ZA200503999B - Process for demineralising coal - Google Patents

Description

Process for demineralising coall

Field of the inventtion

The present imvention relates to a process for deminer—alizing coal.

Background of th invention

Several metheods have been described in the literature for producing demineralized or Jow-ash coal for fuel and other industrial applicatiorms, but none have achieved sustained commercial use.

A process wa s developed in Germany during the 1940's for removing ash-forming mineral matter from physically cleaned black coal concenmrates, involving heating the coal as a paste witha aqueous alkali solution, followed by solid/liquid separation, acid washing and water washing steps. Reports on this process detail a practical chemical demineralizing meth od. German practice showed that a deranineralized coal with an ash yicld of 0.28% coul d be produced from a physically cleared feed coal which had an initial ash yield of 0.8%.

The coal-alkali feed paste was stirred at 40° - 50°C for 30 minutes, then pumped through a heat exchanger to a continuously operable gas-heated tubular reactor in which the paste was expose d to a temperature of 250°C for 20 mimmtes, under a pressure of 100- 200 atmospheres (10-20MPa). The reaction mixture was tlnen passed through the heat exchanger previouslsy mentioned, in order to transfer heat to the incoming feed, then cooled further in a water-cooled heat exchanger.

The cooled paste was diluted with softened water, them centrifuged to separate and recover the alkaline solution and the alkalized coal. The latter was dispersed to 5% hydrochloric acid, then centrifuged to recover the acidified coal and spent acid and . redispersed mm water. The coal was filtered from this slurry, d_ispersed again in another lot of water and centrifuged to recover the resulting low-ash coal as a damp solid product,

American and Indian researchers used broadly simi lar chemical methods, with variations In processing details, to produce low-ash coals from other feed coals, most of which had much higher starting ash levels than the coals than the Geermans used. Another

American group (at Battelle) cl aimed advantages for: (a) Mixed alkali leachants containing cations from at lezast one element from *

Group IA and at least one element from Group IIA of the Periodic Table; (b) Filtration or ceratrifugation of the alkalized coal from the spent alkaline leachant, either at the reaction temperature or after rapid cooling to less than 100°C, in order to minimise the formation of undesired constituents, presumably sodalite or similar compounds; (c) Application of the process to low-rank coals which dissolve in the alkali and which can bee reprecipitated at a different pH fromm the mineral matter, thus allowing separation and selective recovery.

Other researchers had studied scientific aspects of alkaline extraction of sulphur and minerals, including the rel ative merits of different alkalis. Mo st American work has been directed at the removal of sulphur rather than metallic el ements, and the acid treatment step is often omitted . However, an American group (at =Alcoa) has chemically cleaned coal to less than 0.1% ash yield, concurrently achieving large reductions and low final concentrations of iron, silicon, aluminium, titanium, sodium and calcium. The aim was to produce very pure co al suitable for conversion into eleactrode carbon for the aluminium industry. This was achieved by leaching powdered c=oal with hot aqueous alkaline solution under pressur-e (up to 300°C), then successively vevith aqueous sulphuric acid and aqueous nitric acid at “70°-95°C.

Australian patent no. 592640 (and corresponding US -patent no. 4,936,045) describes a process for the preparation of demineralized coal. Thi s process includes the following steps: (a) forming a slurry of coal particles, preferably at lezast 50% by weight of which particles have a maximum dimension of at least 0.5mm, with an . aqueous solution of an alkali, which solution has an alkali content of from 5 to 30% by wei ght, such that the shury has an alkali solution to coal ratio on a weight basi s of at least 1:1;

. (b) maintaining the slurry at a temperature of from 150° to 300°C, preferably 170°C to 230°C, for a period of from 2 to 20 minutes substantially under : autogenous hydrothermal pressure and rapidly cooling the slurry to a temperature of less than 1 00°C; (c) separating the slurry imto alkalized coal and a spent alkali leachant solution; (d) regenerating the alkali leachant solution for reuse in step (a) above by the addition of calcium ox magnesium oxide or hydroxide thereto to precipitate minerals therefrom; (e) acidifying the alkalized coal by treatment with an aqueous solution of sulphuric or sulphurous acid to yield a slurry having a pH of from 0.5 to 1.5 and a conductivity of from 10,000 to 100,000 ss; ® separating the slurry into acidified coal and a spent acid and a spent acid leachant solution; and (2) washing the acidified coal.

Although the process described in Australian patent no. 592640 can produce a demineralized coal product having on ash content of less than 1% by weigint and as low as 0.50% by weight, significant opportunities arise if the ash content can b e reduced to even lower levels. If the ash level can Te reduced to levels even lower than that achieved in Australian patent no. 592640, the demineralized coal product may be ussed as a fuel directly fired into a gas turbine. In this use, the demineralized coal could replace natural gas as a fuel for the gas turbine. Swch demineralized coal could also be used as an alternative to heavy fuel oils and as a high purity carbon source for the production of metallurgical recarbonisers, carbon electrodes for aluminium production arad alternative reductants for high purity silicon manufacture. The contents of US patent mo 4,936,045 . are herein incorporated by cross-reference.

