DEWATERING SLURRIES

Abstract: The specification describes a method of dewatering mineral slurries especially slurries of a fine coal. The method involves adding an anionic flocculant to the slurry and subsequently adding a dewatering aid to the slurry at or just prior to the formation of a filter cake during a filtration step. The specification also discloses that overloading with an anionic flocculant can be avoided if a dewatering aid is used in combination with the flocculant.

Description: DEWATERING SLURRIES This invention relates to improved dewatering methods and agents for use in vacuum filtration operations in mineral and coal processing. The invention is generally applicable to the dewatering of slurries including those encountered in the processing of minerals such as alumina, as well as coal, but it is of particular interest in the dewatering of fine coal and accordingly will be described in detail with particular reference to the latter application. Background of the Invention Vacuum filtration is commonly used in industrial processes to remove water from slurries. Vacuum disc and drum filters are widely used, as well as vacuum belt filters. The efficiency of such processes depends in large part upon the physical properties of the slurries to be dewatered, and extensive studies have been carried out with a view to modifying those physical properties in order to optimise the efficiency of the dewatering process. The properties may be affected, for example, by chemical means such as the use of chemical additives, or by physical means such as the mechanical treatment of the slurries and/or the filter cake which is formed in the vacuum filtration process. As these processes are carried out on a very large scale, economic considerations are also important and what may appear to be small improvements can result in substantial cost savings. Coal processing, particularly the treatment of metallurgical coal, usually incorporates washing steps as part of the beneficiation scheme. The coal-cleaning process produces coal with a broad range of particle sizes. Coarse coal (ie., >600 micron) and fine coal are treated differently to dewater effectively. Water is usually removed from coarse coal by screen drainage or centrifugation and from fine coal (ie., flotation concentrates or refuse) by centrifugation or vacuum filtration. In some cases, thermal drying of fine coal is employed as a necessary step to achieve target moistures. Depending upon the mining operation, the type of coal treated and normal swings in process operations in the plant, the moisture levels in the cleaned coal can vary quite widely. Typical moisture levels in coarse coal range from 2-12%, while fine coal moistures can range from 15-30%. Coal processing plants often have considerable trouble meeting target moisture levels in the product coal they sell. As a result of new mining methods employed in long- wall operations, the proportion of fine coal reporting to the washing & preparation plants has increased significantly. Fine coal is much more difficult and costly to dewater than coarse coal. One method often employed by coal prep plants to achieve moisture specifications is to blend high-moisture fine coal with low-moisture coarse coal in proportions necessary to just meet target moisture. While, in many cases, it would be much more cost effective for a coal prep plant to simply discard the fines and mine more coarse coal, for reasons of resource management/utilisation/conservation it is more prudent to treat the fines. Vacuum filtration is the most commonly used means of mechanical treatment to dewater fine coal. Fine coal, in slurry form, reports to the filtration operation where the water is removed. Vacuum disc and drum filters are the principal type used by the coal industry to filter fine coal, although increasing interest is being shown in vacuum belt filters. To assist in the efficient operation of vacuum filters, reagents are often added to the feed slurry. It is known to use anionic and cationic flocculants in the vacuum filtration of fine coal. These reagents are necessary to "thicken" the slurry as it is fed to the vacuum filter to ensure the proper formation of a filter cake. It is thought that the reagents function by binding the very fine coal particles to larger coal particles in a typical flocculation process, thus producing a more uniform said increased particle size distribution for better filter cake permeability. This results in less "blinding" of the filter cloth and filter cake by the very fine coal particles. Both anionic flocculants (usually high molecular weight acrylamide/acrylate co-polymers) and cationic flocculants (usually low molecular weight polyamines) are used, individually or in combination, to control filter cake formation. These reagents are always added to the slurry feeding the vacuum filters (ie., slurry pre-treatment) in a manner typical of flocculant addition in the minerals industry. There has also been considerable interest by the coal industry in the use of surfactants, in combination with flocculants, to enhance the dewatering of fine coal. Although many studies have been conducted and reported, there are widely varying results/conclusions. Some studies have shown that surfactants do not significantly affect residual cake moisture, while others show they do. Other studies conclude that surfactants can be effective for certain types of coal but not for others. One of the most significant reasons for the variation in the performance of dewatering aids for fine coal is the variable chemistry of coal itself. Unlike all other minerals, coal is an organic material. Coals vary widely in bulk/surface composition, depending upon a multiplicity of factors such as location of the