GB2024250A - Treatment of aqueous slurries of particulate material - Google Patents

Abstract

Process for beneficiating and dewatering an aqueous slurry of particulate material e.g. coal. The slurry is first subjected to froth flotation and the resulting froth provides a concentrate. An agglomeration agent, preferably a liquid hydrocarbon is added to the concentrate to cause selective agglomeration of the particulate material in the concentrate. The concentrate may then be subjected to a shear regime, for example by means of a rotary impeller or agitator, before passing to dewatering equipment such as a screen, filter (drum, disc or belt) or centrifuge. Alternatively the concentrate with added agglomeration agent may be passed directly to the dewatering equipment.

Description

SPECIFICATION Beneficiation and dewatering of slurries Background of the Invention This invention relates to the beneficiation and dewatering of slurries. The invention is particularly applicabie to the recovery of coal fines from aqueous slurries but could also be used in the recovery of other particulate minerals.

Conventional technology has dictated that slurries of fine coal or other minerals are beneficiated by froth flotation and that the concentrate is dewatered by vacuum filters (drum, disc or belt) or to a lesser extent by centrifuges. In some cases a thickener is required before the concentrate is finally dewatered. Very large capital costs are associated with the installation of appropriate dewatering equipment and very often the resulting product has a high water content.

The present invention provides an improved froth flotation/dewatering process which enables an increased degree of benefication to be achieved in the dewatering step and/or enables the capital investment in dewatering equipment to be substantially reduced.

The invention applies to the froth flotation/dewatering process the technique of selective agglomeration.

This technique relies on the fact that certain minerals such as coal are hydrophobic or can be rendered hydrophobic. Consequently, particles of such minerals suspended in an aqueous slurry can be caused to agglomerate by the addition of an agglomeration agent, usually a hydrocarbon oil or other organic liquids, so that the particulate mineral will collect preferentially with the agglomeration agent leaving the non-hydrophobic materials in aqueous suspension. One application of this technique to the extraction of coal fines from aqueous slurries is describd in our copendng British Application No. 18551/78. However, the process of the present invention has wider application and can be used to economically treat larger quantities than would be possible with the process described in the earlier application.

Summary of the Invention According to the invention there is provided a process for beneficiating and dewatering an aqueous slurry of particulate material comprising the steps of subjecting the slurry to froth flotation; obtaining a concentrate from the froth resulting from the froth flotation; adding to the concentrate an agglomeration agent capable of causing selective agglomeration of the particulate material in the concentrate, and dewatering the concentrate.

Preferably, the concentrate is obtained by taking said froth and either causing or allowing it to break to produce a concentrate slurry and the agglomeration agent is added to the concentrate slurry.

Preferably, the agglomeration is a liquid hydrocarbon and is added to the concentrate slurry in an amount falling in the range 0.5% to 10% by weight of said particulate matter in the concentrate.

The concentrate with added hydrocarbon may be subjected to a shear regime prior to dewatering. In this case the shear regime is preferably defined by a Froude Number in the range 2 to 600 and the hydrocarbon is added in an amount in the range 5% to 10% by weight of said particulate matter in the concentrate. In this case too, the hydrocarbon preferably has a viscosity in the range 1 to 10 centipoise and the dewatering step may comprise applying the concentrate, after subjection to the shear regime, to a screen having apertures in the size range 0.15 to 1.00 mm.

Alternatively, the concentrate with added hydrocarbon may be passed to dewatering equipment without being subjected to a shear regime and in this case the amount of hydrocarbon added may be in the range of 0.5% to 5% by weight of particulate matter and the viscosity of the hydrocarbon may be in the range 1 to 3 centipoise.

Brief Description ofthe Drawings In order that the invention may be more fully explained one particular process involving application of a shear regime will be described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic flow sheet of the process; and Figures 2 to 4 are graphs illustrating the improved performance which can be achieved by the invention.

Description of the Preferred Embodiment The illustrated process is particularly suitable for recovery of coal fines of less than 0.5 mm particle size but it may also be applied to the recovery of similarly sized particles of other hydrophobic minerals.

In the illustrated process a slurry input is delivered via a line 10 to froth flotation cells 12. The slurry input may come from direct wet grinding, from wet screening to remove coarse particles for density suparations or from a transportation slurry pipeline. Before the slurry enters the flotation cells 12, reagents that are needed for the flotation to occur (collector and frother) are added through an input pipe 11. The froth flotation step is entirely conventional and will be well known to experts in the mineral recovery field. Further details may be obtained by reference to the text "Handbook of Mineral Dressing" byA.F. Taggart, John Wiley & Sons, 1954, Section 12.

In the froth flotation cells 12, air bubbles attach themselves to the hydrophobic coal particles which then report to the foam. The foam overflows as a concentrate product into the product line 14 and a tailing containing unwanted contaminants is withdrawn through line 13.

When the foam concentrate overflows into product line 14 it breaks to produce a concentrate slurry containing the coal fines in suspension. Line 14 may include a foam breaking device to accelerate the formation of such a slurry. The concentrate slurry is delivered by lines 4 into an agglomeration tank 16 in which it is subjected to a shear regime by rotation of an impeller 21.

