CA1321038C - Method of separating carbonaceous coal from an aqueous slurry - Google Patents

Abstract

TITLE
"A Method of Separating Carbonaceous Coal From An Aqueous Coal Slurry"

INVENTORS
Charles E. Capes Kevin A. Jonasson William L. Thayer ABSTRACT OF THE DISCLOSURE
: In a method of agglomerating carbonaceous coal comprising first agglomerating impurity liberated carbonaceous coal, from an impurity liberated coal slurry, with agglomerating oil to form open structured, chain-like, micro-agglomerates and then forming relatively larger, less open structured more robust agglomerates from a portion of the slurry to provide a mixture of micro-agglomerates, relatively larger, more robust agglomerates, water and any inorganic impurities that may be present. The relatively larger, more robust agglomerates are screened and then the micro-agglomerates are separated using an aerating, skimmer tank leaving an inorganic impurity laden waste water. The micro-agglomerates are rendered buoyant by the aeration and are thus rendered separable from the inorganic laden waste water which may be further treated to remove the inorganic waste to provide water for recirculation.

Description

13210~8 I Thl~ invention relates to a method of separating cArbonaceous coal from an aqueous coal slurry.

It has already been proposed in United States Patent No. 3,655,066, dated May 23, 1972, "Beneflciatlon of Coals", C.E. Capes et al, to add a bridglng liquid ~o an aqueous, clay contalnlng slurry of coal fines, then agitate the resultant mixture to form coal Rgglomerates dispersed in a slurry of the residual clay and ash impurities, and then separate the coal agglomerates by skimmlng them through an overflow spout in a float-sink tank. The separation of the coal agglomerates may be assisted by introducing a multitude of air bubbles at the bottom of the float sink tank. They are then mixed with a binding oil and formed into a balled coal product in a balling device.

It has also been proposed in United States Patent No. 4,284,413, dated August 18, 1981, "In-Line Method For the Beneflclation of Coal and the Formation of A Coal-In-Oil Combustible Fuel Therefrom", C.E. Capes et al, to provide an in-line method for the beneficiatlon of coal and the formation of a coal-in-oil combustible fuel wherein the coal 18 wet pulverized, micro-agglomerated with light oil to dissoclate a large amount of inorganic impurities and some water, agglomerated wlth heavy oll to form relatlvely larger agglomerates and dls~ociate malnly water wlth some lnorganic impurities, and then mixed with further heavy oll to form the coal-ln-oil combustlble fuel.

132~3~
1 While these proposals o~ Capes et al have proved to be useful, it has now been found that a problem exists in processes where a mixture of relatively larger, more robust, carbonaceous coal agglomerates and weaker agglomerates are formed from micro-agglomerated carbonaceous coal.

When carbonaceous coal is separated from inorganic impurities by micro-agglomeration followed by the formation of a portion of relatively larger, more robust agglomerates, a portion of the carbonaceous coal in the form of difficult-to isolate relatively weaker micro-agglomerates often remains in the separated water which also contains inorganic impurities. The proportion of micro-agglomerates which remain in the separated water and inorganics may be dependent upon whether the amount of oil added for agglomeration is sufficient, whether the mixing is optimal or whether the carbonaceous coal is relatively more difficult to agglomerate, such as, for example, when the coal is a low rank or oxidized coal.

There is a need for a process wherein micro -agglomerates may be separated when present in water and inorganic impurities together with relatively larger, more robust agglomerates.

According to the present invention there is provided a method of separating carbonaceous coal from an aqueous coal slurry, comprising a) mixing an agglomerating oil for carbonaceous coal with an aqueous coal, comprising particles of ~321038 carbonaceous coal, particles of inorganic impurities, and ~ater, lmtil oF~n structured, chain-like micro-agglomerates are formed from carbonaceous coa]
particles of the slurry, then S b) mixing the slurry containing the micro-agglomerates under conditions sufficient to form relatively larger, less open structured, more robust agglomerates from a portion of the micro-agglomerates to provide a slurry containing micro-agglomerates and relatively larger agglomerates, then c) separating the relatively larger, more robust agglomerates from the slurry to leave a mixture of water, micro-agglomerates of carbonaceous coal particles and particles of inorganic impurities, and then d) aerating the water containing the micro-agglomerates of carbonaceous coal and particles of inorganic impurities to render the micro-agglomerates of carbonaceous coal particles buoyant, then e) separating buoyant micro-agglomerates of carbonaceous 2G coal particles from a major portion of the water and the particles of inorganic impurities, and then f) mixing the separated micro-agglomerates with the separated relatively larger robust agglomerates to form a combined micro-agglomerate, robust agglomerate product.

