WO2010032212A1

WO2010032212A1 - Treatment of hydrometallurgical process streams for removal of suspended fine particles

Info

Publication number: WO2010032212A1
Application number: PCT/IB2009/054093
Authority: WO
Inventors: Mark Lawrence Langton
Original assignee: Roymec Technologies (Proprietary) Limited
Priority date: 2008-09-18
Filing date: 2009-09-18
Publication date: 2010-03-25
Also published as: AP2011005676A0; AU2009101351A4

Abstract

This invention relates to the treatment of a hydrometallurgical stream, including the steps of: 1 ) adding an absorbent particulate clay, for example a montmorrillonite clay such as bentonite, to the stream to coagulate ultrafine particles (typically particles less than 10 microns in size) in the stream; 2) adding a settling aid such as a flocculant; and 3) separating ultrafine particles coagulated with the clay from the stream. The process is able to consistently, economically and reliably reduce the suspended solids in a hydrometallurgical stream to levels acceptable for effective and efficient operation of downstream processes, particularly processes that require a high throughput rate.

Description

TREATMENT OF HYDROMETALLURGICAL PROCESS STREAMS FOR REMOVAL OF SUSPENDED FINE PARTICLES

BACKGROUND TO THE INVENTION

In almost all hydrometallurgical plants, there are aqueous process streams that contain small amounts of solids. These streams typically originate:

® directly from acid or alkaline heap leaching of an ore,

* from a basic liquid/ solid separation step such as conventional, high rate or ultra high rate thickening, or β a filtration step (disc, drum or belt filters).

The solids contained in these streams are very often a cause of downstream processing problems. The solids are typically colloidal or at least less than 10 micron and frequently cannot be removed effectively by conventional separation processes. For example, one of the major purification and concentration processes in a hydrometallurgical plant is Solvent Extraction (SX). In an SX circuit, an organic phase (typically kerosene and an extractant) is contacted with an aqueous phase (acid or alkaline) and the target metal is extracted. Suspended solids in a leach solution fed to an SX circuit causes crud problems. Crud is an oily accumulation of these entrained solids that collect in the SX settlers and cause phase separation problems between the aqueous and organic phases, or simply settle out in the SX settlers causing residence time reduction (effects the efficiency of separation). Cleaning of these settlers is hazardous and time consuming, resulting in lost production due to extended downtime.

In another metal recovery process, the hydrometallurgical solution is treated with a chemical that results in the precipitation of the target metal. In this case the presence of the suspended solids (usually a waste material) in the supernatant results in the reduction in quality of the recovered product.

Another process adversely affected is Ion Exchange, where the solids cause blocking of the ion exchange resin bed, hindering transfer, and the flow through the system is reduced.

The suspended solids in these hydrometallurgical streams have up to this time been a serious problem to the industry. Numerous different filtration or separation processes have been developed or adapted to remove these solids. The typical processes tried are as follows: a Polishing filter presses, ® Polishing candle filters,

• Sand or multimedia pressure filters, ® Lamella (inclined plate) clarifiers,

® Membrane filtration,

• Floating media pressure filters (Washikimizu filter), and o Pinned bed clarifiers.

There has been considerable work put into using chemical additives, such as natural and synthetic coagulants and flocculants, to bind the ultrafine particle into larger agglomerations (called floccs). These flocculants assist in capturing the solids in a filter, or allowing the solids to settle in a gravity or high centrifugal force system (centrifuge).

None of these processes however have been able to consistently, economically and reliably reduce the suspended solids to levels acceptable for effective, efficient operation of downstream processes.

It is an object of this invention to provide a process that is able to consistently, economically and reliably reduce the suspended solids in a hydrometallurgical stream to levels acceptable for effective, efficient operation of downstream processes, particularly processes that require a high throughput rate.

SUMMARY OF THE INVENTION

This invention relates to the treatment of a hydrometallurgical stream, including the steps of:

1 ) adding an absorbent particulate clay, for example a montmorrillonite clay such as bentonite to the stream to coagulate ultrafine particles (typically particles less than 10 microns in size) in the stream;

2) adding a settling aid such as a flocculant; and

3) separating ultrafine particles coagulated with the clay from the stream.

A hydrometallurgical stream is an aqueous stream containing dissolved metals obtained from the recovery of metals from ores, concentrates, and recycled or residual materials.

Coagulation is a process whereby stable colloidal matter is destabilized by neutralizing the charge on the colloid, thereby allowing the colloids to aggregate, forming much larger particles.

A flocculant is (typically) a long chain polymer, either charged or neutral, that promotes aggregation of suspended particles in a liquid, forming much larger particles (floes). These larger floes improve the settling rate or filterability of small particles. A flocculant is frequently used in conjunction with a coagulant - a flocculant will seldom improve the settling rate of a colloidal suspension.