The concentrate slurry being delivered to agglomeration tank 16 receives an addition of a hydrocarbon agglomeration agent. The hydrocarbon may be added through an inlet 15 but it could alternatively be injected at the exit of the siurry stream within tank 16. The choice of hydrocarbon additive depends on the requirement for the final agglomerated product. If a very strong agglomerate is required, then the hot addition of a heavy hydrocarbon such as tar or pitch is needed. For less stringent requirements of a cheap heavy fuel oil like 'furnace oil" may be used. Although these types of oils are poorly selective they are relatively cheap. In a situation where further beneficiation is possible and desired a lighter, more selective oil such as-industrial diesel fuel, distillate or kerosene may be used. These oils are more expensive but produce cleaner agglomerates.The properties of these hydrocarbons are given in Table 1.

Table 1 Properties of Hydrocarbons Type of Viscosity Specific Gravity Hydrocarbon (Centipoise) (at 15'CJ Kerosene 1.5 0.788 Distillate 2.53 0.829 Industrial Diesel Fuel 2.64 0.841 Furnace Oil 38.0 0.931 Tar =25 =1.1 Pitch =350 The level of hydrocarbon addition is preferably between 5 to 10% on a dry solids basis and the hydrocarbon is preferably emulsified prior to its injection into the slurry.

Impeller 21 is rotated so as to produce a shear regime defined by a Froude Number in the range 2 to 600.

After a suitable residence time within the agglomeration tank, preferably of the order of 5 min. to 30 min., agglomerates and water overflow from the tank in astream 17 onto a dewatering screen orsieve bend 18 having apertures in the range 0.15 to 1.00 mm depending on the material being treated. The agglomerates are captured by the screen and pass from the bottom of the screen at 19 to product handling whilst the water, together with any contaminants, passes through the screen in a stream 20 which may be directed either to a clarified water supply or to a water clarifier system.

An indication of the effectiveness which may be expected of the above process is given in the following working example: Working Example 1 A slurry was made by mixing 100 gm of bituminous coal (ash content 13.4% adb) with approximately 1200 ml of water. This slurry was placed in a Denver Laboratory flotation machine. Collector (diesel oil) and frother (MIBC) were added to the cell at the addition rates of 1 kg and 0.1 kg per tone of solids respectively.

After mixing for one minute, air was introduced into the cell. The froth, containing approximately 25% by weight solids, was collected for dewatering. After the foam had collapsed, the product slurry was placed in a stirred beaker with a shear regime corresponding to a Froude Number of 17.3. Hot "Bunker C" oil was added to the beaker, at an addition of 10% on a solids basis, to agglomerate the coal particles. Dewatering was carried out on a 0.5mm screen. Product yield was approximately 75% and the product ash was 7.2% (adb).

Product moisture was 27.8% but it has been observed that water drains freely from the product if allowed to stand for a shorttime. The product was in the form of small agglomerates which were able to be handled readily.

Table 2 contains drainage rates for typical agglomerates.

Table 2 Drainage for Agglomerates Drainage Time Product Moisture (minutes) (Wt%) 5 15.6 15 13.3 30 12.9 60 12.8 The process described in some detail above with reference to the drawings is advanced by way of example only and could be modified considerably. For example the dewatering screen 19 could be replaced by some other type of dewatering device, for example by a vacuum filter or centrifuge device.

Moreover, the invention extends to froth flotation processes in which the shear regime is omitted but an agglomeration agent is added to the froth concentrate prior to dewatering in order to improve the dewatering efficiency obtainable with conventional dewatering equipment such as vacuum filters. In such cases the agglomeration agent may be a hydrocarbon oil with a viscosity of between 1 and 3 centipoise and is added at the rate of 0.5 to 5% relative to the weight of solids. Sufficient turbulence is present in the pipes or launders taking the product slurry to the dewatering device to mix the hydrocarbon with the solid particles.

The general phenomenology associated with oil addition to filter feed can be qualitatively interpreted in terms of a flocculation process in which size enlargement occurs through groups of particles being held together with oil bridges at their points of contact. This proposed flocculation process is believed to lead to an increase in bed porosity and an effective increase in the particle size of the units around which liquid flow can occur. Both of the factors can be shown theoretically to lead to increased filtration rates and a reduction in the capillary forces causing liquid to be retained by the cake.

The "cleaning" process in which the ash content of the filter cake is reduced can be interpreted in terms of the relative affinities (wettabilities) of the oil phase to the coal material and mineral matter. Particles of mineral matter are generally hydrophilic as a result of the presence of ionisable groups on their surface or, in the case of clay materials, through the electric charge generation caused by isomorphous substitution within the crystal lattice. As a consequence, oil droplets do not adhere to clay particles in contrast to the strong adhesion experienced in the case of hydrophobic coal particles.Furthermore, since the mineral matter contained in coals is largely kaolinitic or montmorillonitic, the particle size of these components is often much less than that of the coal material and is capable of passing through the filter cloth and reporting to the filtrate. This concept is consistent with the increase, in ash content of the filtrate solids seen in the further Working Examples below.

