PLANT FEED CONTROL

2.1 Homogenization
A coal preparation plant operates most efficiently when the raw coal does not vary too much in quality. If there are occasional high proportions of dirt, clean coal, wet coal or fine coal then the plant will not operate efficiently.

One way of reducing variations in raw coal is to homogenize before feeding to the plant. This can be done by using a series of bins knows as homogenizing bins. There are usually four of these bins and the pre-sized feed is arranged to go into each bin in turn, transferring to the next bin when to one being filled is full. The raw coal is extracted at equal rates from all the bins at a controlled total rate equal to the feed into the bins.

Another method which is now gaining favor, is the use of homogenizing stockpiles. They have the added advantage of providing a raw coal stockpile on many thousands of tons capacity, as well as being the ideal method of homogenization for the raw feed to the plant. They can also be used if required, for stocking, homogenizing and reclaiming prepared fuels such as washed or blended smalls, prior to a rapid loading system. There are two main type of system, the circular and the rectangular homogenizing stockpiles. Neither system will handle run-of-mine output, so between the shaft or drift and the stockpile either a screening/crushing or trammel breaker system has to be installed. Whilst both circular and rectangular systems work satisfactorily, the rectangular system is capable of extension should increase stock hold capacity be required at a later date. Lesson 13 Fig. 1a shows the arrangement of a homogenizing stockpile, with component parts labelled as follows:
1. Raw coal feed conveyor
2. Lifting and slowing jib, to feed coal onto stockpile
3. Stocked coal
4. Clearing rake, to dislodge material at reclaim face, to feed onto
5. Reclaiming scraper conveyor
6. Conveyor feeding homogenized coal to preparation plant.

Lesson 13 Fig. 1b shows the method of chevron layering achieved by the slewing of the jib as the machine gradually works its way round the stocked coal. The jib keeps the coal discharge as near as possible to the top of the stocked coal. Irrespective of the shape of the stockpile the method of formation is the critical factor, and in this respect, chevron layering and full face reclamation are vitally important. The operations of the stockpile, homogenizing and reclaiming, can be fully automated whilst the luffing and slewing in the case of the circular pile, or lugging and traversing in the case of the circular pile, of luffing and traversing in the case of the rectangular pile, minimize breakage.

Typical holding capacities of circular type stockpiles are shown in the following table I, where

D = peripheral diameter of stockpile
H = height of coal in stock
V = maximum working of stockpile

Table I Size and Capacity of Circular Homogenizing Stockpiles

By using homogenizing bins and stockpiles in this way it is possible to even out wide variations in feed quality, but it is important to remember that it will not change the average quality.

2.2 Consistent Tonnage Control
When the feed to the coal preparation plant is taken from more than one source, (such as form the pit top and from a coal car tippler), it is important to ensure that feed belts do not become overloaded by taking from both sources at once or that the feed belt is not run empty for excessive lengths of time. This can be prevented by using a form of plant feed control. An example is shown in Lesson 13 Fig. 2.

2.3 Belt Weighers
In order to control what is happening on the coal preparation plant it is necessary for the plant manager to know how much material is on any one conveyor belt. This can be done by using belt weighers. There are many different types of belt weighers available and generally speaking the most accurate are the most expensive. Belt weighers need to be installed correctly and maintained properly so that they will work accurately.