CA1207923A

CA1207923A - Process for preparing high solid concentration coal- water slurry

Info

Publication number: CA1207923A
Application number: CA000426187A
Authority: CA
Inventors: Kazuhiko Nakaoji; Mitsugu Kamao; Kazuaki Shigetomi; Kunizo Shinano
Original assignee: Electric Power Development Co Ltd; Sumitomo Heavy Industries Ltd
Current assignee: Electric Power Development Co Ltd; Sumitomo Heavy Industries Ltd
Priority date: 1983-04-14
Filing date: 1983-04-19
Publication date: 1986-07-15
Also published as: AU1351583A; AU551624B2

Abstract

Abstract of the Disclosure A pulverized coal slurry containing 5-25 weight percent pulverized coal particles, more than 50 weight percent of the particles being minus 200 mesh, is subjected to flotation for deashing and desulfurizing treatments to thereby recover as froth cleaned coal of 15-30 weight percent concentration. This froth is dewatered into a high solids concentration coal-water slurry of more than 60 weight percent. This slurry can be transported by pipe flow as a fluid fuel or it can be burned directly in a boiler without need for dewatering. In addition, the high-degree of deashing of the slurry according to the invention enhances combustion efficiency and reduces abrasion of the boiler.

Description

~7~:3 A PROCESS FOR PREPARING HIGH SOLID CONCENTRATION COAL-WATER SLURRY
B~ckground of the Invention (1) Field of the Invention This invention relates to a process for preparing a hi~h solids concentration coal-water slurry which can be handled as a fluid fuel and further be burned directly in various furnaces including a boiler.

(2) Description of the Prior Art Coal has usually been crushed to minus 2 inches and the crushed coal is handled by bulk transportation by a wagon or the like. In this transportation and further loading and unloading of the crushed coal dusting problems occur, because fine coal particles (less than 0.5 mm) are scattered by wind and the like. In addition, during mining of coal phenomena such as spontaneous combustion caused by reaction of the mined coal with the oxygen in the atmosphere are occasionally observed. This spontaneous combustion depends not only upon chemicsl proparties of the coal itself, but al50 on retention time of the mined coal in the atmosphere, so that it sometimes burns in the storage yard and during transport. As stated previously, bulk transportation of coal involves various problems in handling coal, and various investigations have been carried out to solve thesa problems. A hi~h solids concentration coal-water slurry that can be handled in the fluid state preventin~ dust and spontaneous combustion problems is very attractive. This slurry can be obtained by mixing pulverized co~l with a small amount of water snd it can be handled in the same manner as petroleum. In this CQSe, it is desir~ble that the solids concentration of the coal slurry should be increased as much as possible for economy of fuel.
Coal contains more or less incombustible mineral matter (ash) widely di~tributed therein. Accordingly, in the above mentioned high solids concentration slurry, the previous removsl of ash from coal results in an increased heating value of the fuel ~nd can reduce abrasion of tha walls of a boiler.
Ash sometimes cont~ins sulfur in the form of pyrite and the like. In this case, removal of the ash leads to decreased sulfur in the fuel, and prevents the corrosion of the boiler tube by SOx.
It has hitherto been disclosed to sub~ect fine coal particles less than 0.5 ~m to flotation. However, the conventional flotation treatment has not .

