CA1274686A

CA1274686A - Process for preparation of stable coal-water mixtures

Info

Publication number: CA1274686A
Application number: CA000449907A
Authority: CA
Inventors: Luigi Palumbo; Giansilvio Malgarini
Original assignee: Centro Sviluppo Materiali SpA
Current assignee: Centro Sviluppo Materiali SpA
Priority date: 1983-04-29
Filing date: 1984-03-19
Publication date: 1990-10-02
Anticipated expiration: 2007-10-02
Also published as: AU2611284A; EP0124488A1; BR8401913A; AR230842A1; ZA842004B; IN162513B; IT8348185A0; IT1197637B; EP0124488B1; DE3480850D1; AU568637B2; ATE49016T1

Abstract

A B S T R A C T
A specific type of wet grinding of coal utilizing the total quanti ty of water needed for the final mixture, plus appropriate additives, furnishes a very stable product that is especially suit-able for use as fuel.

Description

Process for pre~aration of stable coal-water mistures.
The present invention relates to a process for the preparation of stable coal_Nater mi~tur~s. Mor~ precisely it relates to the produc tion of coal-water mistures with a high coal content, suitable for use as a substitute for fuel oil or metallurgical coke (e.g. in blast furnaces).
The increase in the price of oil, only slightly mitigated by the recent small reductions, as well as the prospects of possible scarcity of this raw material on the markets, have given a spur to the search for oil substitutes, or at least greater o i 1 savings.
For these reasons, major fuel_oil consuming sectors such as electri city generating authorities and the iron and steel industry are actively engaged in seeking alternative and fuel-savine solutions.
As a result, oil-water, coal-oil, coal-water and coal-oil-water mistures have been proposed.
In particular, coal is coming to play an ever more important role for use as ausiliary fuel injected into the blast furnace via the tuyeres.
Among the various ~olutions one of the most promising for replacing fuel oil, especially for maintaining regularity of blast-furnace operation and reducing production costs, is the use of coal-water mi es with a high coal content.
In this regard it has been found that mixtures containing from 70 to 8 ~ coal are of special interest, both from the fuel consumption and blast-furnace operating quality aspects. ~owever, some practical difficulties are encountered with mistures of this kind, mainly ~Z7468~;

concerning pumping and phase separation, with settlement of the coal on the bottom of the storage tanks.
m is invention proposes to eliminate these difficulties by providing a simple, cheap process for preparing a coal-water mixture with a coal content of 70 to 80% which is easy to pump and is stable timewise.
Thus the present invention provides a process for the prepa-ration of stable coal-water mixtures suitable for use as fuel, in which there is between 70% and 80% coal by weight. In this process, coal is ground together with water in the proportions desired for the final mixture, plus a small quantity of at least one fluidizing agent amounting to between 0.05 and 2~ by weight, the grinding being carried to such as extent that at least 60% of the coal is finer than 74 ~m and less than 10% is coarser than 250 ~m.
The present invention, in particular, provides a process for the preparation of stable coal-water mixtures suitable for use as fuel, in which there is between 70% and 80% coal by weight, in which coal is ground together with the water in the proportions desired for the final mixture, plus a small quantity of at least one fluidizing agent amounting to between 0.05 and 2% by weight, the fluidizing agent being selected from the group consisting of humic acid based ' compounds and lignin sulphonates. The grinding is carried to such an extent that at least 70% of the coal is finer than 74 ~m and less than 10% thereof is coarser than 250 ~m. The grinding is also performed in two stages. The first one of these stages comprises feeding the coal together with the water and the additives in the proportions desired for the final mixture and processing the resulting mixture to obtain a mixture in which at least 50% of the coal is finer than 74 ~m and less than 20% is coarser than 250 ~m. The seocnd grinding stage comprises processing the mixture obtained at the end of the first stage to obtain the desired final mixture in which at least 70% of the coal has a particle size of less than 74 pm and less than 10% thereof is coarser than 250 pm and in uhich the coal particles have a harmonic mean diameter of around 50 ~m.