deposit, rank and mineral (inorganic) matter, degree of weathering, internal structure/porosity, etc. Therefore, depending upon the specific coal treated, dewatering aid performance can be expected to vary widely since both the adsorption (a surface chemical property) and absorption (a bulk chemical property) characteristics of coals with respect to drainage aid interaction will vary widely. The specification of European Patent Application Publication No. 460811 in the name of Unichema Chemie BV describes the addition of anionic flocculants or cationic coagulants to a coal slurry prior to filtration in the presence of a dewatering aid. However, the specification states that the preferred point for addition of the dewatering aid is from a position within the filter cake forming zone. We have now found that when an anionic flocculant is used, the best point for addition of the dewatering aid is at or just prior to the formation of the filter cake during the filtration step. SuTTtmary of the invention According to the present invention there is provided a method of dewatering an aqueous coal or mineral slurry comprising adding a flocculant to the slurry, subsequently adding a dewatering aid, and filtering the slurry by means of vacuum filtration to obtain a filter cake and a filtrate, characterised in that the dewatering aid is added at just prior to formation of the filter cake during the filtration step. Preferably, the dewatering aid is a salt such as a sodium salt of a fatty acid including fatty acids derived from tall oil. The fatty acid may have from 8 to 20 carbon atoms. However, sodium oleate is particularly preferred. The dewatering aid may also be a potassium or ammonium salt of a fatty acid. The dewatering aid may also comprise either a salt of a sulphonic acid such as dodecyl benzene sulphonic acid, an ethoxylated alcohol or an unsaponified fatty acid. The flocculant may be added to the coal slurry prior to the filtration step in a ratio in the range from 10 to 100 and preferably 20 to 50 gms per tonne of mineral or coal contained in the slurry. On the other hand the dewatering aid may be added to the slurry in a ratio in the range from .02 to 1.5 and preferably 0.5 to 1.5 kg per tonne of mineral or coal. In a further aspect of the invention we have found that the expensive cationic flocculants presently used may be replaced by very much smaller amounts of anionic flocculants, without suffering the drawbacks formerly associated with the use of said anionic flocculants, whereby a very large saving in cost is achieved. In yet another aspect of the invention, a two-stage process is provided involving use of anionic flocculants in place of cationic flocculants, in combination with application of the dewatering aid at a particular stage of the process, to provide a greatly improved and much more cost-effective process for dewatering slurries than has hitherto been considered feasible. Detailed description of the invention An experimental rig was established enabling accurate simulation of actual plant operations at the BelIambi Coal Company Pty Ltd, South Bulli, New South Wales, Australia. The performance of the laboratory filter rig was correlated with the performance of a Delkor vacuum belt filter at Bellambi. Current practice involves addition of a cationic flocculant to the feed of the Delkor filters in order to control the formation of the filter cake on the filter belt. This reagent addition represents a significant operational cost but use of the cationic flocculant has been considered necessary for adequate process control and performance of the Delkor filters. It was necessary to accurately assess the performance of the cationic flocculant on filter cake formation prior to undertaking drainage aid testwork. The amount of the cationic flocculant needed to achieve cake formation in the test-rig comparable to that in the plant had to be determined and the effect of the cationic flocculant on drainage aid performance had to be ascertained. Brief Summary of the Drawings Figure 1 is a graph of filter cake moisture content versus cationic flocculant dosage at a fixed drying time and applied vacuum. Figure 2 is a graph of filter cake form time and apparent applied vacuum against cationic flocculant dosage rate, Figure 3 is a graph of filter cake moisture content and form time versus applied vacuum at a fixed dosage rate of cationic flocculant, Figure 4 is a graph of filter cake moisture content versus dosage rate of dewatering aid where the dewatering aid is added at different locations in the filtration step, Figure 5 is a graph of filter cake form time versus anionic flocculant dosage rate for two types of anionic flocculant. Figure 6 is a graph of filter cake moisture content versus anionic flocculant dosage rate for two types of anionic flocculant, Figure 7 is a graph of filter cake moisture content versus dosage rate of dewatering aid showing the effect of incorporating the dewatering aid at various locations in the filtration step, Figure 8 is a graph of filter cake moisture content versus dewatering aid dosage rate for both an anionic flocculated filter cake and a cationic flocculated filter cake; and Figure 9 is a graph of filter cake moisture content versus anionic flocculant dosage rate with and without the addition of a dewatering aid. Figure 1 shows the time required to achieve filter cake formation and the apparent vacuum of the system (at 80 kPa applied vacuum) as a function of coagulant addition. To match the filter-cake formation time (5-60 sec) and apparent vacuum conditions (70 kPa) of the Delkor #2 filter, an equivalent dosage of 250 g/tonne of the cationic flocculant was required as an addition to the test-rig slurry. Figure 2 shows the residual filter cake moistures