~2~7~23 achieved a satisfactory deashing effect.
Summary of the Invention The object of this invention is to provide a process for preparing a high solids concentration coal-water slurry from which ash hss been removed to a high degree.
In other words, the process for preparing a high solids concentration coal-water slurry of this invention is characterized by the steps of subjecting a pulverized coal slurry containing pulverized coal particles in the amount of S-25 wt.~, S0 wt.% or more of said particles being pulverized into minus 200 mesh, to flotation for deashing; recovering froth which contains 15-30 wt.~ of deashed coal; and dewaterin~ the recovered froth at least up to 60 wt.~ solids concentration. According to this process, ash and sulfur can be effectively removed from slurry.
The resulting slurry is advantageous in that the amounts of ash and SOx generated on co~bustion o the slurry in a combustion apparatus such as a boiler in a steam power plant are 50 small that the damage to the appQratus csused by abrasion with ash or corrosion with SOx can be effectLvely reduced, and in that the exhaust gas can be easily treated, the maintenance of the apparatus being simple. In addition, the weight ratio of the combustible components contained in this fluid fuel increase in proportion to the amount of reduced ash and thus the heatin~ value of the fluid fuel increases.
Brief Description of the Drawings Fig. 1 is a flow sheet illustrating one embodiment of this invention.
10 ... flotator 12 ... filter 14 ... mixer 16 ... pump 18 ... thickener 26 ... slurry preparation tank 28 ... conditioner Fig. 2 is a flow sheet illustrating the process for preparing 8 high solids concentrstion coal-water slurry.
Fig. 3 is a graph illustrating the relationship of the slurry concentration and ash content in the froth with the deashing rate.
Fig. 4 is a graph illustrating the relationship of the froth concentration with the deashing rate.
Detailed Description of the Invention Fig. l is a flow sheet illustrating an embodiment according to this invention. A pulverized coal slurry feed is prepared so that it may contain ; - 2 -~2~7923 pulverized coal particles of minus 200 mesh ~74 m) in the ratio of 50 wt.% or more and has 5-25 wt.~ solids. This pulverized coal slurry can be prepared by introducing coal and water into a wet pulverizer such as a wet ball mill, wet-grindin~ the coal and dilutin~, if necessary, to a predetermined concentration in slurry preparation tank 26. The slurry can also be prepared by dry pulverizin~ coal into fine coa7 particles of a given particle size and mi~ing the coal particles with water in tank 26. This pulverized coal slurry is then introduced into flotator 10 through conditioner 28 for deashing.
According to this invention, since the coal has been pulverized, before being subjected to flotstion-treatment, to minus 200 mesh, coal particles come to occupy 50 wt.~ or more, and ash is effectively separsted.
The ash rsferred to denotes incombustible components contsined in the coal, said components being for the most part comprise~ of mainly A 23 sio2 and the like and partly of Fe2S or FeS. Since this Fe2S or FeS is converted into SOs on combustion in the boiler, the deashing of the slurry leads to reduction of S0~ generation.
Flotation is carried out by introducing into a flotation vessel a slurry, or pulp, havin~ 5-25 wt.~ of solids, preferably 5-15 wt.~, in the presence of flotation reagents such as a collector, frother and the like to thereby recover the coal as froth having 15-30 wt.~ of solids, preferably 18-25 wt.%.
If the slurry concentration is more than 25 wt.~ there c&n not be attained ~atisfactory daashing efficiency. If the slurry concentration is not more than 5 wt.~, on the other hand, the deashing efficiency can be improved but the equipment must be large. Therefore, the latter is uneconomical.
The collector, which is added in flotator 10 or conditioner 28, exhibits a selective adsorption to coal. Fuel oils such ns Diesel oil, white kerosine, kerosine, a heavy oil and the like are suitabla collectors. As the frother methyl isobutyl carbinol (MIBC), pine oil and the like are suitable.
With flotation a froth ha~in~ 15-30 wt.~ of solids can be obtained, which is hi~her than that of the pulp. The solids concentration of the froth can be controlled by regulating the amount of air introduced, namely, the amount of bubbles formed, although the solids concentration of the froth also depends on that of the pulp. When the froth concentrat~on is too high, coal recovery deteriorates, and when the froth concentration is too low, the resultant froth cannot be effectively deashed, &nd the dewaterin~ load in the successive ; - 3 -. . ..