In some embodiments of the present invention, after removal of micro-a~glomerates, the water fraction is separated from the inorganic impurities therein and is recirculated as feed water for the a~ueous coal slurry.

A In other embodiments of the present invention, the , 1 ~r\i( mJ ll-Jloln~rlm ~i Iri~ rormecl in 1l rcLatively high shear, im?eller hla}l-~ mi~er, t;'le rel~tively Larger, more robust agqlomerates ,Ire formed in a rekltiv(?1y low shear, impeller blade mixer, and a portion of the water contclinin~ relatively lar~Jer, more robust agglomerates from the relatively low shear, impeller blade miY~er is recirculated for further mixiny therein to enhance nucleation of the relatively laryer, more robust agglomerates.

me combined micro-agglomerate, robust agglomerate product may `oe Eurther mixed with relatively coarser grain, clean coal for ease of transpor-tation, storage and utilization as a combustible fuel.

Alternatively, the carbonaceous fine coal product which comprises the micro-agglomerates and the relatively larger, more robust agglomerates, may be mixed together with a binder therefor for size enlargement of the agglomerates for increased flouability during transportation, storage and utilization.

In the accompanying drawings which illustrate, by way of example, embodiments of the present invention, Fiyure 1 is a flow diagram of a method of separating carbonaceous coal from an a~ueous coal slurry, Figure 2 is a photograph of open structured, chain-like micro-agglomerates obtained from tests using the apparatus described ~ith reference to the flow diagram shown in Figure 1, and Figure 3 is a photograph of relatively larger, less open structured, more robust agglomerates obtained during the tests.

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l In Figure l there 18 shown fl method of separatlng carbonaceous coal from an aqueous coal slurry, comprising a) mixing, ln relatlvely high shear, lmpeller blade mixers 1 to 4, an agglomerating oll for carbonaceous coal wlth an aqueous, lmpurity liberated, coal slurry until open structured, chaln-like micro-agglomerstes a~e formed from carbonaceous coal particles of the slurry, b) mixlng ln relatively 1QW shear impeller blade mlxers 6 and 8, the slurry cOntaining the micro-agglomerates to form relatively larger, les~ open structured, more robust agglomerates from a portion of the mlcro-agglomerates to provide a slurry containlng micro-agglomerates and relatively larger agglomerates, c) separatlng by si~e in a screening device 10, the relatively larger agglomerates from the slurry, d) aerating the slurry in a skimmer tank 12, to render the micro-agglomerates buoysnt, and e) separating, by means of a sklmmer belt-paddle system 14, buoyant mlcro-agglomerates from a ma~or portion of the water and any inorganlc impurities of the slurry. ::

After removal of the micro-agglomerates, the water 18 separated from lnorganlc lmpurltles ln a settllng tank 16 and 18 reclrculated by pump 18 as feed water for the aqueous coal slurry.

~321038 A portion of the water containlng the relatlvely larger, more robust agglomerates and exltlng from the relatlvely low ~hearl impeller blade mixers 6 and 8 is reclrculated, by means of a pump 19 and ad~ustment of valves 21 and 23, for further mlxing therein.

More particularly, a coal slurry feed from, for example, a fine waste ~tream of a coal cleanlng plant (not shown) i8 fed along a feed plpe 20 to a dilutlon tank where the water content i~ ad~usted with make up water from a header tank 24. The coal slurry i8 then pumped by a pump 25 10 to the hlgh shear, lmpeller blade mlxers 1 to 4. The agglomerating oil is fed by feed plpe 5 into the coal slurry being pumped to the high shear, impeller blade mixers 1 to 4, The agglomerating oll is fed by feed pipe 5 into the coal slurry belng pumped to the high shear, lmpeller blade mixers 1 to 4.