Suammary of the invention ,

In a first aspect, the present invention provides a process for d=emineralizing coal commprising: ) (a) forming a slurry of coal particles in an alkali solu tion, ) (b) maintaining the s lurry at a temperature of 150-250°C under a pressure sufficient to prevent boiling; (c) separating the slu ny into an alkalized coal and a spent alkali leachant; (d) forming an acidifmed slurry of the alkalized coal, said acidified slurry having a pH of 0.5-1.5; (e) separating the acidfied slurry into a coal-contairming fraction and a substantially liquid fraction; ® subjecting the coal—containing fraction to a waslming step mm which the coal-containingg fraction is mixed with water and a polar organic solvent or vvater and an organic acid to fo-tm a mixture; and (2) separating the coal from the mixture in step (f).

The coal that is provided to step (a) is suitably a medium to higzh rank coal, most suitcably a bituminous coal.

The coal that is provided to step (a) preferably has a totall mineral content gen erally in the range of 2-15% by weigTht. More preferably, the mine=ral content of the coal should be as low as possible. It has been found that the chemical consumption and hen ce the processing cost is lower for c oals of low ash content fed -to step (a) of the proccess.

It is preferred that the coal that is provided to step (a) of the process of the present invention is sized such that 100% is les s than 1mm, more preferably 100% less than = 0.5m. The coal also preferably contains a minimum of material less than 20 microns, more preferably less than 5% by weight ssmaller than 20 microns. It ha.s been found that excess amounts of fine material, e.g, less than 20 microns, can cause difficulties in the solid/liquid separation step-s used in the present invention. : Steps (a) and (b) o f the present process subject the coal to an alkali (or caustic) digestion. This results in the silicate minerals, including clays, being solubilized with 5 some re-precipitating as acid soluble minerals.

The slurry formed in step (a) suitably has a coal concemtration of from 10% to 30% by weight. Preferably, the coal concentration is about 25% by weight.

The alkali concentr ation in the liquid phase of the slurry iss preferably in the range of 8% to 20% by weight, more preferably 13% to 15% by weight (calculated as NaOH equivalent). The alkali material is preferably NaOH, although oth-er alkali materials could also be used, either singly or as a mixture of two as more alkali materials. The slurry is suitably heated to a temperature of from 150-250 C, more preferably from 220-250°C.

The slurry is preferably maintained at this temperature for a period of from 15 to 60 minutes, more preferably feor about 20 minutes.

It has been found that the rate of heating the slurry should preferably be maintained at a rate of less than 2°C per minute in the temperature range of 150°C to 250°C.

It is preferred in steps (a) and (b) that the caustic slurry is formed and then heated to the desired temperature.

The slurry in step (b) is suitably maintained at the autogenous pressure of the heated slurry to prevent thes slurry from boiling.

It is also preferred that the slurry be subject to agitation, e specially mild agitation, in step (b). The degree of agitation is preferably such that deposition of sodium aluminosilicates, of whicth one form is sodalite (NasSi3A130,=(OH)), on the process vessel walls is minimised or avoided. Agitation may be achieved Wby any suitable agitation . means known to the persor of skill in the art. Alternatively or in combination, the use of recycled caustic solution containing small seed crystal of sodium aluminosilicates can be used to encourage sodium aluminosilicates crystal growth in the sslurry rather than on the process vessel walls.

Step (c) of the process of the present invention separates the caustic slurry from , step (b) into an alkalized coal and a spent alkali leachant. Thiss separation step preferably takes place at a temperature of from 30°C to 80°C. It is especially preferred that the slurry . from step (b) is cooled at a cooling rate of less than 20°C/ninute more preferably less than 5°C/minute, even more preferably less than 2°C/minute whilst the temperature of the slurry is in the range ©f 240°C - 150°C.

Step (c) may suitably comprise a filtration step. As memntioned above, the filtration step preferably is conducted at a temperature of from 30°C to S0°C.

The spent caustic/leachant from step (c) is preferably t-reated to regenerate caustic and recover minerals. For example, the spent leachant ma-y be mixed with sufficient calcium oxide or calciuna hydroxide to precipitate the soluble silicate and aluminate ions as their insoluble calcium salts, while simultaneously formings soluble sodium hydroxide, thus regenerating the alkaline leachant for recycling. Ins—tead of calcium oxide or hydroxide, the corresporiding magnesium salts may be use:d, or the mixed oxides or hydroxides of calcium an d magnesium derived from dolomite may be used.

The alkalized coal recovered from step (c) is preferably washed to remove excess alkali. The coal is prefer ably washed with a minimum of 3 poarts by weight of water for each part by weight of dry coal, more preferably S parts by weight water for each part by- weight of dry coal.

The alkalized coal from step (c) may also be treated to remove sodium. aluminosilicates such as sodalite therefrom prior to sending to the acid soak step. The= sodalite may be separated from the alkalized coal by physicaml methods such as selective= screening, heavy media float-sink methods, or frotla flotation. The sodium aluminosilicates, such as sodalite, may provide a valuable by-product whilst removal thereof reduces the amownt of acid required in step (d). .

Step (d) of the process of the present invention may suitably involve mixing the coal from step (c), mores preferably washed coal from step- (c), with water or an acid solution to obtain a slurry. The slurry preferably has a coal concentration that falls withir the range of 5% to 20% "by weight, more preferably about 108% by weight. Generally, the greater the ash content of the starting coal the lower the canal concentration in the aciad slurry, with a 10% sRury being suitable for a starting coal with an ash level o=f . approximately 9%. If the slurry is formed by mixing wisth water, it may be suitably acidified by mixing it with an acid.

