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12th International Coal Preparation Congress, May 23-27, 1994, Cracow, POLAND
NEW CONCEPTS IN DEWATERING AND THICKENER CONTROL
Doug Shuttleworth
Mineral Processing Division
Moor, Bradford, Yorkshire, England
ABSTRACT
The use of chemical additives and treatments has become an accepted part of the coal preparation scene over the past few years. Flotation chemicals to increase fine coal yields, flocculants to aid separation of fines in thickening and filtration processes are accepted as essential to the operation and performance of many plants.
This paper describes new developments in the use of chemical additives in the beneficiation processes in the coal industry, in addition, a fully interactive thickener control system is described to enhance the performance of conventional flocculants and thickener/clanfiers.
INTRODUCTION
This paper is in three parts: .
1. Surfactant chemical moisture reduction optimisation in small coal basket centrifuges.
2. The use of solid cationic flocculants to improve cost efficiency where continuous belt filters are used.
3. The description of a fully interactive thickener control system.
SMALL COAL DEWATERING
Basket centrifuges have been used successfully for dewatering of coal in the size range plus 0.5 mm to -13 mm for many years. They give a cost effective and reliable method of improving the quality of this coal size fraction. They have been successful on many coal types from anthracite to subbituminous coals.
From time to time, various bodies have suggested the use of chemical additives in these machines to improve their performance. These have included simple additions of water to various complex chemicals, intended to modify the way in which water is released from coal.
In the latter category, sulpho-succinate compounds, usually in the form of solutions in combinations of organic solvents and water, have shown to give reductions in moisture content in basket centrifuges. Unfortunately, their performance could never be guaranteed, and as a result, high dosages were often applied giving rise to problems of frothing and foaming in other parts of the washery circuit, especially to the detriment of the flotation circuit. This was especially the case when flotation was being used.
The reduction of moisture at this point in the coal beneficiation process can have a great effect on the final quality of the saleable product. A power station fuel, for example, will consist of upwards of 70% of fuel in this category (-0.5 to +15 mm) and, therefore, reductions in moisture can have dramatic effects on the final calorific value of the saleable fuel. The sale of the finished product in term of heat content is becoming more and more relevant, and therefore^ improvements in the dewatenng process can be of great value.
Chemical Addition
The optimisation of the use of sulpho-succmate to reduce moisture in basket centrifuge was carried out on low rank fuel being supplied for industrial purposes.
The sulpho-succinate in liquid form was diluted to give a 1% solution in water, and pumped to a spray bar, situated at a strategic point where the coal enters the centrifuge. Moisture contents were determined using standard methods before addition of sulpho-succmate and after. The differences are represented in Table 1.
Table 1. Total moisture, with and without additive.
| Run № |
Moisture with out additive,% |
Moisture with out additive,% |
Difference |
1 |
18.2 |
17.1 |
-1.1 |
2 |
21.3 |
18.0 |
-3.3 |
3 |
18.2 |
16.1 |
-2.1 |
4 |
18.3 |
17.1 |
-1.2 |
Average |
19.0 |
17.1 |
-1.9 |
Conclusions
Moisture reductions in the range 1.1% points to 3.3% points with an average of 1.9% points was obtained over and above the normal running moisture levels being obtained on the coal washing plant.
The dosage of sulpho succinate required was in the range 10CH-200 g/tds.
The key to giving consistent performance with the additive was in correct choice of addition point. This ensured that a consistently low addition level could be used to give the desired moisture reductions, at the same time preventing excessive quantities of sulpho succinate returning to the washery circuit via the centrifuge effluent. During the whole trial period, no problems with frothing in any part of the washery circuit were noted.
Additional Note
Product from these trials was left to stand in bulk in a stocking situation. Further reductions in moisture were noted in line with the reductions that would be expected on untreated stockpiles.
This means that the sulpho succinate effect is in addition to any further drainage reduction that might be expected.
SOLID GRADE CATIONIC FLOCULANTS ON PRESSURE BELT FILTERS
The environmental problems associated with the coal industry have been reduced to some extent with the introduction of continuous pressure belt filters for treatment of coal slurries and tailings. The success of these devices have been very much dependant upon development of new flocculant systems which have enabled the separation and dewatering process to be viable. These new flocculants have been developed with improved polymerisation technique, quality control etc. However, limits in terms of cost effectiveness have been a problem, particularly in terms of the use of additional canonic flocculants, to enable cake to be released successfully from the filter.
Cationic flocculants are usually required when the coal slurry or tailings are very fine. Also when pH control of the fines slurry has had to take place using lime or caustic soda.
Tradition polyamine cationics are limited in terms of the molecular weights that can be achieved in the polymerisation process. The development of other cationic monomers has enabled higher molecular weight products to be manufactured, which have resulted in higher efficiency in the flocculation process. These products are available in a solid form, but unlike conventional flocculants, they require less ageing time to give a satisfactory and usable solution.
Trials were carried out with the improved cationic flocculant, compared with conventional poly amines.
The underflow from this thickener, with a solids content of between 250+500 g/1 is passed to a buffer tank prior to feeding to the filter belt press.