The relatlvely hlgh shesr, impeller blade mixers 1 to 4 are preferably of the type described and clalmed ln United States Patent No. 4,610,547, dated September 9, 1986, "Apparatus For Di~perslng A

Partlculate Material In A Liquid", Bennett et al, wherein, as shown in 20 that patent, the slurry passes upwardly through a cylindrlcal contalner past a lower, flat lmpeller blade type turblne rotor, an intermediate knlfe impeller blade type turbine rotor and an upper pitched impeller blade type turbine rotor. As prevlously stated, micro-agglomerates of lmpurlty liberated carbonaceous coal particles are agglomerated from the 25 coal slurry in the relatlvely high shear, impeller blade mixers 1 to 4, wherein high lntensity mixing, distributing and dispersion of the agglomerating oil with the carbonaceous particles of the slurry occurs.

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13210~8 1 The micro-agglomerates, water and any inorganlc lmpurltles origlnally present in the coal slurry are passed to the relatlvely low ~hear, impeller blade mdxers 6 and 8 each having four, radial flow, flat impeller blades, two of whlch are shown and deslgnated 26, 28 and 30 and 31, respectlvely. The relat~vely low shear, impeller blade mixers 6 and 8 are provlded wlth four baffles, two of whlch are shown for each mlxer 6 and 8 and designated 34, 36 and 38, 40, reduce any flow a~ound the impeller blade shaft, caused by the flat lmpeller blades such as those deslgnated 26, 28 and 30 and 32, of the micro-agglomerates, water and any inorganic impurities present 80 that the predominant flow ls radially outwardly f~om the lmpeller blades such as those deslgnated 26, 28 and 30, and 32, and then lnwardly reboundlng along curved paths over and under the impeller blades, such as those designated 26, 28 and 30 and 32, generally towards a central point between them. The baffles such as tho~e 15 deslgnated 34, 36 and 38 and 40 sre spaced from the contalners in which they are situated to avoid the formatlon of stagnant areas between the baffles, such as those deslgnated 34, 36 and 38 and 40 and their respectlve containers.

As previously stated, a portion of the slurry containing larger, more robust agglomerates from the relatively low shear, lmpeller blade mixers 6 and 8 18 recirculated, by means of pump 19, for further mixing thereln.

By optimizing, for the particular coal being treated, the carbonaceous coal particle size, the amount of oil used and, the recirculatlon amount, 1~21~8 l i) the escape of carbonaceous coal through the relatlvely low shear, impeller blade mixers 6 and 8 ln dlscrete partlcle form, that ~8 carbonaceous coal partlcle~ whlch have not been agglomerated, is mlnlmlzed, and ii) the ratlo of micro-agglomerates to relatlvely larger, more robust agglomerates ln the dlscharge from the relatively low shear, impeller blade mlxers 6 and 8 can be optlmlzed.

The deslred ratio of reclrculatlon of the discharge from the relatively low shear, impeller blade mixers 6 and 8, i8 to a large extent dependent upon the amount of agglomeratlng oil that is used.

There are a number of factors lnvolved whlch determlne whether there i8 a predomlnance of relatlvely larger, more robust agglomerates formed from the slurry, or a predominance of micro-agglomerates formed thereln. Among these factors are whether or not the carbonaceous coal is oxidlzed, whether the coal is a low rank coal or a bitumlnous coal and the residence tlmes ln the mixers 6 and 8. However, lt i8 important to note that:
a) a predominance of the relatively larger, more robust agglomerates 18 obtalned when, for example, 1) the ash release partlcle slze of the carbonaceous coal being treated 18 large enough for a relatlvely low agglomerating oll content to be used to produce the relatively larger, re robust agglomerates, or 1~21038 l il) the coal ~lurry being treated la a taillngs contalnlng sufflclently low agglomerating oll content to be used to produce the relatlvely larger, more robu~t agglo~erates, and b) a predomlnance of mlcro-agglomerates is obtalned when, for example, a mlnimal agglomeratlng oll content ls used.

The pump l9 also provldes some relatlvely low shear mixing for agglomeratlng carbonaceous coal psrticles that have not been agglomerated ln the relatively low shear, impeller blade mixers 6 and 8.

