WO1999064163A1

WO1999064163A1 - Materials separation

Info

Publication number: WO1999064163A1
Application number: PCT/GB1999/001785
Authority: WO
Inventors: Stephen Thomas Hall
Original assignee: The University Of Nottingham
Priority date: 1998-06-09
Filing date: 1999-06-07
Publication date: 1999-12-16
Also published as: AU4277399A; GB9909553D0

Abstract

A method of materials separation and particularly coal separation. The method comprising incorporating landfill-derived liquid such as landfill leachate in a separation process such as froth flotation. The incorporation of this liquid means that at least some of the conventional chemical additives can be reduced in volume or omitted altogether.

Description

MATERIALS SEPARATION

This invention relates to a method of materials separation and particularly but not exclusively mineral and coal separation; and also a method of treating landfill-derived liquids (leachate, gas condensate and processed fractions thereof).

Landfill leachates are highly polluted wastewaters resulting from the ingress of water to landfilled wastes and the degradation and leaching of materials from the wastes. They are highly variable in composition between sites and with time. They contain biodegradable compounds; inorganic salts; viruses such as salmonella and e-coli; and trace recalcitrant pollutants such as solvents, paints, oils etc. As a minimum they require treatment for the reduction of their BOD/COD (biological oxygen demand and chemical oxygen demand) contents prior to release to the environment. Various wastewater treatment technologies have been applied to landfill leachates, with varying degrees of success and associated levels of cost.

Landfill gas condensates arise whenever landfill gas is collected for flaring or power generation. LFG is usually saturated with water when it enters the collection pipework. The temperature at which it is formed is generally around 30-40°C. The gas emerges from the body of the fill, it is drawn along the collection pipelines, where it will cool and condense. Gas condensates offer a possibly concentrated and purified source of volatile fatty acids (VFAs) transferred from the body of the waste. Current U.K. practice is to allow the condensate to continuous discharge back to the landfill; however, it can be collected and treated.

Conventionally landfill sites are provided with a substantially impermeable lining or base. Wells are engineered into the sites in the form for example of concrete collars extending into the site, such that landfill gas and leachates can be obtained through the wells. Fine coal (typically -0.5 mm) can be efficiently recovered from unwanted shales by froth flotation. The shales are non-combustible and lead to a high ash content of fine coal-bearing materials if not removed. These ash-bearing minerals are rejected in the froth flotation process.

The coal flotation process involves the selective attachment of coal particles to air bubbles in an aerated system. The air bubbles, stabilised by the addition of a surfactant ("frother"), float to the top of the tank, form a froth and are removed as a fine coal concentrate. The attachment of the air bubbles to coal particle surfaces is assisted by the addition of a hydrocarbon "collector", which preferentially absorbs on the coal surface increasing the hydrophobic nature of the coal surface.

The ash minerals form a waste ("tailings") stream which is disposed of as a slurry to a lagoon or is filtered and dumped. Stabilisation by revegetation of such dumped materials is though difficult due to the lack of nutrients and soil structure. Coal flotation systems are costly to install and operate, and consequently fine coal is often simply disposed of untreated to a tailings lagoon.

The recovery and selective separation of many other minerals is achieved in a similar manner with a variety of collector chemicals being employed, in addition to a frother. Often further chemicals are introduced ("promoters"), which are necessary to achieve the desired separation. These promoter chemicals can be selected from a wide variety of chemical types.

Other materials, including recycled wastes (e.g. paper, polluted soils) can be treated by froth flotation processes in a similar manner.

Other surface chemistry separation processes can be used such as for example selective agglomeration. In this process chemical conditions are controlled so as to promote the agglomeration, i.e. bonding together of particles, of a particular species, i.e. the mineral, allowing their subsequent separation from the other materials present. Conventionally in view of the additives required, this process has not been commercially viable to date for coal separation.

According to the present invention there is provided a method of material separation, the method comprising incorporating landfill-derived liquid in a surface chemistry separation process.

The amount of frother, collector, promoter, or other chemical additive which would otherwise be required in the process except for the presence of the landfill-derived liquid, may be reduced. In some circumstances one or more of the frother, collector, promoter or other chemical additive may be dispensed with.

The landfill-derived liquid may be analysed prior to material separation. The amount of frother, collector, promoter or other chemical additive, and/or other parameters of the separation method, may be selected in the light of the analysis.

The surface chemistry separation process may comprise froth flotation or alternatively selective agglomeration.

The invention further provides a method of coal separation, the method being according to any of the preceding four paragraphs.

Another aspect of the invention provides a method of treating landfill- derived liquid, the method comprising incorporating said liquid in a mineral separation method according to any of the preceding five paragraphs.