Bentonite is an absorbent aluminium phyllosilicate, generally impure clay. There are a few types of bentonites and their names depend on the dominant elements, such as K, Na, Ca, and Al. In the present invention, the preferred form of bentonite is in the sodium form. The bentonite may be added to the hydrometallurgical stream either in powder form or in a slurry form, preferably in slurry form, and typically at an addition rate of 10 - 500 mg/l, usually 10 - 60 mg/l, for example 10 to 50 mg/l.

The settling aid is preferably added in a two-step mixing process: a) a high energy mixing step where a portion of the settling aid is mixed into the PLS stream at a power input of 200-400, typically 250-350 Watts per m³ of tank volume; followed by b) a low energy mixing step where a portion of the settling aid is is mixed into the PLS stream at a power input of 20-60, typically 40-50 Watts per m³ of tank volume.

The stream may have a retention time of 2 to 20, typically 2 to 10 seconds in the high energy mixing step, and a retention time of 30 to 120, typically 40 to 60 seconds in the low energy mixing step.

The settling aid is preferably a flocculant, for example a non-ionic long chain polymer, for example a non-ionic polyacrylamide having a size of from 5 to 20, typically 10 to 12 Daltons, and may be added at an addition rate of 1 - 100 mg/l, usually 1 - 10 mg/l, preferably 1-5 mg/l.

The separation step 3) typically takes place in a separation device that has a high throughput rate of 5 to 100 m³/hr of solution per m²of device.

The separation step 3) may be conducted in:

® Gravity clarifier/s,

• Polishing filter press/es,

•> Polishing candle filter/s,

® Sand or multimedia pressure filter/s, β Lamella (inclined plate) clarifier/s,

<» Membrane filter/s,

® Floating media pressure filter/s (Washikimizu filter), or • Pinned bed clarifier/s.

For example, a pinned bed clarifier may have a throughput rate of 5 to 30 m³/hr of solution per m² of clarifier area, preferably 10 to 15 ntVhr of solution per m²of clarifier area.

Preferably, a portion of coagulated particles separated from the stream in the separation step 3) is re-circulated to at or before step 2) of the process.

Preferably, in the separation step 3), coagulated particles form a sludge bed and the stream is injected into the sludge bed.

The hydrometallurgical stream may be an alkaline stream having a pH of greater than 7, typically 10 to 11 or preferably an acid stream having a pH of less than 7, typically 2 or less.

In accordance with a preferred embodiment of the invention, the hydrometallurgical stream is an acid stream having a pH of 2 or less, typically a pregnant leach solution (PLS) from a leaching process.

After the separation step, the clarified stream may be subjected to a metal recovery or purification process such as a solvent extraction (SX), electro- winning or precipitation process.

BRIEF DESCRIPHON OF THE DRAWINGS

Figure 1 is a process flow diagram for the removal of ultrafine particles from a pregnant leach solution;

Figure 2 is a graph showing the results of TSS tests on a PBC feed and PBC outlet using bentonite as a coagulant in the process of the present invention; Figure 3 is a graph showing the results of turbidity tests on a PBC feed and PBC outlet using bentonite as a coagulant in the process of the present invention;

Figure 4 is a graph showing the results of TSS tests on the PBC feed and PBC outlet where bentonite is not used in the process; and

Figure 5 is a graph showing the results of turbidity tests on the PBC feed and PBC outlet where bentonite is not used in the process.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to the present invention, the inventor has, surprisingly, determined that it is possible to use a naturally occurring day type material referred to as bentonite to coagulate ultrafine particles in hydrometallurgical aqueous process streams that require high throughput rates. This is surprising because tests conducted with commercially available synthetic coagulants and flocculants used for treating hydrometallurgical process streams in the past failed to provide any significant reduction in total suspended solids (TSS) or turbidity.

TSS analysis is conventionally carried out by a filtration method. It involves shaking the sample to disperse the solids, then pouring the contents into a measuring cylinder and recording the volume. The sample is then filtered under vacuum filtration. The un-dissolved solids that were in suspension are now left behind on a pre-dried and pre-weighed filter paper. The measuring cylinder is washed using double deionised water and the contents added to the filter. The filter cake is also then washed to remove any residual solution. The filter paper and cake are then dried in a drying oven for 3 h and the mass of the filter cake recorded. The mass of the filter cake is divided by the volume (using appropriate units) to obtain a TSS value in mg/l. Turbidity is the measure of the relative sample clarity. The technique involves passing a light beam through a solution containing suspended solids and then measuring the amount of scattered light using a photocell. When a beam of light passes through a solution containing solids several things can happen: the beam can be transmitted through the solution or it can be scattered by the suspended solids. The colour of the solution can also affect the amount of light transmitted as, depending on the colour, it can absorb part of the light. The intensity of the scattered light depends on several factors: the wavelength of the light beam, the particle size, colour and shape.