75~
dewatering step is increased.
Subsequently, the froth i5 introduced into filter 12 for dewatering up to st least 60 wt.% of solids. As filter 12 there can be employed a suitable device such 8S a vacuum filter, a centrifuge, a filter press or the like. In the vacuum filter, it i5 possible to employ au~iliary means to enhsnce dewatering efficiency such as compression dewatering means with roll and steam or the li~e. Still further, it is possible to treat the froth with a thickener prior to treatment with filter 12 .
A dewatering agent may be added to the froth on dewatering in filter 12.
Since the froth, which contains finely divided particles, is dewatered with rela~ive difficqlty, the addition of the dewatering agent is preferable in that the dewatering efficiency is anhanced and a wide variety of filters is usable for various requirements.
As the dewatering agents there can be employed substances which are capable of promoting separation of water to facilitate dewatering, for instance surface active agents such as sodium dioctylsulfosuccinate, sodium didecylsulfosuccinate, sodium ditrimethylhexylsulfosuccinate and the like and a coagulating agept.
It is desirable that the thus obtained coal-water slurry ha~ing 60 wt.~ or more of solids is then introduced into mixer 14 and mixed with a dispersing agent so 8S to stabilize slurry fluidity. As the dispersing agent there can be used surface active aBents such as water-soluble sulfonates and sulfates, natural or synthetic water-soluble hi8h molecular compounds and the like. The resulting stabilized high-concentration slurry is supplied directly to a boiler, is stored in a tank, or is transferred by way of a pipe line.
For this in~ention, thare is the necessity of introducing a relatively low solids concentration slurry into the flotator in order to conduct the deashing treatment effectively by flotation, whil~t it is desirable to make a final product slurry have a high solids concentration, so far as its fluidity ss maintained. In order to meet this requirement, it is essential to dispose a dewatering step in succession to the deashing step. In this dewatering step, much water is separated from the ~lurry and discharged. The water used in the separation and removal of ash from the tails of the flotator is also discharged. Accordingly, it is preferable that these exhaust waters should be re-used in recycling to the flotator, the slurry preparation tank, or the mill.

:lZ1~7~Z3 Referring to Fig. 1, the water separated by filter 12 cycles through line 22 to slurry preparation tank 26. The tail from flotator 10 is introduced to thickener 18 to separate water from ssh. The separated water may be cycled through lines 24 and 22 to tank 26. The water separated from filter 12 and thickener 18 may be allowed to cycle, wholly or partially iP needed, to tank 26. The thickener 18 may be replaced by other separating means such as the above-mentioned filters.
To obtain a high solids concentration coal-water slurry efficiently a dewatering agent can be added to the slurry which is dewatered by means of a filter. That is, coal and water are mixed to~ether and pulverized by means of a wet pulverizer so as to prep~re a coal-water slurry containing the pulverized coal particles, 50 wt.% or more of the part~cles being minus 200 mesh. This slurry can be made into a 60 wt.% or more concentration coal-water slurry by dewatering the former slurry by means of a dewaterin~ device such as a vacuum filter, a filter press or a centrifu~e. It i8 8190 possible to improve the dewatering efficiency by adding 8 dewaterin~ a8ent thereto. It i5 preferable that this slurry shGuld be stabilized by the addition of dispersing a8ent.
Fig. 2 is a flow sheet illustrating the process. First, coPl and water are fed to wet pulverizer 28 and pulverized therein, thereby obtaining a coal-water slurry containing the p~lverized coal particles, 50 wt.~ or more of the particles being minus 200 mesh. The thus obtAined slurry is fed into vacuum filter 12' and a dewatering sgent added for dewatering. The obtained high concentration cake is fed into mi~er 14 and a dispersing agent added, thereby obtaining a high concentration slurry. The nature of the dewatering agent and dispersing agent and the results caused by their addition or the filter used herein are explained previously.
According to this invention, coal is pulverized beforehand so that 50 wt.%
or more of the coal particles are minus 200 mesh.
Thereafter, the slurry containing the pulverized coal particles in fixed concentration is introduced into the flotator for deashing. The thus treated slurry i5 recovered as froth of specific concentration. Subsequently, this froth is dewatered to obtain a high solids concentration coal-w&ter slurry.
This slurry permits coal to be used as a fluid fuel in like manner to petroleum and contains ash in a very small amount. Therefore, on combustion _ 5 _ ~,'`