achieved in the lab test rig as a function of cationic flocculant addition. Use of the cationic flocculant at the dosage rate determined above (for proper form time/apparent vacuum) results in a residual moisture level of 22-23%. This result is identical to the residual moisture levels of samples of fine-coal taken from the Delkor #2 filter, confirming the ability of the test rig to accurately simulate Bellambi plant operations. Excessive cationic flocculant addition was found to have no detrimental effect on filter cake moisture. Figure 3 shows the variation in filter-cake moisture and formation time as a function of the applied vacuum (at a fixed dosage of cationic flocculant) . These tests were conducted to test the sensitivity of the test rig to ensure that changes in residual moisture during the evaluation of the drainage aids were due to chemical parameters and not to minor changes in vacuum which inevitably occur during test work (i.e., due to slight variations in sampling or test procedure) . As seen, form time is virtually independent of applied vacuum (as expected) while moisture varies only slightly over the variations in applied vacuum expected. Tests on the Bellambi system indicate that a particular drainage aid, namely sodium oleate, had the most significant potential to achieve the criteria for technical success established prior to undertaking the experimental work. In attempting to optimise the performance of sodium oleate, considerable effort was spent to examine the most effective method of reagent addition to the vacuum-belt filter. Consideration was given to a variety of reagent-addition techniques. Firstly, there is often a detrimental effect on cost-performance of adding drainage aids to the filter feed or to the supernatant liquor too early in the filter cake formation stage as a result of dilution of the reagent in the slurry. However, a loss in reagent performance due to reagent dilution must be weighed against the problems of (1) insufficient penetration of reagent-treated liquor into the filter cake, (2) a reduction in filter-cake dry-time and (3) the need to recycle filtrate if the drainage aid reagent is applied as a wash after filter cake formation occurs. Figure 4 shows that at the preferred dose rate 0.50 kg/tonne the most effective methods of reagent application are to ensure that the reagent is applied to the Delkor filter (from a trough or spray system) at or close to the point of formation of the filter cake. Moisture reduction of approximately 3 to 5% (22-23% to 18-19%) can be expected if the reagent is properly applied. There is, however, a severe detrimental consequence of applying the drainage aid to the Delkor filters too early in the filter cake formation stage. This is due to a precipitation reaction between the cationic flocculant and specific components of the drainage aid. This problem is seen clearly from the results presented in Figure 4. When the drainage aid is added to the cationic-treated slurry, prior to the slurry reporting to the filter belt, a dramatic increase in cake moisture is observed. This problem is reduced if the reagent is added to the supernatant liquor after the slurry is on the belt. Anionic Flocculant Addition Unfortunately, due to swings in Bellambi process operations, particularly as more fines reach the filter due to long-wall mining, it is extremely difficult to control the Delkor filters to ensure that the filter cake form zone remains at the same point during filtration, even with flocculant addition. In an effort to overcome a potential process control problem resulting from the need to apply reagent at a critical point in the process operation, a method was sought to eliminate the use of the cationic flocculant in order to achieve rapid filter cake formation. Testwork was conducted on samples of fine-coal taken from the plant prior to being treated with the cationic reagent, using both of the liquid anionic flocculants (i.e., acrylate/acrylamide copolymers) currently used by Bellambi in other process operations. The results, given in Figure 5, show that by replacing the cationic flocculant with an anionic flocculant the filter cake formation time can be dramatically reduced (from 60 sec to 20 sec) using less than one-tenth of the amount of reagent (15 g/t anionic flocculant vs 250 g/t cationic flocculant) . As the cost per weight unit is identical, the saving is substantial. There is, however, the potential to overdose the fine-coal slurry with an anionic flocculant. This may result in process problems not encountered with the use of the cationic flocculant. As shown in the cake moisture vs reagent dosage data given in Figures 6 and 9, overdosing the anionic flocculant can result in an increase in the filter cake moistures. However, this problem is not unique to Bellambi. Most unit operations involving flocculant addition, especially thickeners, have similar problems if too much flocculant is added to a mineral slurry. Fortunately, with proper control of the dosing of the flocculant to the slurry, this potential problem can be virtually eliminated. Sodium oleate/Anionic Flocculant Performance In addition to possible process savings by changing from the cationic flocculant to an anionic flocculant to control filter cake formation, we have found that there are significant benefits to the performance of sodium oleate and the control of filter operations. Firstly, there is no danger of a detrimental interaction of the reagent with the anionic flocculant. Unlike the catastrophic result shown in Figure 4, addition of dewatering aid to anionic flocculated slurry produces no dramatic increase in filter cake moisture (as shown in Figure 7) . However, comparison of the moisture vs reagent dosage profiles in Figure 7 