AU2001293315A1

AU2001293315A1 - Slurry treatment

Info

Publication number: AU2001293315A1
Application number: AU2001293315A
Authority: AU
Inventors: Basil Kransdorff
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-08-24
Filing date: 2001-08-24
Publication date: 2002-03-04
Also published as: WO2002016262A3; WO2002016262A2

Description

SLURRY TREATMENT

BACKGROUND OF THE INVENTION

This invention relates generally to the treatment of slurry and to a method of treating a slurry and disposing of waste gypsum from an industrial process in an environmentally friendly manner. More particularly the invention is concerned with the treatment of slurries which contain colloidal clays which are in a stabilised form and which therefore stay suspended. It also has application where water recovery from waste produced in a mining process is important, and in situations in which organic flocculation techniques are used with high rate thickeners.

A slurry in a stabilised form can be troublesome and costly to treat. Charged mineral particles in the slurry effectively repel one another and prevent or inhibit gravitational separation and settlement. If the slurry is particularly stable, e.g. a situation which results in the treatment of kimberlite to extract diamonds, then the slurry may require the addition of large quantities of expensive flocculent or the addition of acid to reduce the pH of the slurry. Even this approach however may not always be successful and may be costly, not environmentally friendly and cause plant to corrode.

Gypsum on the other hand is a by-product in the production of certain fertilizers. Relatively small quantities of gypsum are used in the manufacture of ceiling boards, panels and the like, and other low volume applications, but large amounts of gypsum remain in stockpiles, presenting environmental problems.

A member of techniques are described in the prior art for treating mineral slimes and solutions containing suspended, finely-divided slimes, and reference is made, for example, to the disclosures in US3932275, US4051027, US4069152, US4363749, US4704989 and US4997573.

US447781 describes the use of hydratable calcium sulphate prepared from co- produced waste phospho-gypsum particularly for coalescing phosphatic clay slimes to form a strength-bearing matrix. Use is made of gypsum in the hemihydrate form which reacts with water in the slimes and solidifies. A binding or coalescing effect results which could possibly be described as a form of inorganic flocculation achieved by attaching the suspended particles to one another so that they are heavy enough to settle out. The hydratable material is added to the slimes in a quantity of about 1 to 9 times the weight % slimes solids in suspension, and thus a substantial amount of the material is required to treat a large slimes dam effectively.

SUMMARY OF THE INVENTION

The invention provides a method of treating a stabilised slurry which includes the step of adding a de-stabilising, non-hydratable agent to the slurry or to a material from which the slurry is produced.

The method may include the step of thereafter adding an organic flocculent to the slurry. This promotes rapid settlement in the slurry.

The method of the invention may be employed in diverse applications eg. to improve the settlement of particles in a slimes dam, to enhance water recovery from the slimes and to increase the stability of walls in a slimes dam.

The de-stabilising, non-hydratable agent is preferably phospho-gypsum Gypsum is produced as a by-product in the production of phosphoric acid eg. during fertilizer manufacture. In small quantities gypsum may be beneficial to the conditioning of soil. However phospho-gypsum stored in large quantities in stockpiles can release pollutants to the environment in relatively large quantities, for example due to the leaching action of rainwater or the like, and can have undesirable side effects for it increases the acidity level and sulphate content in the water table. Certain chemical processes, scrubbers and water purification processes produce large quantities of hydrated gypsum, often referred to as dihydrate phosphogypsum or desulphurized gypsum. The expression "hydrated gypsum" as used herein, without being limiting, includes dihydrate gypsum from all sources including in particular phospho-gypsum, desulphurized gypsum and produced from water purification processes.

A need therefore exists for utilising gypsum in a cost efficient and environmentally friendly manner. Consequently the use of gypsum as a de-stabilising agent, to be added to a stabilised slurry, holds two primary benefits. Firstly the gypsum destabilises the slurry and allows the water to be more readily clarified. The slurry is also rendered more susceptible for treatment using other techniques. Secondly, a process results wherein the gypsum is consumed in a cost-effective manner.

The de-stabilising agent may be added to the slurry at a rate which varies according to operating conditions, requirements, the solids content and the nature of the material to be de-stabilised. Typically the de-stabilising agent is added at a rate of from 0.25kg/m³ to 2.5kg/m³ slurry. At higher dosages the excess quantity of agent does not solubilise. In some situations it has been found to be beneficial to add the gypsum in smaller dosages accumulatively, as opposed to a single, relative large, batch.