3 ~D7~3 of this slurry in a boiler or the like, there is little possibility ~or the ash content to cause abrasion and blockage of the apparatus or corrosion of the apparatus. Furthermore, the deashed ælurry according to the invention, which contains much combustible effective component, has high combustion efficiency, being econo~ical in fuel.
Preferred Embodiment of This ~nvention E~amples 1 to S
Bituminous Coals containing ash in the range of 8.4-10.3 wt.% were pulveri~ed together with water. The resulting coal-water slurries (feeds) containing minus 200 mesh particles in the rate of 70-80% and havin~ solids concentrations of 10.0-16.2 wt.2 were each introduced into a flotator, a collector and 8 frother were added thereto for flotation, and the resulting froth was recovered. The results are as shown in Table 1. It is apparent from Table 1 that the ash is effectively removed by flotation.
Examples 6 to 8 High solids concentration coal-wat0r slurries were obtained by adding 8 dewatering a~ent, i.e., sodium dioctylsulfosuccinate to 500 cc of froth recovered respectively from the flotation and then vacuum-dewatering the froth using a vacuum filter. The results are as shown in Table 2.

Ta~le 2 No. Froth Dewatering _ Ratio of Concen- Amount of Pres- Solld minus tration dewater- sure tration 200 mesh ing agent (mmHg) after particles (% per dewater-_ (wt%) coal ton) -500 in(wt~) 6 80 17.0 0.4 to 79.4 7 80 16.~ 0.4 to 76 3 8 80 18.2 1 0 -3t8oo - 69.1 e~,' __ __ O I ~dP CO ~ ~ C~ ~
~ ~ . . . .
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O~lo~ o ,~ ~ co a~
O O h~rl 3 ~ ~ ~ ,~
V V

E ~ o o o o o I
o o _ ~__ _ 0 ~ VH ~ : ~
~: _ _ _ _ ~1 ~- _J ~ ~ CO OD
~0 0 r~ t- t~ ~ In ~o o 3~ 1 i o i ~¦ ~ u ~ a R ~ O -I J~d~ ~ ~r o a~ ~
~¢ V~3 CO C~ ,~ C~ O
- - - -0~ 3 ~ ,1 O O O
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o l r~ o o o o ~ o o o o o ,~ O O~ In ~o ~ u~ u~
_ _ _ _ ~ ~ ~ ~ ~ u XO,z _ _ _ 7 ~-~l2~7~23 8xample 9 and E~ample 10 Hi~h solids concen~ration coal-water slurries were obtained. A dispersins s~ent, i.e., sulfonic acid type surface nctive agent, was added to th~
slurries. Thus, there were obtained stabilized hi~h concentration slurries.
The results are as shown in Table 3.

Table 3 am- High concentration Stabil zed high ple coal-water slurry coal-water slurry Ratio of Amount Solid Vis-Amount Amount minus of dis- con~ cosity ofof 200 mesh persing cen~ of coal water parti-agent tra- slurry (g) (g) cles(~ per tion ~C.P.) (wt~) coal) (wt%) . _ _ 1. 400 161 7~-80` 0.5 71 560 400 195 70 800.5 71 S90 E~ample 11 Blair Athol was pul~erized into minus 65 mesh particles and into minus 250 mesh particles respectively. Coal-water slurries containin~ these fine particles were each introduced in a flotator, to which was added 1660 g/ton of Diesel oil as a collector and 620 g/ton o~ MIBC as a frother for flotation.
Froth was recovered therefrom. The deashin~ rate and the recovery of cosl were compared with reference to these recovered froths. The results are as shown in Table 4. It i9 apparent therefrom that when flotation is applied to finer particles, the deashin~ rate can be improved from 35.7 ~ to 52.5 % with substantially equivalent recovery.
Example 12 By using 1020 ~/ton of Diesel oil and 330 ~/ton of MIBC, a Tatun~ Coal ~2~79Z~