lZ74686 - 2a -According to this invention, coal preferably finer than 3 mm in size, is fed into a mill together with the quantity of water desired in the final mixture. Typically 70-80% by weight of coal is added to 20-30% of water.
At this stage between 0,05 and 2% by weight of known fluid-i~ing agents such as humic acid based compoundsior their derivates or lignin sulphonates are added.
Grinding must result in a coal-water mixture where the harmonic mean diameter of the coal particles is around 50 ~m. It is also necessary that at least 60% and preferably at least 70% of the coal should be finer than 74 ,um, while less than 10% should be coarser than 250 ym.
In the experiments carried out, it has been found useful to perform the grinding in two stages in a disc mill. While the final grading must be as indicated above, the first stage of grinding must provide a mixture in which the harmonic mean diameter of the coal particles is around 60 ~m, at least 50% of the coal being finer than 74 ,um, and less than 20% coarser than 250 um.
Of course, the type of mill is not binding according to the invention. Typically a mixture produced in this manner containing 73% by weight of coal having a mean diameter of 54 ,um, has an apparent ~Z74686 ~ 3 ~

5 viscosity of about 300 cP ~Brookfield at 30 rpm).
This mi~ture i5 e~tremely stable. After 45 days the suspension is still of e cellent quality, is readly pumpable and has a virtu~lly negligible vertical concentration gradient.
It i8 important to note that the addition of fluidizing agerts has a very marked effect, especially on the viscosity of the suspension.
However, depending on the particle size and the total quantity of coal, ~a~imum efficiency is attained with a given quantity of fluid izing agent, beyond which the viscosity may rise even markedly. In our esperiments it has been seen that the ma~imum efficiency is 15 attained for additions of between 0,3 and 1~3~o.
The present i~vention will now be illustrated in relation to a series of practical e~periments performed on a medium-high volatiles American coal commonly used in iron and steel making, having the following characteristics: volatile matter 30~8~o~ fised carbon 64~ 2~o~ ash 5%
(the percentages are calculated on a dry-weight basis).
~he minus 3 mm coal was wet ground in a disc mill, as described above, feeding to the first mill the coal, water and additive in the quantities desired for the final suspension.
A mi~ture of activated salts of humic acid and phosphates was added 25 at a fi~ed rate of 0, 5%.
The results obtained are reported in the following table:

12~4686 _ _ _ _ _ _ _ _ I = ,, ~ ~= ~ ~ ~o o ~o .
U O O h h O 0 æ O c ~ ~
--5 _____ -~ _ ~ (~ 0~ _ n m5 _ _~_ _ 0 _ ~ ~ ~0 0 _ _ _ _ _ U~ U~ C~J CO ~t ~o n ~N . ~ N r~l ~ 0~ ~ ~0 u~

n ~ '~ 3 3 _ . . _ 1.~.'~ _ __ ~ c ~ ~ u~ u~ ~ ~
3u~12~ o ~ u~ ~ ~ ~ c ~ ~ 1~ u~ ~ ~
n ~ m e ~c o~ o~ 2 o uo~ o 0~ u~ o uo~ ~ o ~~ r _ _l _ _ _ O CO ~ ~ O ~ O ~ ~ .