The slurry, once destabilised by the addition of the de-stabilising agent, may be further treated in any appropriate way. For example a flocculent of any suitable kind may be added under conditions and at a rate which are determined by operating requirements. The destabilised slurry is more readily treated by lower dosages of the flocculent and rapidly separates into solid and liquid phases.

The method of the invention lends itself particularly to the treatment of kimberlite and similar slurries e.g. derived from, or formed by, alluvial ilmenite deposits or where a particular type of clay creates stable colloidal particles.

It has been shown that the method of the invention is reversible through a chelating effect and it is believed that the calcium ions, as ions, are probably the cause of the deactivation. At a concentration of about 2.5kg/m³ of water it appears that the gypsum solubilizes itself and forms the calcium ions in solution. It is likely that the ions then attach themselves to the clay particles or substitute themselves for sodium ions. This gives rise to a change in the charge distribution in the particles which keeps the particles repelled from one another and hence in suspension. As a consequence of the charge change the particles are no longer repelled and are able to settle.

The aforementioned effect is therefore different from that achieved in US4457781 wherein hydratable calcium sulphate is employed to achieve solidification of clay-like material. BRIEF DESCRIPTION OF THE DRAWING

The invention is further described by way of example with reference to the accompanying drawing which is a flow sheet illustrating the treatment of a kimberlite ore using the method of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

The accompanying drawing illustrates in block diagram form the use of the method of the invention in treating kimberlite ore.

Kimberlite ore 10 is subjected to a process 12, which is known in the art, for the recovery of diamonds. In the process 12 water 14 is added to the ore once it has been ground and, ultimately, a waste slurry 16 is produced. These aspects are generally known in the art and consequently are not further described herein.

The slurry 16 contains a stable suspension of kimberlite in water. The particles in suspension are charged and are not susceptible to being treated by the addition of flocculents. Even if the flocculation approach does work the quantity of flocculent which is required is excessively high and this technique is therefore expensive and can be difficult to control.

Many diamond mines are in arid regions and consequently the availability of water 14, for adding to the process 12, may therefore be restricted. In other words it is also highly desirable, in many cases, to be able to recover the water content of the slurry for reuse. If the slurry were to be pumped to a slimes dam and exposed to the atmosphere the water content would be evaporated and clearly would be unavailable for reuse. Also slimes dams are costly to maintain, and where clays stay in suspension, they reduce the amount of water that can be evaporated. This can cause enormous problems where mass balances of generated waste result in large and expensive storage facilities. Where such slimes dams leak, the addition of gypsum to the area around the leak will de-stabilise the suspended clay particles which will then settle and help seal those areas of the dam at which leaking takes place. This is a beneficial aspect which can be realised as a secondary effect when using the principles of the invention.

In the method of the invention gypsum 18 from any appropriate source, e.g. a stockpile created during the production of phosphoric acid, is transported to the treatment site and is added in a step 20 to the slurry, leading to the destabilisation of the slurry in a cost-effective way.

The gypsum may be added in any appropriate form, e.g. as a solid pulp, but preferably is injected as a liquid slurry directly into the slurry flow system. The addition of organic flocculents can further enhance the process by increasing the size ^" or weight of the attached particles in a network giving better water clarity.

The gypsum pulp can be first turned into a slurry by mixing with water on a 50:50 basis or any other appropriate ratio, using a slurry pump or high speed shear mixer and then injected at a controlled rate directly into a slurry. A dispersant may be used in order to improve the pumping of the gypsum slurry. Where settlement of the gypsum slurry is an issue, eg. during transport or in long pipe lines, the slurry may be gelled using bentonite at a dosage of about 7% to 8% by weight of bentonite to the amount of water used. When the gypsum is added as slurry it is important to ensure that proper flushing systems are used in the pipes to flush out settled gypsum which might block the pipes.

The gypsum slurry can be injected into the process water prior to settlement in a thickener or dam or any other holder of the slurry. This can also be done prior to the addition of a flocculent in a thickener. Another possibility is to add the gypsum as a pulp, eg. by conveyor belt or bucket, to an ore prior to grinding, or into a slurry circuit where there is good agitation. For example the gypsum 18, combined with the water 14 to form a pulp, may be added to the process 12 prior to the stage at which the ore 10 is ground.