The dewa~ered, relatively larger, more robust agglomerates from the screenlng devlce 10 are pa~sed to two screen bowl type centrifugal separators 44 and 46 which further dewater them and pass them to a clean coal conveyor 48 whlch conveys them for storage or further treatment into a fuel.

The mlcro-agglomerates from the sklmmer tank 12 are pas~ed to the screen bowl type centrlfugal separators 44 and 46 for dewatering with the relatively larger, more robust agglomerates and eventual deposition therewlth on the clean coal conveyor 48.

As previously stated, the clean water from the settllng tank 16 1~ pumped by pump l8 for recirculation. Inorganic impurities, such as ash, whlch have settled out of the water ln the settllng tank 16 are pumped by a pump 50 to a dewaterlng device 52 where the dewatered inorganlc impurlties are passed to a conveyor belt 54 for dlsposal.

1~21038 I In test~ to verify the present inventlon, 1) the relatively high shear impeller blade mlxers 1 to 4 each had a 600 lltre mlxing capacity and a 55 KW mixer, ii) the relatively low shear impeller blade mlxers 6 and 8 each had a 12,000 litre mlxing capacity and a 7.5 KW
mlxer, iii) the pump 19 and the valves 21 and 23 reclrculated 6,000 to 12,000 litres/mlnute of water contalning the relatlvely larger, more robu~t agglomerates to the relatively low shear lmpeller blade mlxers 6 and 8, iv) the cla~sification cut of the screenlng device lO was 60 - 100 mesh, . v) the screen bowl type centrifugal separatora 44 and 46 each had about 20 metrlc tonne~/hour capacity, vl) the dewatering device 52 was a solid bowl centrifuge havlng a capacity of about lO metric tonnes/hour capacity, vii) the aqueou~ coal slurry fed by the pump 25 to the relatively high shear impeller blade mixers l to 4 contalned 15 wtX solids of which 30 wt~ was ash.

In a fir~t set of the tests, 2,600 litres/mlnute of the aqueous coal slurry containing about 5 wt% agglomeratlng oil ba~ed on the total sollds content of the aqueous coal slurry was fed by the pump 25 to the relatlvely high shear impeller blade mixers 1 to 4.

1~21~3~

I The screening device 10 captured 80 wt% of the agglomerates formed leavlng 20 wtX to be captured by the sklmmer belt 14 from the skimmer tank 12, The screen bowl type centrifuge separators 44 and 46 delivered relatively larger, more robust agglomerates contalnlng 17.5 wt% moisture and 10.7 wtX ash, together wlth 4.8 wt% agglomerating oil~ to the conveyor 48.

The solid bowl centrifuge S2 produced a refuse product containing 28.9 wt% moisture and 69 wt~ ash (dry basis) from the feed thereto, for deposition on the conveyor belt 54.

In a second set of tests, 3,200 litres/minute of the aqueous coal slurry contalning about 8 wt% agglomerating oil (based on the total solids content of the aqueous coal slurry) was fed by the pump 25 to the relatlvely hlgh shear lmpeller blade mixers 1 to 4.

Thls second set of tests produced a greater preponderance of the relatlvely larger, more robust agglomerates than the first set of tests wlth the consequence that 95 wtX of the agglomerates formed were captured on the screening device 10 leavlng 5 wtX to be captured by the sklmmer belt 14 from the sklmmer tank 12.

The screen bowl type centrifugal separators 44 and 46 dellvered relatively larger, more robust agglomerates containing 15 wt% molsture and 10 wt~ ash to the conveyor 48.

l The solld bowl centrlfuge 52 produced a refuse product cootalnlng 35 wt~ molsture and 79 wt% ash (dry basls) from feed thereto, for deposltlon on the conveyor belt 54.

Flgure 2 ls a photograph of typical open structured, chaln-like mlcro-agglomerates that are produced in the slurry of the relatlvely high 6hear, lmpeller blade mlxers l to 4. It is clear from Figùre 2 that these micro-agglomerates are relatively weaker and consequently are difficult to separate from water and any lmpuritles present.