Il ~ '~ ~ ~ co ~i o ~o O~ c~i u~
'. ~ _ _ _ ~ ___ 12746B~ii ~he static stability of the mi~tures over the course of time is indicated by the variation in ths number of seconds required for a 20 g rod 3 mm in dia~eter to penetrate under its own ~eight through a 180 mm depth of mixture stored in the undisturbed state.
In the first three tests the coal had all settled out a~ter one week, so penetration of the rod was stopped by the coal layer.
The assessment was made as objectively as possible, beingbased on the ratio of the penetration time a~ter five weeks compared with that when the mi~ture had just been made up (~ero weeks). With a ratio of less than 4 the stability of the mi~ture was considered ve~y good, while if it was between 4 and 10 it was classed as good.
Of course, viscosity also has a bearing on the assessment, stable mi~tures with a vi æosity of less th2n about 500 cP being classed as good. This i8 why Test 4 was considered to be only satisfactory, because although its stability was good its viscosity was 1400 cP.
A~ is evident from the Table, as soon as more than 6~o of the coal is finer than 74 ~m, time-stable mi~tures with good viscosities are obtained. Of course, if mi-tures with very high stability are not needed, because they are to be used immediately after being prepar ed, then it is not secessary to grind the coal any finer t~an 50~0 minus 74 ~m. In this description the average diameter of the particles is calculated as the harmonic and not the arithmetic mean.
The mi~tures thus prepared are suitable for substituting fuel oil and similar petroleum derivatives in applications such as, for instance, fuel in thermal-electric power stations or as au~iliary fuel for injection into blast furnaces via the ~uyeres. Ihe mi~tures appear to be very i~teresting for the latter application, not only from the economic point of view but also because they permit part-icularly uniform, efficient blast-furnace operation.

3o

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the preparation of a stable coal-water mixture suitable for use as fuel and containing between 70% and 80% of coal by weight, comprising grinding coal together with water in the proportions desired for the final mixture, plus a small quantity of fluidizing agent amounting to between 0.05 and 2% by weight, said grinding being carried to such an extent that at least 60% of the coal is finer than 74 µm and less than 10% thereof is coarser than 250 µm.

2. A process according to claim 1, wherein the grinding is performed in two stages, and wherein during the first one of these two stages, the coal is fed together with the water and the fluidizing agent in the proportions desired for the final mixture and the resulting mixture is processed to obtain a mixture in which at least 50% of the coal is finer than 74 µm and less than 20% is coarser than 250 µm.

3. A process according to claim 2, wherein the coal feed is finer than 3 mm.

4. A process according to claim 2, wherein, after the first grinding stage, the coal particles have a harmonic mean diameter of around 60 µm.

5. A process according to claim 2, wherein, during the second grinding stage, the mixture obtained at the end of the first grinding stage is processed to obtain a final mixture wherein at least 70% of the coal has a particle size of less than 74 µm.

6. A process according to claim 3, 4 or 5, wherein at the end of the second grinding stage, the coal particles have a harmonic mean diameter of around 50 µm.

7. A process for the preparation of a stable coal-water mixture suitable for use as fuel and containing between 70% and 80% of coal by weight, comprising grinding coal together with water in the proportions desired for the final mixture, plus a small quantity of a fluidizing agent amounting to between 0.05 and 2% by weight, said fluidizing agent being selected from the group consisting of humic acid based compounds and lignin sulphonates, said grinding being carried to such an extent that at least 70% of the coal is finer than 74 µm and less than 10% thereof is coarser than 250 µm, and being performed in two stages, the first one of said stages comprising feeding the coal together with the water and the fluidizing agent in the proportions desired for the final mixture and processing the resulting mixture to obtain a mixture in which at least 50% of the coal is finer than 74 µm and less than 20% thereof is coarser than 250 pm, the second grinding stage comprising processing the mixture obtained at the end of the first grinding stage to obtain a final mixture wherein at least 70% of the coal has a particle size of less than 74 µm and less than 10% thereof is coarser than 250 µm, and in which the coal particles have a harmonic mean diameter of around 50 µm.

8. A process according to claim 7, in which the coal feed is finer than 3 mm.

9. A process according to claim 7, in which, at the end of the first grinding stage the coal particles have a harmonic mean diameter of around 60 µm.

10. A process according to claim 7, in which the fluidizing agent is present in an amount of 0.3 to 1.3% by weight.

11. A process according to claim 5, wherein, at the end of the first grinding stage, the coal particles have a harmonic mean diameter of around 60 µm.

12. A process according to claim 5, wherein the coal feed is finer than 3 mm.

13. A process according to claim 5, 11 or 12, wherein at the end of the second grinding stage, the coal particles have a harmonic mean diameter of around 50 µm.