Step (d) preferably forms a slurry that contains a mmaineral acid, more preferably sulphuric acid or hydrochloric acid.

The acidified slurry has a pH that falls in the range =f 0.5 to 1.5, more preferably pH about 1.0.

The temperature of the slurry in step (d) preferably falls within the range fromm 20°C to 90°C, more pre=ferably from 30°C to 60°C.

The slurry may be suitably agitated in the acid solution.

The coal is preferably maintained in contact with the acid solution in step (d) for =a period of at least 1 minwte, more preferably for at least 20 m inutes, even more preferabl—y about 60 minutes.

In one embodim ent of the present invention, after an appropriate time, the coal im the slurry of step (d) is separated in step (e) and passed to =step (f). In a more preferred embodiment, the coal fraction from step (€) is re-slurried with water and acid and brough_t to a pH of between 0.5 and 1.0, more preferably about pH 0.55, for a further period of timee of greater than 1 minute. In the more preferred embodiment the first acid treatment wil 1 be with a pH of 1.0-1.5 for the minimum time sufficient to =chieve essentially completes sodium aluminosilicate dissolution. The second acid treatment is preferably at a pH of 0.5-1.0 for times betwee=n 10 minutes and 3 hours.

The step of re-s lurrying the coal may be repeated beetween one and four times. .

Fresh acid solution may be used for the re-slurrying.

Alternatively, the re-slurrying may comprise a counterscurrent mixing stage.

Step (e) involves. separating the acidified slurry into a «coal-containing fraction anc a liquid fraction. This nay be achieved using any suitable solids/liquid separation means known to the skilled pesrson. Filtration is preferred. If the fi ltercake is to be re-shurriecd

: with acid, it does not require washing so long as the time between. step (e) and the second . acid treatment is kept to a minimum, preferably less than 5 minutes. After the final stage of acid re-slurrying, the filter-cake may be given a minimal water vovash such that when the . filtercake is re-slurried in fressh water, the pH of the solution is pre=ferably about 2.

The spent acid may be treated to regenerate an alkali soliution and to obtain the controlled precipitation of muinerals as by-products. For example, the spent acid may be treated with calcium oxide tos regenerate a caustic solution and pre=cipitate the minerals.

The wash step of steps (f) involves two possible options. Omne of these is to mix the coal from the last of the acid soak steps with a solution of wa ter and a polar organic solvent. The polar organic solvent is suitably miscible with wzater. The polar organic solvent is preferably an alcohol, more preferably ethanol, amithough methanol and propanol may also be used.

The coal is preferably mixed with the solution of water arnd polar organic solvent such that a slurry having a solids content of 10-30% by weight_, more preferably about 25% by weight. The residual acidity from the acid soak step(s) is preferably such that the pH of the slurry is from 1.5 teo 2.5, and more preferably about 2.0.

The slurry is preferably heated to a temperature of from 240°C to 280°C, more preferably 260°C to 270°C, dn step (f). The slurry is preferably k—ept at temperature for a period of between 1 minute zand 60 minutes, more preferably aboust 5 minutes.

The slurry of coal/wa-ter/polar organic solvent is preferably. heated at a heating rate of between 2°C per minute and 20°C per minute.

The pressure of thes slurry is such that boiling is prevented. The slurry is preferably heated under aumogenous pressure. At the preferrecl temperature specified above, the autogenous pressuare is approximately 8 MPa.

As mentioned above, the presently preferred polar organic solvent is ethanol. It is especially preferred that the liquid phase mixed with the coal toe produce the slurry is a , 50% by weight ethanol in water solution

Optiora 1 of the washing stage reduces the level of the Na, Si, Fe and Ti, bumt it is primarily act®mve in reducing Na and Si. If onky Na is required to be reduced, the . temperature wmsed in the wash stage can be as losw as 10°C, with operation at anabient temperature b-eing especially suitable.

The second option for the washing stage mnvolves mixing the coal from thee acid soak step(s) -with an aqueous solution of an organic acid. Citric acid is present ly the preferred org anic acid, with chloroacetic acid, maalonic acid and malic acid also being able to be use=d.

The catric acid solution preferably contains between 5% and 20% by weigh® citric acid (hydrate d basis), more preferably about 10% by weight. The coal concentration in the slurry is preferably in the range of 10% to 30% by weight, more preferably aboumt 25% by weight. The slurry is preferably heated to a te-mperature of between 240°C to 280°C, more preferably between 250°C to 270°C. The p ressure should be maintained at a level sufficient to —prevent boiling. The pressure 1s suitably the autogenous pressure whiech, for the temperatwire range specified above, is approx®mately 8 MPa. The slurry is preferably kept at the el evated temperature for a period of beetween 1 minutes and 60 minutes., more preferably at=out 5 minutes. The shury is preferatoly heated to the elevated temperamture at a heating rate of between 2°C per minute and 20°€C per minute.

In amother embodiment of the second Option, the slurry may be heate d to a temperature of between 150°C and 160°C. In thais embodiment, Na and Fe will not be removed.

When step (f) is conducted at elevated temperature, it constitutes a hydrothermal wash step.