Flgure 3 is a photograph of a typical slurry produced by the relatively low she~r impeller blade mixers 6 and 8 and containlng relatlvely larger, more robust agglomerates. It is clear from Figure 3 that these relatively larger, more robust sgglomerates can be separated from the slurry, while any open structured, chain-like micro-agglomerates present in the slurry will to a large extent be lost in the water and inorganics separated from the relatively larger, more robust agglomerates.

In different embodiments of the present invention, the slurry contalnlng the mlcro-agglomerates and the relatively larger, more robust agglomerates i8 passed dlrectly from the relatively low ~hear, impeller blade mixers 6 and 8, by the pump 19, to the skimmer tank 12, where all of the agglomerates are aerated and separated by means of the skimmer belt 14 ln one operation.

1~21038 1 In other embodlments of the present invention, relatively coarse grain, clean coal, from, for example, the coal cleaning plant (not shown) from which the coal slurry feed along feed pipe 20 i9 derived, i8 fed along a conveyor 56 to the conveyor 48. The conveyor 48 then conveys the relatively coarser coal, the dewatered mlcro-agglomerates and the dewatered relatlvely larger, more robust agglomerate& to a hopper 58, whlch passes them to a mixer 60 where all of them are mixe~ for use, for example, as a combustible fuel, for carbonization or for making a coal liquld fuel.

In yet other embod~ments of the present invention, when only mlcro-agglomerates and the relatively larger, more robust agglomerate~ are fed to the hopper 58, the mixer 60 may be used to add a binder such as, for ~xample, asphaltic oil, bitumen, coke oven tar or a polymeric emuls10n for size enlargement of the agglomerates for lncreased flowability.

In yet other embodlments, where the type of coal or agglomerating oil or both permit it, the formation from the orlginal slurry of a slurry contalnlng a mlxture of agglomerates may be carried out ln the same mixer or the same group of mixers in parallel.

Further tests were carrled out, addlng a frothlng agent to improve recovery of the micro-agglomerates by the ~kimmer belt paddle system 14. The frothlng agent used was that marketed under the trademark "Aerofroth" by Cyanamld, Montreal, Canada. The amount of frothing agent added was up to about 0.5 kilograms/metric ton of feed to the skimmer tank 12.

1~21~3~
I These tests showed an improvement ln recovery, and a more reproduceable higher recovery, of micro-agglomerates than was obtainable when no frothing flgent was used.

It is within the scope of the present inveDtlon to use other conditionlng agents than frothing agents to enhance the performance of the method for the separatlon the carbonaceous coal from the aqueous slurry.

For example, long chain alcohols such as that marketed under the trademark 'ACCOAL' by Cyanamid, Montreal, Canada, may be used, or any other surface active agent for the carbonaceous coal may be used which will, for example, enhance the wettabllity of the carbonaceous coal by the agglomerating oll.

The present invention has been found to be useful for separating carbonaceous coal from low rank, oxidized coals and bituminous coals.

Claims (2)

1. A method of separating carbonaceous coal from an aqueous coal slurry, comprising (a) mixing an agglomerating oil for carbonaceous coal with an aqueous, impurity liberated, coal slurry, comprising particles of carbonaceous coal, particles of inorganic impurities, and water, until open structured, chain-like micro-agglomerates are formed from carbonaceous coal particles of the slurry, then (b) mixing the slurry containing the micro-agglomerates under conditions sufficient to form relatively larger, less open structured, more robust agglomerates from a portion of the micro-agglomerates to provide a slurry containing micro-agglomerates and relatively larger agglomerates, then (c) separating the relatively larger, more robust agglomerates from the slurry to leave a mixture of water, micro-agglomerates of carbonaceous coal particles and particles of inorganic impurities, then (d) aerating the water containing the micro-agglomerates of carbonaceous coal particles and particles of inorganic impurities to render the micro-agglomerates of carbonaceous coal particles buoyant, and then CLAIMS (Cont) 16 (e) separating buoyant micro-agglomerates of carbonaceous coal particles from a major portion of the water and the particles of inorganic impurities, and then f) mixing the separated micro-agglomerates with the separated relatively larger robust agglomerates to form a combined micro-agglomerate, robust agglomerate product.

2. A method according to claim 1, wherein the combined micro-agglomerate, robust agglomerate product is further mixed with relatively coarse grain, clean coal to form a combustible fuel.