The gypsum is added to the slurry at a rate which depends on the operating conditions. It has however been found through experimentation that in most instances the gypsum should be added at a rate of from 0.25kg gypsum/m³ slurry to 2.5kg gypsum/m³ slurry. These values are however given only as non-limiting examples for dosage rates in practice depend upon the actual operating conditions and the loading of solids in the slurry. If, as indicated, gypsum is added to an ore prior to grinding then this may be at a dosage rate of from 5kg to 10kg of gypsum per ton of ore treated, depending on the type and quantity of clay content of the ore.

Where a dam of stabilised colloidal particles needs to be treated, a gypsum slurry which is produced using dam or other water can either be sprayed over the dam or circulated back into the dam by pumping.

The gypsum/slurry mixture can be fed to a counter current flow thickener 22 of a type which is known in the art. Flocculent 24 of a suitable type, eg. a long chain polyacrylate, can be added to the mixture. The flocculent works more effectively and rapidly on the destabilised mixture, than on the stable mixture ie. without the gypsum, and results in efficient separation of the mixture into a liquid phase 26 and a solid phase 28.

The liquid phase is substantially clear water. The water may be recovered and, if necessary, be further treated in any appropriate way before being recycled to the process 14.

The solid phase 28 is a fluent waste mixture. This is collected at the bottom of the thickener. The waste may be pumped directly to a tailings dam or slimes dump 30 but, preferably, a viscosity modifier 32 is added to the waste to reduce the power consumption required for pumping to the slimes dam.

The invention holds particular promise in utilising gypsum, which is available in large quantities in stockpiles, for the treatment of kimberlite slurry. Not only is the gypsum consumed, and it is available as a raw material at a relatively low cost apart from transport, but the treatment process reduces the quantity, and hence cost, of the flocculent which may be required, and leads to the recovery of water from slimes dams which is an important aspect in all regions and which is beneficial when mining takes place in arid regions.

Another benefit of the invention is that de-stabilization of the slurry can be achieved at a substantially neutral pH, which limits adverse environmental aspects.

The invention is not limited to the treatment of kimberlite and other slurries, e.g. slurries produced in the processing of alluvial ilmenite deposits and in processes where a colloidal clay can be a problem can be similarly treated. The invention thus helps to reduce the quantity, and hence cost, of expensive organic flocculents when such flocculents are used and improves the rate and efficiency of settlement. This is of substantial importance in the design specification of modern high rate thickener processes like the ECAT process. Such design specifications may thus be reduced also saving money on the capital cost of equipment.

It has been observed that the de-stabilisation effect of the gypsum is not destroyed by shear forces. To the applicants' knowledge many organic flocculents which are used to clarify water or thicken slurries are adversely affected by shear forces. The feature that gypsum's effects are not materially affected by shear forces means that the gypsum slurry can be pumped to dams where the de-stabilisation will still be effective for settlement. Such treated recovered water can be re-circulated in circuits without materially affecting the ability of the gypsum to de-stabilise slurry. It also means that gypsum pulp or slurry can be added into an ore-grinding process without adverse results. Further, as is shown by dotted lines in the drawing, water recovered from the slimes dam 30 can be returned to the process 12 to assist in de-activating stabilised ore or particles which otherwise are produced in the process.

An important potential application of the invention is to de-stabilise a slurry which is passed to a rake thickener or settlement pond. No flocculent is added and the solids are allowed to settle. Clarified water is removed from the slurry. The de-stabilised pulp is pumped to a settlement dam from which additional water is recovered. Depending on the volume of water requiring treatment and the size of the thickener or settlement pond use may be made of a batch process or a continuous process. If the process is to be speeded because of a high slimes volume then organic flocculation may be used and more slimes can then be treated using the available thickening equipment because of the increased settlement rate.

In other words a saving is achieved by not having to use flocculents if the equipment is capable of producing enough settled material related to the volume of slurry. If flocculation is required then the balance of the water can be recovered from the slimes dam because even if the flocculent is destroyed the de-stabilisation effect survives the pumping and the particles can settle in a relatively short time.

