This invention relates to a process for the removal of water from solid materials, particularly from solid fuels and minerals. In certain parts of the world, for example Victoria,

Australia, large deposits of brown coal exist near the surface and can be recovered at comparatively little cost by open- cut mining. Unfortunately, this material has a very high moisture content, up to 65% by weight, which makes it uneconomic to transport as a fuel or raw material. As a result, its exploitation is limited to combustion at or near the coal field, chiefly in power stations. A high proport¬ ion of the energy in the coal is utilised in evaporating the water. Up to 257 ₀ of the energy may be consumed in this way, largely because of the high latent heat of vaporisation of water.

Water is associated with brown coal both physically and chemically. The bulk of the water is physically adsorb¬ ed in pores in the structure of the coal. Some, however, is chemically combined in functional groups in coal. In both cases, the water can be driven off by raising the temper¬ ature since heating the coal causes the pore system of the coal to close up and thus expel the physically adsorbed water and also results in the chemical decomposition of the funct^- ional groups to carbon dioxide and water.

Although these effects begin to take place around 60 C they are undesirably slow at this temperature and temp¬ eratures considerably in excess of this are currently requir¬ ed which normally result in evaporation of the water and large energy requirements as previously discussed.

Another approach for coal drying disclosed in United States Patent specification 3,327,402 (Lamb) involves the use of an organic solvent miscible in water. This process slurries the coal in a low-boiling, water-miscible solvent such as methanol. After separation of the solid and liquid phases, the solvent is regenerated by distillation from the

water/solvent stream and by stripping the solid product. The effectiveness of this process is based upon the substan¬ tially lower heat of vaporisation of the organic solvent compared with water (e.g. _s methanol -262 cal/g; water -539 cal/g) . However, it has the disadvantage that all the solvent used is vaporised during organic solvent recovery and, there¬ fore, the quantity of solvent that can be used is limited if any significant energy saving is to be achieved.

U.S. Patent 4,014,104 (Murphy) discloses a method of drying particular coal using methanol or other low boiling point solvent. This method involves distillation to recover methanol from the methanol water mix. Methanol is not a particularly good choice of solvent and this process is also not economical in relation to energy use. United States Patent Specification 4,223,449

(Bodle) discloses a process for treating a hydrocarbonaceous solid containing water to remove at least a portion of the water from the solid which comprises:

(i) contacting the water containing solid with a liquid solvent relatively free of moisture at an elevated temp¬ erature to transfer at least a portion of the water in the solid into the solvent, said solvent being charact¬ erized by its ability to dissolve only relatively small amounts of water at low temperatures and its ability to dissolve substantially greater amounts of water at higher temperatures;

(ii) removing at least a portion of the solvent contain ing water from the resultant solids at an elevated temperature to provide a solvent stream containing water and a product solid stream having a diminished water content;

(iii) cooling the solvent containing water stream to a lower temperature to cause the water to separate from the solvent and to provide a liquid solvent stream relatively free of water and a water stream relatively free of solvent; and

(iv) passing at least a portion of the liquid solvent stream relatively free of moisture to contacting step

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(v) to provide at least a portion of the liquid solvent utilised therein.

The major drawback of this process is that the temp¬ erature required to obtain reasonable solubility of the water in the washing solvent is in the range 100 -300°C and, there¬ fore, to eliminate substantially the vaporisation of water, the coal/solvent contacting and separation procedures have to be carried out at the pressures of up to 40 bar.

Peat, which has a higher water content than brown coal, suffers from similar drying problems to an enhanced exten .

We have now discovered a process for the dewatering of solid materials, such as fuels and minerals which uses a solvent which has a lower critical point in its water solvent phase equilibrium and which has relatively low energy requirements. This process is applicable not only to low rank coals such as lignite, peat and brown coal but also to high rank coals and some minerals.

According to this invention the water and solvent are completely miscible at relatively low temperature, e.g., about ambient, but separate into two phases at higher temper¬ atures. This is to be contrasted with the more common type of equilibrium in which the water and solvent have a higher critical point, i.e., they are immiscible at low temperature but dissolve into a single phase at higher temperatures.

Thus according to the present invention there is provided a process for the removal of water from a solid material which process comprises:

(a) contacting a wet, solid material with an organic solvent, having a lower critical point in its water solvent phase equilibrium, at a temperature below its critical point, so that water from the solid material is taken up by the solvent,

(b) separating the solid phase from the liquid phase, (c) heating the liquid phase to a temperature above its critical point, so that the water and solvent separate into two liquid phases,

(d) separating the solvent and water phases, and

(e) recycling the solvent at a temperature below its critic¬ al point and optionally removing residual solvent and water from the solid material by heating the solid material within the range of 170°C to 200°C. Preferably the critical temperature Is below 100 C so that all unit operations can be carried out at atmospheric pressure.