The liquid may comprise landfill leachate.

An embodiment of the present invention will now be described by way of example only. Froth flotation of fine coal, i.e. with a particle size predominantly less than 0.5 mm, was carried out in a conventional manner employing a batch laboratory flotation system as follows. In a first sample a frother MIBC (methyl isobutyl carbinol) and a collector kerosene was added. The yield of coal obtained, i.e. that floating, was 36.196 by weight of the original feed. When this product was analysed it was found to have an ash content of 10.4%. The tailings ash content was 59.6%.

In a second sample the flotation was carried out in landfill leachate with no additional chemical additives. This produced a coal yield of 30.5% to the clean coal product; with an ash content of 11.096, and a concentration of ash in the tailings of 54.296. In a third sample using landfill leachate kerosene was added as a collector, with half the proportion of kerosene relative to the first sample being used. This provided a coal yield of 55.796; with an ash content of 12.296, and a concentration of ash in the tailings of 77.496.

This example illustrates the increased yield and also higher ash content in the tailings achieved by using leachate with a reduced proportion of kerosene as a collector, and no frother. Even without any additives, the leachate provides a reasonable yield.

The above described example is purely illustrative and various modifications may be made without departing from the scope of the invention. The leachate is one example of a landfill-derived liquid that can be beneficially employed in such a process, others include landfill gas condensate or processed fractions arising from the treatment of landfill leachate or landfill gas condensate. Treatments could include membrane filtrations or distillation. Different chemical additives could be used and/or in different proportions. Also, the landfill-derived liquid could be analysed prior to flotation, and the proportion and type of additives then chosen in response to this analysis.

Different surface chemistry separation processes could be used such as selective agglomeration. The method is also applicable to separation of other materials, including minerals other than coal, and also recycled wastes (e.g. paper, polluted soils).

There are thus provided methods which provide for a number of advantages. For instance, the landfill-derived liquid is a highly polluted wastewater which to date has served no useful purpose and has required relatively costly treatment processes. Accordingly, this material would be available at low cost, or the flotation process operator may in fact be paid to take this material. Following use of the landfill-derived liquid in mineral separation, there has been found a significant reduction in the BOD and other analytical values of the process water relative to the raw liquid.

The use of landfill-derived liquids in the separation processes permits the quantity of chemical additives such as frothers, collectors and promoters in many instances to be reduced, and in some instances such additives can be dispensed with. This can considerably reduce the cost of the separation methods. For instance, selective agglomeration for coal is generally not economically viable due to the quantities of oil required in the process.

In separation processes and especially froth flotation, conventionally collectors such as kerosene, paraffin or other hydrocarbons have been used which add to the cost of the separation. As these are long chain hydrocarbons, it has been generally found that relatively fresh landfill-derived liquid permits greater substitution of such materials. Frothers are conventionally in the form of chain alcohols such as pine oil. These again add to the cost of the separation methods and therefore their reduction or omission reduces the costs involved.

Difficulties can be encountered in disposing of fine solids from the coal industry. Where the coal is not separated out the waste tends to be structurally not particularly stable, and there is a potential fire risk. Where the coal has been removed due to lack of nutrients etc., revegetation can be difficult, and conventionally fertiliser such as ammonia fertiliser is used. In the absence of vegetation dumped material will dry out and cause a dust problem. When landfill leachate is used in the separation method this provides an intimately mixed nutrient supply to the tailings which can alleviate the requirement for fertiliser, or at least reduce the quantity required.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A method of materials separation, characterised in that the method comprises incorporating landfill-derived liquid in a surface chemistry separation process.

2. A method according to claim 1, characterised in that the amount of frother, collector, promoter, or other chemical additive which would otherwise be required in the process except for the presence of the landfill-derived liquid, is reduced.

3. A method according to claims 1 or 2, characterised in that one or more of the frother, collector, promoter or other chemical additive is dispensed with.

4. A method according to any of the preceding claims, characterised in that the landfill-derived liquid is analysed prior to material separation.

5. A method according to claim 4, characterised in that the amount of frother, collector, promoter or other chemical additive, and/or other parameters of the separation method, are selected in the light of the analysis.

6. A method according to any of the preceding claims, characterised in that the surface chemistry separation process comprises froth flotation.

7. A method according to any of claims 1 to 5, characterised in that the surface chemistry separation process comprises selective agglomeration.

8. A method of coal separation, characterised in that the method is according to any of the preceding claims.

9. A method of treating landfill-derived liquid, characterised in that the method comprises incorporating said liquid in a mineral separation method according to any of the preceding claims.

10. A method according to claim 9, characterised in that the liquid comprises landfill leachate.