With=out wishing to be bound by theory, the present inventors have postulatted that : 25 two mecharmisms may be taking place in the vevashing step to further reduce the ash content, thesse being: (1) the residual acid in the coal from &he acid soak step(s) results in thes slurry of step (d) teing acidified, eg, to a pH of betw=een 1.5 and 2.5. This promotes further mineral diss-olution;

(ii) itis thought that humic compounds aree formed by interaction between the . coal and the alkali Rn steps (a) and (b). In the acid soak step(s), these humic compounds “collapse” and tie Lp some of the Na. In the washimg step, option 1, the alcohol allows . the humics to hydreolyse to release the Na. The Na reports to the water phase fol lowing alcohol/water sepamation. The alcohol can be recycled, essentially in a closed loop recycling step, thias minimising alcohol consumpstion. In option 2, the citric acid facilitates release off the Na from the humics.

Still without wishing to be bound by theory, an alternative mechanism postulated by the inventors is that the Na is scattered amongst functional groups ard also incorporated into the coal structure, especially the graphitic structures. This is bome out : by the higher resi dual Na found in processed higher rank coals, which have fewer humic/functional gmroups but an increased proportion of graphitic structures.

It is suggested that the Na is bound to and/or ~trapped within the coal structwure, and that the ethanol swells the structure and allows the Na to migrate out, or in the case of functional groups «lower rank coals), participates in an esterification reaction. Organic acids, such as citric acid, would have incomplete dissociation in water, so that the dissolved yet undisssociated citric acid molecules alsso swell the coal. Heat also helps to give the Na the kimetic energy to escape any bonds holding it to the coal. Diffusion of the

Na out of the coal structure is also believed to play a_ part.

Step (g) of the process of the present inventJon involves separating the ceoal from the mixture or slurry in step (f). This solid/liquid separation may be achieved by any means known to bes suitable by a person of skill in the art. Filtration is preferred.

It is preferred that the coal recovered from step (g) be washed. Preferably the washing uses a mimnimum of one part of clean water for each part of coal, by weig=ht.

The processs in accordance with the first aspect of the present invermtion can ‘ produce a demineralised coal preduct having an ash content of from 0.01-O.2%, by weight. The process also removes Na and Si from the coal and thus by lowerimg the Na ) content the ash fussion temperature of the ash remairming in the coal is also advantageously increased by the porocess. The ash fusion temperat-ure is important if the demi_neralised

. coal is to be used as a fue] for gas turbines as these require that #he ash fusion temperature be greater than 1350°C, rmore preferably greater than 1500°C. ) The process of the first aspect of the present inventio n is capable of achieving demineralised coal having an ash content of less than 0.2% by weight preferably from 0.01% to 0.2% by weight,, with trials involving some coals achieving an ash content of 0.01% by weight. Steps (a) to (€) of this process of the first aspect of the invention are capable of producing a Aemineralised coal having an ash content as low as 0.3-0.4% by weight. For some uses, this ash content is acceptable and the further processing of the : washing step may not be necessary.

Accordingly, in a second aspect, the present invention provides a process for demineralising coal comprising steps (2) to (€) of the process d_escribed with reference to the first aspect of the present invention.

The washing stage has also been shown to reduce the asth content of the coal. This also suggests that the washing stage can be used as a stage in a demineralisation process that includes steps other than steps (a) to (€) as described with reference to the first aspect of the present invention.

Accordingly, in a third aspect, the present inventior: provides a process for demineralising coal comprising the steps of alkali digestion followed by acid soaking and wherein coal from the acid soaking step is subjected to a further step as described with reference to step (f) of the first aspect of the present invention.

The demineralised coal may be subjected to a binderlesss briquetting process to form a final product of erahanced handleability.

Brief descripti on of the drawings . Figure 1 is a proc ess flowsheet of an embodiment of a process for demineralising coal in accordance with the first aspect of the invention; ) Figure 2 is a process flowsheet of one embodiment of th.e acid soak step of Figure

Figure 3 is a process flowsheet of an alternative embodiment of the acid soak st-ep of Figure 1;

Figure 4 is a process flowsheet of an embodirment of a process for demineralist.ng : coal in accordance with the second aspect of the inveration; and

Figure 5 is a process flowsheet of an embodirment of a process for demineralis3ng coal in accordance v=vith the third aspect of the inventi on.

Detailed duescription of the drawings

In considerirag the drawings, it will be appreciated that the drawings are provided for the purposes off illustrating preferred embodiments of the invention. Therefore, the invention should not be considered to be limited to the features shown and described with reference to the dra—wings.

A flow sheet for a demineralisation process in accordance with the pre sent invention is shown in figure 1. In figure 1, a slurry 11 of coal and caustic solution iss fed to a caustic digestion vessel 10. Caustic digestion v=essel 10 is suitably an autoclave ora pressure vessel that allows the slurry of caustic solut ion and coal to be heated.

The caustic solution 12 that is fed to causstic digestion vessel 10 comprises a sodium hydroxide solution having a sodium hydroxide concentration of 13 to 15%. The coal 11 and soditam hydroxide solution 12 are fe=d to caustic digestion vessel 10 in amounts such that a slurry containing 25% coal is achieved.