Suitable solvents are those possessing both hydro- phobic and hydrophilic groups in the same molecule. Preferred solvents are the mono alkyl ethers of lower alkyl glycols, particularly ethylene and propylene. Specifically, these include 2-butoxy ethanol (critical point 49°C), iso-butoxy ethanol (critical point 24.5°C, 1-propoxy propane-2-ol (critical point 34 C) and 2-propoxy-propane-l-ol (critical point 40°C). The heat of vapourization of these solvents are also low so that the total energy required for the water removal operation is reduced.

The efficiency of the water extraction step may be increased by manipulating the phase equilibria by the addition of a third component to reduce the amount of water in the recovered solvent and thereby increase the amount of water removed.

Suitable third components include aliphatic and aromatic hydrocarbons such as kerosene. Preferably, a sur- factant such as polyethylene glycol can be used to enhance the effect of this third component.

A low concentration of an inorganic salt, e.g. sodium chloride may enhance the phase separation of water and the solvent. Contacting the fuel with the solvent in Stage (a) may be achieved by simple counter-current mixing.

The solid phase may be separated from the liquid phase in Stage (b) by screening, vacuum filtration or cent- rifugation. The product quality of brown coal fuel produced by this invention can be improved by further heat treatment to a temperature below 400 C in an inert atmosphere. The proper-

ties which can be improved in this way are strength, resist¬ ance to spontaneous combustion, resistance to moisture re absorption, and net specific energy. The degree of improve¬ ment can be controlled by the rate at which the fuel is heat- ed and the maximum temperature to which it is raised.

The invention is illustrated with reference to Fig. 1 which is a flow diagram of a process according to the present invention.

Run of the mine brown coal containing 65% by weight of water is fed by line 1 to a counter current washer A where contacting is effected at 20°C with recycled 2-butoxy-ethanol supplied through line 6. Coal and solvent are then separated in a solid/liquid separator B.

The treated coal which contains some adsorbed solvent is then passed through line 2 to a stripper C where it is stripped of remaining solvent and water within the temp¬ erature range of 170°C to 200°C. Dry coal product is removed by line 3.

Brown coal dried to 200°C will re absorb a large amount of water, is mechanically weak and is generally un¬ stable with respect to its resistance to spontaneous combust¬ ion. The coal is therefor passed for heat treatment to heat¬ er G where hot inert gases are used to directly heat the coal in a controlled manner, and at a specified rate and a speci- fied maximum temperature. The specific rates and tempera¬ tures (below 400°C) will depend on coal type but the cond¬ itions need to be sufficient to reduce the content of undesir¬ able oxygen containing functional groups (eg. carboxylic acid groups) and to collapse the pore structure of the coal. This results in an improvement of the above listed properties.

Vapours from the stripper are cooled to 120°C and passed by line 4 to a knockout drum D. Pure 2-butoxy ethanol bottoms from this drum are taken off by line 5, mixed with 2-butoxy ethanol coming from the phase separator E via line 10 and recycled to the mixer A through line 6. The tops from the knockout drum at the azeotropic composition are taken by line 8 and mixed with the effluent stream coming from the

solid/IIquid separator B by way of line 7 and the combined stream is then passed through line 9 into the phase separator E where the liquid separates into a water rich phase and a 2-butoxy ethanol rich phase at 75°C. The latter is removed by line 10, mixed with 2-but- oxy ethanol from the knockout drum D and recycled to the mixer A through line 6.

The water rich phase flows through line 11 to a water treatment plant F containing activated charcoal which removes 2-butoxy ethanol from the water. Pure water leaves the plant by line 12.

2-butoxy ethanol Is removed from the charcoal by means not shown and returned by line 13 to join the products in lines 7 and 8 and flow to the phase separator through line 9.

Mass balance data is given in the following Table.

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TABLE

Stream No

Stream ROM Coal 2-Bu0ET0H Dry Coal Recovered Pure liquid Feed Treated Coal Product Solvent 2-BuOETOH from

Vapour knock-out drum

Coal 1.269 1.269 1.269 0 0

2-BuOETOH 0 1.408 0 1.408 1.216

Water 2.356 0.495 0 0.495 0

Total liquid 2.356 1.903 0 1.903 1.216

Stream No. 8 10

Stream Solvent Feed Spent Azeotropic Phase 2-BuOETOH Rich

Contacting Solvent Vapours Separator Phase

Stage Feed

All flows are shown in mass units/hr.