clean coal from a Baum Jig contained small particles of shale, what might be the cause, and how would you rectify the matter?

MODEL ANSWERS TO EST PAPER 9
(on Lesson 13)

LESSON 13

1. Question:
What are the benefits of homogenization of raw coal prior to feeding to a coal preparation plant?
Answer:
Homogenization of the raw coal will average out all the variable factors due to seam differences, high and low proportions of dirt, clean coal, wet coal and fines content so that a uniformly consistent raw material can be fed to the plant for preparation. This may enable a reduction in plant cost and will improve plant performance and the quality of products.
2. Question:
Describe briefly two types of automatic control which are associated with Baum Jigs.
Answer:
Two of the following should be described:
CAROL FEEDER for measuring the bulk density of the feed and regulating and limiting he input so that an overload of clean coal or reject does not occur.
NO-FEED CONTROL to reduce the operating air pressure under no-feed conditions and gradually re-introduce full air pressure when the feed is restored, to prevent loss or disruption of the jig bed.
AUTOMATIC SHALE DISCHARGE EQUIPMENT to control the amount of shale in the jig bed according to the shale tonnage entering the jig and the capacity of the shale elevator. For a description of the common types of controllers, see Lesson 7, Section 3.
3. Question:
Draw a simple sketch showing the two main items of a nucleonic density gauge.
Answer:
See Lesson 13, Fig. 5(b)
4. Question:
What is the ash content of a blended coal containing 150 tons of washed smalls at 7% ash, mixed with 300 tons of untreated coal at 28% ash?
Answer:
150 tons of washed small at 7% + 300 of untreated coal at 28% ash = (150 + 300)t of blended coal at a% ash.
(150 + 300)a = (150 x 7) + (300 x 28)
450 a = 1050 + 840
Therefore: a = 9450/450 = 21
Therefore the ash content of the blend will be 21%.
5. Question:
State briefly the principles of the AERE ash monitor.
Answer:
The AERE ash monitor relies on the radiation emitted from a radioactive isotope. This radiation is beamed onto a prepared bed of coal, and the back-scattered radiation is measured. Low ash coal constituents back scatter well, whereas high constituents back scatter well, whereas high ash constituents absorb more radiation and the back scatter is less. By measuring the back scattered radiation, a determination of the ash content of the coal sample can be made.
6. Question:
Explain the use of a clarometer.
Answer:
A clarometer takes a sample of the feed passing into a thickener, and measures the settling rate of the solids. The addition of flocculant to the thickener feed can be automatically increased or reduced, depending on whether the settling rate is too slow or too fast.
7. Question:
Draw a simple flow diagram showing the equipment necessary to produce a consistent blend of washed and untreated small coal.
Answer:
See Lesson 13, Fig. 7.
8. Question:
Why is a liquids flowsheet necessary to a Coal Preparation Plant Manager?
Answer:
To show the liquids circulating systems in a concise and readily understandable form.
To simplify the task of explaining the liquids circuit to other people.
To examine possible ways of carrying out any operation, eg alternative flows which may be used in emergency.
9. Question:
List three factors you would recommend to a plant designer as being of primary importance when designing the layout of a coal preparation plant.
Answer:
No model answer provided.
10. Question:
Suppose your own coal preparation plant is subject to periods when the amount of refuse in the feed is too high for efficient cleaning. To improve the situation what steps would you take:
(a) immediately; and
(b) in the longer term?
Answer: No model answer provided.
Revision Questions
These require essay-type answers, and no model answers are provided.
11. Question:
Describe briefly what types of screen you would favor for the following duties, and give reasons for your choice:
(a) sizing 150mm – 25 washed coal into saleable
grades
(b) dewatering a 25mm – o Baum-washed coal
(c) extracting 6mm – 0 dry fines from a damp raw coal feed
12. Question:
Describe briefly what steps should be taken to provide a reasonable working environment in a coal preparation plant.
13. Question:
If samples of clean coal from a Baum jig contained small particles of shale, what might be the cause and how would you rectify the matter?

END

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