The slurry of coal and caustic solution in veessel 10 is heated to a temperature of from 150-250 C, rmore preferably from 220 to 250= Celsius. The slurry is maintaimed at this temperature for a period from 1 minute to 60 minutes, with 20 minutes Tbeing especially suitable. The slurry is maintained under autogenous pressure so thaat the solution does not oil. ,

The slurry of caustic solution and coal is h eated such that the rate of increase of temperature does mot exceed 2° Celsius per minute when the temperature of the coa falls within the temperaature range of 150 to 240° Celsius.

After the required ressidence time has passed, the shury— is cooled at a cooling rate of less than 20 C per minute , more preferably less than 5° Celzsius per minute, even more suitably less than 2° Celsius per minute, whilst the temperatu-re is in the range of 240 to 150° Celsius. The slurry is memoved from caustic digestion vessel 10 and passes via line 15 into filtration unit 20. Filtration unit 20 may be any suitaable filtration unit that can achieve separation of coal fom the caustic solution. Belt filters and drum filters are especially useful. It will also be appreciated that other soliad/liquid separation devices may be used in place of filtration unit 20. For example, thickxeners or decanters may be used.

The spent caustic solution 22 recovered from filtratiomn unit 20 is sent to caustic recovery 24. In caustic re covery 24, the spent caustic sol ution is regenerated. For example, the spent caustic solution may be contacted wit’h calcium oxide, calcium hydroxide, magnesium oxide or magnesium hydroxide to precipitate minerals therefrom and regenerate sodium hydroxide. The regenerated sodium hydliroxide can be reused.

The alkalised coal 26 is then washed with water in wamter wash vessel 30. Water wash vessel 30 may be any suitable vessel for mixing liquidss and solids. Altematively, and preferably, water wash 30 is effected by washing the filte—r cake on the filtration unit 20. In this regard, if a belt filter is used, a filter cake comprising alkalised coal and residual caustic solution is formed on the filter belt. This filter cake may be sprayed with wash water 32. As the filter cake is still in contact with the filtration unit, the wash water is removed as removed wash water 34. The wash water 34 ray also be sent to caustic regeneration 24.

The washed filter cake, comprising washed alkalised coal 36, is then fed to the acid soak process 40. In the acid soak process 40, alkalised acoal from filtration unit 20 and water wash 30 is mixed with water to give a slurry concertration in the range of 5 to 25% by weight coal, preferably 10% by weight coal. The slurmry is acidified with acid 42, preferably sulfuric acid, to obtain a pH in the range of from 0.5t0 1.5, preferably pH 1.0.

The temperature of the acid slury is maintained in the raroge of 20° to 90°C, more suitably in the range of 30° to 60° Celsius, for a period of gmreater than 1 minute, more preferably greater than 20 mminutes. It has been found that 60» minutes is a suitable time for maintairing the coal in contact with the acid solution. The coal should be agitated to promote mixing of the coal with the acid solution.

The acid wash soak process 40 may comprise a single contact between the acid ’ solution and the coal. However, it is preferred that the acid soak process invo lves contacting ®he coal with acid solution more than once. Preferably, the coal is contacted with the acid solution under the conditions of temmperature and residence time outimned above. Thee coal and acid solution are then sepamated and the coal further contact with acid solution on one or more occasions. Figures 2 and 3 show schematic diagrams of some possilble embodiments of the acid soak process 40.

After the acid soak process 40, the coal and acid solution are separated in separation wit 50. Separation unit 50 is suitably a filtration unit, especially a belt filter or a drum falter. The spent acid solution 52 is removed.

The recovered coal 54 is then subjected to a water wash 60. Water wash 60 is suitably achieved by spraying the filter cake of the belt filter or the drum filter with a wash water 62. The wash water is removed fromm the filter cake through the filtration unit, and the removed wash water is shown as refexence numeral 64.

The washed filter cake 66, which comprises treated coal and a small amoumt of residual acid solution, is then passed to hydrothermal washing process 70. The wa shed coal 66 tha t is provided to hydrothermal washing process 70 has residual acid present in an amount such that when the washed coal 66 is Teslurried in fresh water, the pH o f the liquid phasee will be approximately 2.

In lmydrothermal washing process 70, watex 72 and ethanol 74 are mixed with the coal. Preferably, the water and ethanol are mixed. such that a solution of 50% ethan_ol in water is olbtained. The amount of water, ethariol and coal fed to the hydrothesrmal washing pr-ocess 70 is such that a slurry having a solids loading of 25% by weigzht is ‘ achieved. Suitably, the water, ethanol and coal are mixed prior to feeding to vessel 70.

In a most preferred embodiment of the present invention, the slurry in hydrothermal washing process 70 is heated t0 a temperature of 240 to 280° Celsius, especially 260 to 270° Celsius, by heating the slurry at a heating rate of between 2°

Celsius per minute and 20° Celsius per minute. Heatin g is conducied under autogesnous pressure such that boiling is prevented. At the maximum temperatures reached i n the hydrothermal wasshing process 70, the autogenous pressure is approximately 8 MPa. The slurry is suitably kept at the elevated temperature for a pesriod of between 1 minute amnd 60 minutes, suitably 5 minutes. Under these conditions, tlhe hydrothermal washing preocess reduces the leved of sodium, silicon, iron and titanjurm in the coal, with the pri_mary activity being red uction of sodium and silicon levels.

If only sodium is required to be reduced in hydrothermal washing process 70, the temperature usedl the hydrothermal wash stage can be as low as 10° Celsius amnd is suitably ambient temperature. In this case, the hydrothermmal washing stage can be si mply described as a wa shing stage.