clearly indicates that the reagent is best applied as a concentrated solution at or just prior to cake formation. Secondly, as shown in Figure 8, at 0.5 kg/tonne dosage rate, there is a demonstrable improvement in the performance of the reagent (i.e., 1.0-2.0 % improvement in moisture reduction capability) using it in conjunction with an anionic flocculant rather than with a cationic flocculant. Furthermore, the potential problem of over flocculation of the fine-coal slurry is eliminated with a combination reagent/flocculant programme. Lab tests indicate that, when using sodium oleate as a drainage aid, excess anionic flocculant addition has little effect on residual filter cake moisture levels. It will be apparent that a number of process benefits are achieved by the invention. Firstly, if the dewatering aid is applied to the filter cake via a spray at or just prior to cake formation, this results in a reduction in the amount of drainage aid required to effect a 3 to 4% drop in residual moisture in the fine coal filter cake. Secondly, the application of the drainage aid reagent to the vacuum filter as a filter-cake spray at or around the point of cake formation on the filter belt/disc enables an anionic flocculant to be substituted for a cationic flocculant for filter feed thickening. This two-stage approach to fine-coal dewatering has been found to completely eliminate problems associated with process control if an anionic flocculant is used to thicken the filter feed slurry. The reagent spray has been found to ensure that filter cake permeability during the dewatering phase of filtration (i.e., once air begins penetration of the filter cake) is maintained regardless of any slight overdosing of the anionic flocculant due to process swings. This permits an anionic flocculant to be substituted for a cationic flocculant with enormous process savings. Figure 9 shows the impact on final product cake moisture using an anionic flocculant pretreatment step prior to filtration with and without the aid of the applied reagent. High product cake moistures are encountered where the flocculated pretreatment is too high and the corresponding curve shows the rise in moisture after this phenomenon occurs (i.e., around 20 g/T flocculant dose). However, in a dual flocculant/reagent system, the problems encountered with flocculant overdosing are corrected by way of a synergism with the reagent addition. The end result is a lower cake moisture with a far more cost effective pretreatment step. Summary Although the experimental work described above was carried out in connection with the development of fatty acid derivatives and particularly sodium oleate as dewatering aids, it is apparent that the concept of applying a solution of the dewatering aid at or about the point of formation of the filter cake in combination with use of a flocculant is novel per se and generally applicable using conventional dewatering aids. The present invention therefore extends to this concept as applied to the dewatering of slurries using dewatering aids other than fatty acid derivatives. Application of the above identified concept enables substitution of anionic flocculant, hitherto believed to be impractical, for the much more expensive cationic flocculant hitherto employed in the dewatering of slurries, particularly in the dewatering of fine coal. As detailed above, this is of considerable economic significance. It will be clearly understood that the invention in its general aspects is not limited to the particular details set out in the experimental work disclosed above.

Claims: CT.ATMfi 1. A method of dewatering an aqueous, coal or mineral slurry comprising adding an anionic flocculant to the slurry, subsequently adding a dewatering aid and filtering the slurry by means of vacuum filtration to obtain a filter cake and a filtrate characterised in that the dewatering aid is added at or just prior to formation of the filter cake during filtration. 2. A method according to Claim 1 wherein the dewatering aid comprises a salt of a fatty acid having from 8 to 20 carbon atoms. 3. A method according to Claim 2 wherein the salt is selected from the group consisting of sodium, potassium and ammonium salts. 4. A method according to Claim 2 wherein the fatty acid is oleic acid. 5. A method according to Claim 2 wherein the fatty acid is a tall oil fatty acid. 6. A method according to Claim 1 wherein the anionic flocculant is added to the slurry at a rate in the range from 10 to 100 grams/tonne of coal or mineral. 7. A method according to Claim 1 wherein the anionic flocculant is added to the slurry at a rate in the range from 20 to 50 grams/tonne of coal or mineral. 8. A method according to Claim 1 wherein the dewatering aid is added at a rate in the range from 0.02 to 1.5 kg/tonne of coal or mineral. 9. A method according to Claim 1 wherein the dewatering aid is added at a rate in the range from 0.5 to 1.5 kg/tonne of coal or mineral. 10. A method according to Claim 1 wherein the dewatering aid comprises a salt of a sulphonic acid. 11. A method according to Claim 1 wherein the dewatering aid comprises a fatty acid having from 8 to 20 carbon atoms. 12. A method according to claim 1 wherein the dewatering aid comprises an ethoxylated alcohol. 13. A method of dewatering an aqueous mineral or coal slurry comprising adding an anionic flocculant to the slurry, subsequently adding a dewatering aid and filtering the slurry by means of vacuum filtration to obtain a filter cake and a filtrate characterised in that the dewatering aid is a salt of a fatty acid having from 8 to 20 carbon atoms.

Inventors: KEENEY MARK EDWARD, KEENEY, MARK, EDWARD
Applicants: UNILEVER PLC
Application year: 1992
Main classification: B01D37
IPC codes: B01D12, B01D37, C02F1