Working Example 2 A sample of flotation concentrate from a coal preparation plant cleaning bituminous coal, was taken. This sample had a pulp density of 32% by weight. This material was filtered using a vacuum leaf filter with a cloth aperture of 0.4 mm at a suction of 68 kPa. Figures 2 and 3 show the improved filtration results that can be obtained by the use of small amounts of oil.

The results show that considerable decrease in fi Itelr cake moisture from 26.6% to 14.1 % can be obtained for an oil addition of 1% on a feed basis. Furthermore, this decrease in cake moisture is accompanied by an equally significant increase in filtration and cake pick-up rates. Figure 3 shows that a considerable "cleaning" of the filter cake was taking place with the result that a lowering of the filter cake ash from 18.0% to 10.9% vivas also achieved for an oil addition of 1%. This lowering of filter cake ash was accompanied by a corresponding increase in the ash content of the filtrate.

Working Example 3 Research into different flotation circuits (Firth, B.A., Swanson, A.R., and Nicol, S.K. Flotation Circuits for Poorly Floating Coals, Int. J. Mineral, Process, vol. (1979) pages 321 - 334) has shown that advantages can ibe obtained by floating coarse and fine particles separately. However, in the past the filtration of very fine material has been extremely difficult. The data in figure 4 shows that the addition of a small quantity of kerosene (1%) to a minus 76 F m feed can improve filtration dramatically. This oil addition enabled the cake moisture to be lowered from 40% down to 16% at 68 kPa suction of a 38 ffi m mesh filter cloth.

Working Example 4 Flotation concentrates from a washery preparing bituminous coal for coking purposes, was fed to a continuous pilot scale drum filter of approximately 1 m2 filtration area. The applied vacuum was 67kPa and the filter mesh was 0.4 mm. The feed had a pulp density of 25% and an ash content of 14%. Without oil the product moisture was 26% and the product rate was 1 kg/min. The ash of the filter cake and the solids in the filtrate were 12.6% and 16.1% respectively. The filter cake was not evenly spread over the drum indicating inefficient use of the filter area.

After the addition of 1% of kerosene the production of filter cake jumped to 22 kg/min and the cake was thick and even. The product moisture fell to 2221%. The ash of the filter cake was 11.3% whereas the ash of the filtrate solids was 18.5%.

It is to be understood that the invention is not limited to the specific process and conditions detailed herein and that many modifications and variations will fall within the scope of the appended claims.

Claims (15)

1. A process for beneficiating and dewatering an aqueous slurry of particulate material comprising the steps of subjecting the slurry to froth flotation; obtaining a concentrate from the froth resulting from the froth flotation; adding to the concentrate an agglomeration agent capable of causing selective agglomeration of the particulate material in the concentrate, and dewatering the concentrate.

2. A process as claimed in claim 1, wherein the concentrate is obtained by taking said froth and either causing or allowing it to break to produce a concentrate slurry and the agglomeration agent is added to the concentrate slurry.

3. A process as claimed in claim 1 or claim 2, wherein the agglomeration agent is a liquid hydrocarbon.

4. A process as claimed in claim 3, wherein the hydrocarbon is emulsified prior to its addition to the concentrate.

5. A process as claimed in claim 3 or claim 4, wherein the liquid hydrocarbon is added to the concentrate slurry in an amount falling in the range of 0.5% to 10% by weight of said particulate matter in the concentrate.

6. A process as claimed in any one of claims 3 to 5, wherein the concentrate with added hydrocarbon is subjected to a shear regime prior to dewatering.

7. A process as claimed in claim 6, wherein the shear regime is defined by a Froude Number in the range 2 to 600.

8. A process as claimed in claim 6 or claim 7, wherein the hydrocarbon is added in an amount in the range 5% to 10% by weight of said particulate material in the concentrate.

9. A process as claimed in any one of claims 6 to 8, wherein the hydrocarbon has a viscosity in the range 1 to 10 centipoise.

10. A process as claimed in any of claims 6 to 9, wherein the dewatering step comprises applying the concentrate, after subjection to the shear regime, to a screen having apertures in the size range 0.15 to 1.00 mm.

11. A process as claimed in any one of claims 3 to 5, wherein the concentrate with added hydrocarbon is passed to dewatering equipment without being subjected to a shear regime.

12. A process as claimed in claim 11, wherein the amount of hydrocarbon added is in the range 0.5% to 5% by weight of particulate matter in the concentrate.

13. A process as claimed in claim 11 or claim 12, wherein the viscosity of the hydrocarbon is in the range 1 to 3 centipoise.

14. A process for beneficiating and dewatering an aqueous slurry of particulate material substantially as hereinbefore described in any of the examples.

15. A process for beneficiating and dewatering an aqueous slurry of particulate material substantially as hereinbefore described with reference to the accompanying drawings.