Process for preparation of stable coal-water mixtures

Abstract

 A specific type of wet grinding of coal utilizing the total quantity of water needed for the final mixture, plus appropriate additives, furnishes a very stable product that is especially suitable for use as fuel.

Description

[0001] The present invention relates to a process for the preparation of stable coal-water mixtures. More precisely it relates to the produc tion of coal-water mixtures with a high coal content, suitable for use as a substitute for fuel oil or metallurgical coke (e.g. in blast furnaces).

[0002] 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-saving solutions. As a result, oil-water, coal-oil, coal-water and coal-oil-water mixtures have been proposed.

[0003] In particular, coal is coming to play an ever more important role for use as auxiliary fuel injected into the blast furnace via the tuyeres.

[0004] Among the various solutions 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 mixes with a high coal content.

[0005] In this regard it has been found that mixtures containing from 70 to 80% coal are of special interest, both from the fuel consumption and blast-furnace operating quality aspects. However, some practical difficulties are encountered with mixtures of this kind, mainly concerning pumping and phase separation, with settlement of the coal on the bottom of the storage tanks.

[0006] This invention proposes to eliminate these difficulties by providing a simple, cheap process for preparing coal-water mixture with a coal content of 70 to 80% which is easy to pump and is stable timewise.

[0007] According to this invention, coal preferably finer than 3 mm in size, is fed into a mill together with the quantity of water desir ed 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 fluidizing agents such as humic acid based compounds or their derivates or lignin sulphonates are added.

[0008] Grinding must result in a coal-water mixture where the harmonic mean diameter of the coal particles is around 50µm. It is also necessary that at least 60% and preferably at least 70% of the coal should be finer than 74µm, while less than 10% should be coarser than 250 µm.

[0009] 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 µm, at least 50% of the coal being finer than 74 µm, and less than 20% coarser than 250 µm.

[0010] 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 µ m, has an apparent viscosity of about 300 cP (Brookfield at 30 rpm).

[0011] This mixture is extremely stable. After 45 days the suspension is still of excellent quality, is readly pumpable and has a virtually negligible vertical concentration gradient.

[0012] It is important to note that the addition of fluidizing agents has a very marked effect, especially on the viscosity of the suspension. However, depending on the particle size and the total quantity of coal, maximum efficiency is attained with a given quantity of fluid izing agent, beyond which the viscosity may rise even markedly. In our experiments it has been seen that the maximum efficiency is attained for additions of between 0,3 and 1,3%.

[0013] The present invention will now be illustrated in relation to a series of practical experiments performed on a medium-high volatiles American coal commonly used in.iron and steel making, having the following characteristics: volatile matter 30,8%, fixed carbon 64,2%, ash 5% (the percentages are calculated on a dry-weight basis).

[0014] The 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.

[0015] A mixture of activated salts of humic acid and phosphates was added at a fixed rate of 0, 5%.

[0016] The results obtained are reported in the following table:

The static stability of the mixtures over the course of time is indicated by the variation in the number of seconds required for a 20 g rod 3 mm in diameter to penetrate under its own weight through a 180 mm depth of mixture stored in the undisturbed state. In the first three tests the coal had all settled out after one week, so penetration of the rod was stopped by the coal layer.

[0017] The assessment was made as objectively as possible, beirgbased on the ratio of the penetration time after five weeks compared with that when the mixture had just been made up (zero weeks). With a ratio of less than 4 the stability of the mixture was considered very 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 mixtures with a viscosity of less than about 500 cP being classed as good. This is why Test 4 was considered to be only satisfactory, because although its stability was good its viscosity was 1400 cP. As is evident from the Table, as soon as more than 60% of the coal is finer than 74 µm, time-stable mixtures with good viscosities are obtained. Of course, if mixtures with very high stability are not needed, because they are to be used immediately after being prepar ed, then it is not necessary to grind the coal any finer than 50% minus 74 µm. In this description the average diameter of the particles is calculated as the harmonic and not the arithmetic mean.

[0018] The mixtures 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 auxiliary fuel for injection into blast furnaces via the tuyeres. The mixtures appear to be very interesting for the latter application, not only from the economic point of view but also because they permit particularly uniform, efficient blast-furnace operation.

Claims

1. 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), characterized by the fact that the coal is ground together with the water in the proportions desired for the final mixture, plus a small quantity of known fluidizing agents amounting to between 0,05 and 2% (by weight), the grind ing being carried to the point where at least 50% of the coal is finer than 74 µm.

2. Process as per Claim 1, characterized by the fact that the grind ing is performed in two stages, during the first of which the coal is fed together with the water and the additives in the proportions desired for the final mixture, thus obtaining 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. Process as per Claim 2, characterized by the fact that the coal feed is finer than 3 mm.

4. Process as per Claim 2, characterized by the fact that after the aforesaid grinding the coal particles have a harmonic mean diameter of around 60 µm.

5. Process as per Claim 1, characterized by the fact that after the second grinding at least 70% of the coal has a particle size of less than 74 µm.

6. Process as per Claim 5, characterized by the fact that after the second grinding the coal particles have a harmonic mean diameter of around 50 µm.