The slumrwy from hydrothermal washing proces-s 70 is passed via line 76 to filtration unit 80. In filtration unit 80, the slurry from time hydrothermal washing process is separated into aa coal fraction 82 and a liquid fraction 8-4. The liquid fraction 84 m=ay be sent to an ethanol recovery unit 90, which is suitably a distillation column. In etlanol recovery unit 90. the liquid fraction 84 is split into a water rich fraction 92 and an etlaanol nch fraction 94. Ethanol rich fraction 94 is suitably’ returned as stream 74 to the hydrothermal was hing unit 70.

The coal fraction 82 is washed in washing proc ess 100 using fresh wash vovater 102. The wash w ater is removed via stream 104 and a re=covered ultra clean coal product 110 1s recovered.

The ultra «clean coal product is preferably subjescted to a binderless brique=tting process to produces a product having enhanced storage ancl transport properties.

The ultra clean coal product recovered from the process shown in figure 1 will typically have an ash content of between 0.01 and 0.2% by weight, with an ash fiasion temperature sufficciently high to enable use of the ultra clean coal as a fuel for gas turbines. When the ultra clean coal is used to fire directly into gas turbines as part of a gas turbine combiried-cycle power station, the ultra clean coal has the potential to reeduce the greenhouse gas emissions by 25% when compared. to modern coal fired the-rmal power stations. When the extra processing involved in obtaining the ultra clean coal is , taken into account, greenhouse gas emissions are still reduced by nearly 10% on an overall life-cycle basis.

As mentioned above, the acid soak process 40 may compmrise a first slurying of the coal with an acid solution, followed by re-slurying of the coall between one and four times. Figure 2 shows one possible flow sheet for the acid soak p-rocess 40. In figure 2, the alkalised coal 36 is fed to a first acid soak vessel 140. An acid solution 142 is mixed with the alkalised coal 36 in vessel 140 for the desired time -and under the desired temperature conditions. The acidified slurry of coal 144 then passses to a separator 146.

The spent acid solution 148 is removed and the coal containing frzaction 150 is thereafter fed to second acid soalk vessel 152. Spent acid solution may be s ent to caustic recovery step 24 for NaOH regeneration and recovery of minerals. Fresh acid solution 154 1s mixed with the coal containing fraction in vessel 152 under the required conditions. The acidified slurry 156 is sent to second separator 158. The acid solution 160 is removed and the coal containing fraction 162 sent to either separation unit 50 ass shown in figure 1 or, if further is re-slurrying steps are required, sent to a further acid soak vessel 164. Broken lines 165 indicate that the sequence of soaking with fresh acidl solution followed by separation may be repeated one or more times.

In vessel 164, the coal containing fraction 162 is mixed vevith fresh acid solution : 166 for the desired time and under the desired conditions. The rermoved slurry 44 (which corresponds to slurry lime 44 shown in figure 1) is then passed to separator 50 and water wash 60, which corresp ond to the respective separator 50 and wate=r wash 60 of figure 1.

The re-slurrying of the coal with fresh acid solution —preferably takes place between one and four ti mes.

Figure 3 shows an alternative embodiment of the acid soak process in which a . number of contacts are made between the acid solution and the coal fraction. In figure 3, the acid soak process is achieved by a multi stage, counter current contacting between the : coal and the acid solution. The process involves contacting thes coal fraction with the acid solution in a number of contacting vessels 240, 242. The broken lines 244 indicate that there may be more contacting vessels than the two shown in figure 3. The coal 36 is fed to contacting vessel 24(B. The coal containing fraction 250 from vessel 240 is fed to contacting vessel 242. The coal containing fraction 252 from contacting vessel 240 iss : then fed to either separatio-n unit 50 (as shown in figure 1) or to one or more further contacting vessels (not shown).

Similarly, fresh acid solution 260 is fed to the dowrastream contacting vessel (242 in figure 3). The liquid fraction from 262 from vessel 242 i s then fed to contacting vesse=l 240. The liquid fraction 26% from contacting vessel 260 is- removed. The spent acid 264 may be sent to caustic regeneration (eg 24 in Figure 1) to regenerate an NaOH solutiomn and recover precipitated mirerals.

The process shown #n figure 3 may utilise any apparatus known to be suitable te the man skilled in the art for counter current contact between solids and liquids. Such apparatus will be well know=n and need not be described further.

Figure 4 shows a flew sheet of a process in accordance with the second aspect o=f the present invention. For some uses, the coal product obtained from water wash 60 shown in figure 1 has sufficiently low ash content to be used without needing to undergeo the hydrothermal washing p tocess. Therefore, the process Shown in figure 4 is essentially identical to that shown in fi gure 1, except that the coal fraction 66 from water wash 60 i_s not fed to the hydrothermal washing process, but rather goess to water wash 100, where mt is washed with wash water 102 to obtain an ultra clean coal product 110. The ultra clean coal product 110 of figure 43 will have a somewhat higher zsh content that the ultra clean coal product 110 of figure 1 .

The remaining featuares of the process shown in figu re 4 are essentially identical t © those of figure 1 and the same reference numerals have b een used in figure 4 for thosse features. ) 25 Figure 5 shows a flow sheet in accordance with thes third aspect of the inventior.