A conventional anionic flocculant was normally used on this plant in conjunction with a liquid polyamine. This polyamine was replaced with the solid cationic coagulant prepared as a 0.18% solution using a conventional flocculant eductor. This was dosed into the same addition point as a polyamine, the results of the tests are shown in Table 2a and 2b.
Table 2a. Cost of treatment, solid anionic and conventional liquid cationic.
| Sample ref |
Cost per dry tonne( Anionic),$ |
Cost per dry tonne(Liquid cationic ),$ |
Total cost,$ |
1 |
0.72 |
0.39 |
1.11 |
2 |
0.73 |
0.37 |
1.10 |
3 |
0.74 |
0.40 |
1.14 |
4 |
0.69 |
0.39 |
1.08 |
5 |
0.68 |
0.44 |
1.22 |
6 |
0.66 |
0.41 |
1.07 |
7 |
0.66 |
0.37 |
1.13 |
8 |
0.72 |
0.38 |
1.01 |
9 |
0.73 |
0.41 |
1.14 |
10 |
0.64 |
0.40 |
1.04 |
11 |
0.68 |
0.40 |
1.08 |
12 |
0.69 |
0.39 |
1.18 |
Average |
0.70 |
0.40 |
1.11 |
Table 2b. Cost of treatment, solid anionic and new solid cationic.
| Sample ref |
Cost per dry tonne( Anionic),$ |
Cost per dry tonne(new solid cationic ),$ |
Total cost,$ |
1 |
0.64 |
0.23 |
0.87 |
2 |
0.58 |
0.19 |
0.77 |
3 |
0.50 |
0.20 |
0.70 |
4 |
0.56 |
0.18 |
0.74 |
5 |
0.66 |
0.23 |
0.89 |
6 |
0.55 |
0.23 |
0.78 |
7 |
0.60 |
0.19 |
0.79 |
8 |
0.60 |
0.19 |
0.79 |
9 |
0.58 |
0.17 |
0.75 |
10 |
0.59 |
0.23 |
0.82 |
11 |
0.54 |
0.21 |
0.75 |
12 |
0.60 |
0.20 |
0.80 |
Average |
0.58 |
0.20 |
0.79 |
Conclusions
Significant cost reductions were obtained using the solid product compared with the liquid polyamine, which meant a considerable cost saving to the operator. This type of coal washing operation can be very marginal in terms of cost efficiency.
A further advantage of a solid cationic is the Health & Safety considerations when using conventional polyamines. These products, though safe to use if handled correctly, are only available in liquid form, and due to the manufacturing methods used, have a slight acidity in solution (pH 3+4). Furthermore, they are usually marketed as 40% or 50% solutions, compared with solid cationics at 100%.
An additional benefit noted during trials was the reduction in moisture of the press cake obtained. Thought that this reduction cannot be guaranteed.
INTERACTIVE THICKENER CONTROL
Devices to improve the performance of conventional tailings, thickeners in coal preparation plants have been available for many years. The clarometer developed by British Coal M.R.D.
Bretby some years ago, has been used for some time by the coal and other mineral industries. More importantly, the device now has taken advantage of new improved technology and reliability has been increased. The ability to interface the device with other detection and control circuits to give a fully interactive control system for thickeners is now possible.
Description
Using the clarometer concept as the basis of a thickener control system, means that the main control parameter is the settlement rate of the flocculated solids in the thickener. This is a true measure of flocculant requirement, since settlement rate is a composite of the variables present in the thickener feed. Amongst these, are the flow rate, solids concentration, particle size distribution, solids surface chemistry.
The system utilises a programmable controller communicating with field devices, utilising amulti task software program controlling a personal computer to provide real time process monitoring. Performance is monitored graphically giving the process data in a clear and intuitive manner to the plant operator.
There are five main monitored areas.
These are:
1. Settlement rate - this is monitored and controlled by the clarometer, to ensure optimum
settlement rate is maintained. This can be set to maintain an optimum level, within limits.
This is the main controlling influence on the performance of the control circuit.
2. Feed solids concentration.
3. Sludge level.
4. Rate mechanism torque.
5. Overflow turbidity.
The above are used within the software Algorithm, to give control of the thickener, with outputs for the underflow pump speed and the flocculant addition dosing pump. Where two flocculants are applied this can be catered for within the software.
Conclusions
Where the thickener control system has been installed the main benefit has been financial in terms of the saving of an operator for thickener adjustment. However, additional advantages have shown in the control of underflow density, which is improved due to the monitoring of the settlement rate very regularly coupled with the sludge level and control of the underflow pump. Much more consistent underflow quality can be obtained.
Where overflow clarity is inconsistent the system has built in monitoring of overflow turbidity. Since the system operates basically on flocculant performance being accurately con¬trolled, it is found that turbidity is, therefore, controlled as a result.
Usually the main benefit of accurate control of the thickener is in optimisation of chemical costs. These can be significant, since the control circuit follows the accurate flocculant requirements of the plant dependant upon the quality of the material being fed to the thickener