_, _ ~;~ ~UU- U~ ~D

',,', _ .~

~JloP, ID ~
. ~ U7 E~ ~ ¦ ~ a 5 . a , . j _ 9 _ ,`t ,`~

~Z~7!~Z3 slurry was subjected to flotstion with the slurry concentration to recover a froth of 18.2 18.9 wt.~ concentration. Fig. 3 illustrates the relationship of the Rlurry concentrntion snd ssh content in the froth with the deashing rate.
E~ample 13 A Tatung Coal slurry having 9.3 wt.~ slurry concentration wQs introduced into a flotstor and subjected to flotation by using 750 g~ton of Diesel oil and 300 g/ton of HIBC. Fig. 4 illustrates the relationship of the froth concentration with the deashing rste.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a high solids concentration coal-water slurry which comprises subjecting to flotation a pulverized coal slurry containing coal particles in an amount of 5-25 wt.%, 50 wt.% or more of said particles being pulverized into minus 200 mesh, for deashing treatment; recovering as froth deashed coal of 15-30 wt.% solids and dewatering the thus recovered froth to a 60 wt.% or more of solids slurry.

2. A process according to claim 1 wherein said dewatered slurry having 60 wt.% or more of solids is stabilized as to fluidity by the addition of a dispersing agent.

3. A process according to claim 1 wherein said pulverized coal slurry having 5-15 wt.% of solids is introduced into a flotator and subjected to flotation, thereby recovering froth having 18-25 wt.% of solids.

4. A process according to claim 1 wherein 65 wt.% or more of said particles in the slurry are pulverized into minus 200 mesh.

5. A process according to claim 1 wherein said pulverized coal slurry contains coal particles in an amount of 5-15 wt.%, 65 wt.% or more of said particles being pulverized into minus 200 mesh, and is subjected to flotation to recover as froth deashed coal of 18-25 wt.% concentration.

6. A process according to claim 1 wherein the froth is dewatered by means of a filter press, a vacuum filter or a centrifuge filter.

7. A process according to claim 6 wherein the dewatered slurry having 60 wt.% or more of solids is stabilized as to fluidity by the addition of a dispersing agent.

8. A process according to claim 1 wherein the froth is dewatered by the addition of a dewatering agent.

9. A process according to claim 8 wherein the dewatered slurry having 60 wt.% or more of solids is stabilized as to fluidity by the addition of a dispersing agent.

10. A process according to claim 1 wherein the froth is dewatered by the addition of a dewatering agent and by means of a filter press, a vacuum filter or a centrifuge.

11. A process according to claim 10 wherein the dewatered slurry having 60 wt.% or more of solids is stabilized as to fluidity by the addition of a dispersing agent.

12. A process for preparing a high solids concentration coal-water slurry which comprises subjecting to flotation a pulverized coal slurry containing coal particles in an amount of 5-15 wt.%, 50 wt.% or more of said particles being pulverized into minus 200 mesh, for deashing treatment; recovering as froth deashed coal of 18-25 wt.% solids; dewatering the thus recovered froth into 60 wt.% or more solids slurry; and adding a dispersing agent into the slurry to stabilize the fluidity thereof.

13. A process according to claim 12 wherein 65 wt.% or more of said particles in the pulverized slurry are pulverized into minus 200 mesh.

14. A process according to claim 12 wherein the froth is dewatered by means of a filter press, a vacuum filter or a centrifuge.

15. A process according to claim 12 wherein the froth is dewatered by the addition of a dewatering agent.

16. A process according to claim 12 wherein the froth is dewatered by the addition of a dewatering agent and by means of a filter press, a vacuum filter or a centrifuge.