In the flow sheet shown in figure 5, the coal 300 is subjected to a caustic digestion 302, and then 10 an acid wash or acid soak stage 304. The caustic digestion 302 and acid wassh stage 304 of figure 5 may be the same or different to the re=spective stages described with reference to figure 1. The acoal fraction 66’ from acid soal< 304 is fed to a hydrothermaal washing process 70", follow ed by separation in filtration unit 80' i nto a liquid fraction 84' and a coal containing fraction 82'. Liquid fraction 84' is framctionated into a water containing fraction 92' and =; recovered ethanol fraction 94". :

Coal containing fraction 82' is washed in washing unit 100" and an ultra clean coal product 100' is recovered. The processing steps and conditions o=f hydrothermal washing process 70' shown in figure 5 is essentially identical to the hydrotlhermal washing process 70 with reference to figure 1.

Those skilled in the art will appreciate that the invention clescribed herein may be subject to variations and modifications other than those specificalAy described. It is noted that the hydrothermal washing process may use an organic ac=id instead of the polar organic solvent, with citric acid being preferred. If citric acid is u sed in the hydrothermal washing process, the prefermed conditions are as set out under thes description of the first aspect of the present inventi on and the ethanol recovery process nmay be omitted.

The particular apparatus used in the present process includes any suitable apparatus known to the person skilled in the art. For examples, the caustic digestion vessel 10 may comprise any suitable reactor including tubular concurrent-flow reactors, stirred autoclaves operating batch wise, or with continuous inflov= and outflow, in single or multi stage configurations, or counter current or cross phase systems. As the apparatus that may be used in the process of the present invention will be well known to the person of skill in the art, it need not: be described further.

It will be understood. that the invention disclosed and defirned herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims (51)

Claims.

1. A process for demineralizing coal comprising: (a) formmng a slurry of coal particles in an alkali =solution, (b) maintaining the slurry at a temperature o—f 150-250°C under a press ure sufficient to prevent boiling; (©) separating the slurry into an alkalized coal and a spent alkali leaclm ant; (d) forming an acidified slurry of the alkalize=d coal, said acidified slurry having a pH of 0.5-1.5; ce) separating the acidified slurry into a coal-cortaining fraction and a subst antially liquid fraction; 63) subjecting the coal-containing fraction to a vovashing step in which the coal-containing fraction is mixed withh water and a polar organic solvent or water and an organic acid t-o form a mixture; and (2) separating the coal from the mixture in step (#f).

2. A process as claimed in claim 1 wherein the coal provided to ste-p (2) 1s sized such that 100% is less than 1mm.

3. A process as claimed in claim 2 wherein the coal provided to ste p (a) is sized such that 100% less than 0.5mm. .

4. A process as claimed im claim 2 or claim 3 wherein the coall provided to step (a) contains 5% by weight sma ller than 20 microns.

5. A process as claimed in any one of the preceding claims wherei_n the slurry formed in step (a) has a coal concentration of from 10% to 30% by weight.

6. A process as claimed in claim 5 wherein the coal concentration in the slurry is about : 25% by weight.

7. A processs as claimed in any one of thee preceding claims wherein an alkali concentration in a liquid phase of the shury is in the range of 8% to 20% Wby weight (calculated as NaOH equivalent).

8. A process as claimed in claim 7 wherein the alkali concentration is from 13 % to 15% by weight (calculated as NaOH equivalent).

9. A process as claimed in any one of the prece=ding claims wherein the slurry is heated to a temperatuare of from 220-250°C in step (b).

10. A processs as claimed in any one of the preceding claims wherein the slurry 1s maintained at an elevated temperature in step (b) for a period of from 15 to 60 rminutes.

11. A process- as claimed in any one of the preceding claims wherein a rate of Ineating the slurry is mairtained at a rate of less than 2°C per minute in the temperatur-¢ range of 150°C to 250=C.

12. A processs as claimed in any one of the pre-ceding claims wherein the shumry in step (b) is maintaiined at the autogenous pressure off the heated slurry to prevent the slurry from boiling.

13. A proces.s as claimed in any one of the prece=ding claims wherein step (c) takes place at a temperature of from 30°C to 80°C.

14. A processs as claimed in claim 13 whereira the slurry from step (b) is cooled to a . temperature of from 30-80 C at a cooling rate of less than 20°C/minute and at 2 C per minute whilst: the temperature of the slurry is in the range of 240°C - 150°C.

15 A process as claimed in any one of the preceding claims wherezin the aikalized coal recovered from step (c) is washed to remove excess alkali.

16. A process as claimed in any~ one of the preceding claims wherein the alkalized coal from step (c) is treated to removee sodium aluminosilicates therefrorm prior to sending to step (d).

17 A process as claimed in any «one of the preceding claims wherein step (d) comprises mixing the coal from step (c) with water or an acid solution to obtain a slurry having a coal concentration that falls withim the range of 5% to 20% by weight.

18. A process as claimed in clair 17 wherein the slurry has a coal concentration of about 10% by weight.

19. A process as claimed in any one of the preceding claims wherezin the slurry in step (d) contains a mineral acid.

20. A process as claimed in claim 19 wherein the mineral acid ds sulphuric acid or hydrochloric acid.

21. A process as claimed in any one of the preceding claims where-in the slurry of step (d) has a pH that falls in the range of 0.5 to 1.5.