The process of de-stabilisation can be reversed by the use of calcium chelating agents such as Calgon and EDTA. This can be important where a clarification of suspended particles needs to be removed first and then returned to its original stable state. It could also be important for applications where viscosity is important. Normally these colloidal suspensions have a low viscosity and are therefore ideal for pumping. Thus if it is required to reduce the cost of pumping clarified sludges, the process can be reversed to obtain lower viscosities. As is indicated by dotted lines gypsum 18A can be added to the slurry before it enters the slimes dam 30 to ensure settlement if the process has been reversed with a chelating agent for pumping purposes.

If the process of de-stabilisation is reversed a significant lubricating effect results which could be beneficial in processes such as brick manufacture, for the extrusion of clay, from which bricks are cut before baking, is greatly facilitated.

The material which is pumped to the slimes dam 30 contains particles of a particular clay with a size grading normally of about 15 microns or smaller, and thus represents classified product which, under other circumstances, would be difficult and costly to separate eg. by sieving. The ability to settle this material without organic contaminants has the possibility to create a unique source of well classified, clean and homogeneous clay material in an inexpensive way. Applications of the clay material include ceramic, tile and brick manufacture. The material also exhibits a unique property of very high absorption of water which opens up other applications for its use eg. in tile adhesives and cat litter pellets.

Claims

1. A method of treating a stabilised slurry which includes the step of adding a destabilising, non-hydratable agent to the slurry.

2. A method according to claim 1 which is employed to improve the settlement of particles in a slimes dam to enhance water recovery from the slimes.

3. A method according to claim 1 or 2 wherein the de-stabilising agent is hydrated gypsum manufactured from a chemical process.

4. A method according to any one of claims 1 or 3 wherein the de-stabilising agent is selected from phospho-gypsum and desulphirized gypsum.

5. A method according to any one of claims 1 to 4 wherein the de-stabilising agent is added at a rate of from 0.25 kg/m³ slurry to 2.5 kg/m³ slurry.

6. A method according to any one of claims 1 to 5 wherein the slurry is selected from the following: a kimberlite slurry; a slurry formed from or by an alluvial ilmenite deposit; and a slurry which contains stable colloidal clays in suspension.

7. A method according to any one of claims 1 to 6 wherein the de-stabilising agent is added to the slurry using a technique selected from the following:

(a) adding the agent in the form of a pulp to the slurry which is agitated;

(b) adding the agent to an ore prior to grinding the ore to produce the slurry, and

(c) turning the agent into slurry form and then injecting the agent in slurry form into the slurry which is to be treated.

8. A method according to claim 7 wherein step (c) takes place before settlement of the slurry which is to be treated.

9. A method according to any one of claims 1 to 8 which includes the step of adding a viscosity modifier to the de-stabilising agent.

10. A method according to any one of claims 1 to 9 which includes the step of adding an organic flocculent to the slurry.

11. A method according to claim 10 wherein the flocculent is added after the addition of the de-stabilising agent.

12. A method according to claim 1 which is followed by the steps of allowing solids in the de-stabilised slurry to settle, to form a pulp, removing water from the slurry, and transferring the pulp to a dam where additional water is recovered from the pulp.

13. A method of producing particulate material which includes the steps of treating a stabilised slurry using the method of any one of claims 1 to 11 , separating the slurry into a liquid phase and a solid phase, and drying the solid phase to produce solid particulate material.

14. A method of recovering clear water from a stabilised slimes dam which includes the steps of treating the slimes using the method of any one of claims 1 to 11 thereby to separate the slimes into water and a solid phase, and recovering the water.

15. A method of de-activating stabilised ores which includes the step of returning water, recovered in accordance with claim 12 from a stabilised slimes dam, to an ore-treatment process from which the said slimes are produced.

16. A method of sealing a leaking dam containing a slurry of highly stabilised colloidal suspended clay particles, which includes the steps of treating the slurry in accordance with the method of any one of claims 1 to 10, and allowing particles, which are thereby separated from the slurry, to settle and help seal areas of the dam at which leaking occurs.

17. Use of the solid particulate material produced by the method of claim 13.

18. A method of disposing of hydrated gypsum which includes the step of using the gypsum to treat a stabilised slurry in accordance with the method of any one of claims 1 to 12.