22. A process as claimed in claim 21 wherein the pH of the slurry 1s about 1.0.

23. A process as claimed in any one of the preceding claims whereira the temperature of the shury in step (d) falls within thee range from 20°C to 90°C. }

24. A process as claimed in clair 23 wherein the temperature falls within the range of from 30°C to 60°C.

25. A process as claimed in any one of the gpreceding claims wherem the ceal is maintained in econtact with the acid solution in step (d) for a period of at least 1 minute.

26. A process as claimed in claim 25 wherein the= coal is maintained in contact with the acid solution ir step (d) for a period of about 60 mmnutes.

27. A process as claimed in any one of the preceeding claims wherein the coal fraction from step (€) i s re-slurried with water and acid arad brought to a pH of between 0. Sand

1.0 for a furthe=r period of time of greater than 1 mi nute

28. A process as claimed in claim 27 wherein the step of re-slurrying the coal is rep»eated between one ard four times.

29. A process as claimed in any one of the prececling claims wherein step (f) compprises mixing the coa_l-containing fraction with a solution of water and an alcohol selected from ethanol, methamol, propanol or mixtures thereof.

30. A process =as claimed in claim 29 wherein the Organic solvent is ethanol.

31. A process as claimed in any one of the preceding claims wherein, in step (f), thee coal is mixed with vevater and polar organic solvent such that a slurry having a solids cont-ent of 10-30% by wei_ght is formed.

32. A process as claimed in claim 31 wherein the s lurry has a pH of from 1.5 to 2.5.

33. A process as claimed in any one of claims 2S to 32 wherein the slurry heateci to a ) temperature of from 240°C to 280°C in step (f).

34. A process as claimed in claim 33 wherein the shury is kept at elevated tempematuare for a period of oetween 1 minute and 60 minutes.

WE) 2004/039927 PCCT/AU2003/001409

35. A process as claimed in claim 33 wherein the slurry of coal”water/polar organic solvent is heated at a heating rate of between 2°C per minute and 20°CC per minute.

36. A process as claimed in any one of claims 1 to 28 wherein. step (f) comprises 3 subjecting the coal-containing fraction to a hydrothermal washing step in which the coal- containing fraction is mixed with water and an organic acid select ed from citric acid, chloroacetic acid, malonic acid, malic acid or mixtures thereof.

37. A process as claimed in claim 36 wherein the organic acid is citric acid and a citric acid solution containing between 5% and 20% by weight citric acidll (hydrated basis) is added to the coal-containing fraction.

38. A process as claimed in claim 37 wherein the slurry is heated to a temperature of between 240°C to 280°C.

39. A process as claimed in claim 37 wherein slurry is heated to a temperature of between 150°C and 160°C.

40. A process as claimed in claim 38 oer 39 wherein the pressure is mnaintained at a level sufficient to prevent boiling.

41. A process as claimed in any one off claims 38 to 40 wherein the slurry is at elevated temperature for a period of between 1 minutes and 60 minutes.

42. A process as claimed in any one of claims 38 to 41 wherein the slurry is heated to the ) elevated temperature at a heating rate of “between 2°C per minute and 20°C per minute. .

43. A process as claimed in any one of the preceding claims the coal mrecovered from step (g) is washed with water.

44. A process as claimed in any one of the preceding claims wherein deminesralised coal recovered from step (g) has an ash content of from 0.01-0.2%, by weight.

45. A -process for demineralising coal comraprising the steps of alkali digesticon followed by acid soaking and wherein coal from the acid soaking step is subjected tO a washing step in ~which the coal-containing fraction is mixed with water and a polar org=anic solvent or watewr and an organic acid to form a mixtiare, and separating the coal from the mixture.

46. A process for demineralizing coal comperising: (a) forming a slurry of coal particles in an alkali solution, said srry containing 10-30% by~ weight coal; (®) maintaining the slurry at a temperature of 150-250<°C under a pressure sufficient to porevent boiling; (c) separating the slurry into an alkalized coal and a sspent alkali leachant; (d) forming an acidified slurry of the alkalized coal, sat d acidified slurry having a pH of O.5-1.5; and (e) separating the acidifie«d slurry into a coal-containing fraction and a substantially liquid fra«<ction.

47. A process as claimed in claim 29 wherein the temperature used in step (f) is from 10 C to zambient temperature.

48. A p-rocess as claimed in any one of claimms 1 to 44 wherein the spent alkali leachant from step (c) is treated to regenerate caustic aand to recover minerals. ‘

49. A porocess as claimed in claim 48 wherein the spent alkali leachant is treated by ' mixing “with one or more of calcium oxide, calcium hydroxide, magnes1 um oxide, magnesivam hydroxide, or mixed oxides or hysdroxide of calcium and magnesivam derived from dolomite to precipitate soluble ssilicate and aluminate ions and fi-om soluble sodium ~~ hydroxide.

50. A process as claimed in any one of claims 1 to 44 or 48 —to 49 wherein the substantially liquid fraction of step (€) is treated to regenerate a causstic solution and to recover minerals.

51. A process as claimed in claim 5€ wherein the substantially liquid fraction is mixed with one or more of calcium oxide, calcium hydroxide, magnesium oxide, magnesium hydroxide, or mixed oxides or hydwoxide of calcium and magnessium derived from dolomite.