With reference to Figure 1 , in accordance with an embodiment of the invention, raw pregnant leach solution (PLS) 10 from a zinc leaching process having a pH of 2 is fed to a holding tank 12 under pump manifold pressure. A PLS stream 14 is extracted from feed tank 12 and bentonite 16 added to the PLS stream 14. The bentonite may be added to the stream in a powder form or in a slurry form. If used in a slurry form, the slurry can be made up at any pumpable concentration of bentonite, typically less than 10%, for example from 2 to 8, typically 5% (weight/weight). The liquid for making up the slurry can be the aqueous solution itself, water or any other compatible aqueous stream on the plant. The addition rate of the bentonite depends on the suspended solids level in the stream, but would typically be less than 500 mg/l, usually 10 -60 mg/l, for example 10 to 50 mg/l. It should be appreciated that this amount could vary depending on the charge, size and chemical composition of the suspended solids in the PLS stream. It must be noted that there are a number of different forms of bentonite, and different types may be required for different applications. For a PLS stream, sodium bentonite is preferred.

After the addition of bentonite, the stream is passed through an in-line mixer 18 to thoroughly mix the Bentonite into the PLS and to ensure effective contact of the bentonite with the solids in solution. A settling aid (natural or synthetic flocculant) is added to increase the settling rate of the bentonite "flocc".

In accordance with the present invention, the flocculant is added in a two- step: 1 ) high energy and 2) low energy mixing process.

In the high-energy mixing process, the PLS stream 20 from the in-line mixer 18 is passed to a high energy mixing tank 22, where the a flocculant 24 (a non-ionic polyacrylamide having a size of from 10 to 20, typically 12 to 12 Daltons such as Floerger™ 920VHM) is mixed into the PLS stream containing bentonite at a power input of 200-400 Watts per m³, preferably 250-350 Watts per m³, typically 300 Watts per m³ of tank volume. The retention time of the PLS stream in the high energy mixing tank 22 may be 2 to 20, typically 2 to 10 seconds.

it is beneficial for a recycled sludge stream 26 to be added at or before the high energy mixing tank 22.

From the high energy mixing tank 22, the PLS stream 28 is passed to a low energy mixing tank 30 where a flocculant 32 (preferably the same non-ionic polyacrylamide having an atomic mass from 10 to 20, typically 12 to 12 Daltons such as Floerger™ 920VHM) is added at or before the low energy mixing tank. The flocculant is mixed into the PLS stream containing bentonite at a power input of 20-60 Watts per m³, preferably 30-50 Watts per m³, typically 45 Watts per m³ of tank volume. The retention time of the PLS stream in the low energy mixing tank 30 may be 30 to 120, typically 40 to 60 seconds.

The addition of the recycled sludge 26 at or before the high energy mixing tank 22 promotes an effective flocculation of the solids and together with the high and low mixing steps produces a bulky flocc of 1 - 10mm in size that is capable of forming an effective sludge bed described below, are fast settling and have sufficient settling velocity to be effective in ensuring separation of the flocc from a hydrometallurgical process stream that requires a high throughput in the separation step described below.

From the low energy mixing tank 30, the PLS stream 34 containing the bulky flocc is passed to a separation device 36 to separate the coagulated fine particles from the stream.

The separation device 36 is typically a clarifier, for example a pinned bed clarifier (PBC). In a preferred embodiment of the invention, the PBC 36 comprises a settling vessel with a top opening 38 and a cylindrical portion 40 which extends into and a cone portion 42 at the base thereof. A sludge zone 44 is provided within the settling vessel 36 at the cone portion 42. A clarifying zone 46 is provided above the sludge zone 44. The clarifying zone 46 is formed with polystyrene beads (which are 0.5 to 5mm, typically 2-3mm in diameter) which are held in place by a screen 48 which traps the polystyrene beads but through which a PLS solution may pass. A clarified supernatant solution zone 50 is provided above the clarifying zone 46. A feeder pipe 48, for feeding the PLS stream 34 into the PBC 36, has a feed opening 52 located within the sludge zone 44, i.e. within the sludge bed.

The PLS stream 34 flows into the PBC 36 through the feeder pipe 48, where the bulky flocc forms a stable bed of partially settled floccs into which the PLS stream 34 is injected through the feed opening 52. From the feed opening 52, PLS solution 54 passed through sludge in the sludge zone 44 where the bulky floccs are captured and settle at sufficient velocity to be effective in ensuring separation of the floccs. From the sludge zone 44, the PLS solution passes into the clarifying zone 46 where polystyrene beads trap solids that may have passed through the sludge zone 44, and through the screen 48 to provide the clarified PLS supernatant solution 54 which is drawn off as a clarified PLS stream 56. The PBC 36 has a high throughput rate of PLS solution of 5 to 30 m³/hr of solution per m² of clarifier area, typically 10 to 15 m³/hr of solution per m²of clarifier area. The clarified PLS stream 56 may be subjected to a metal recovery or purification process such as a solvent extraction (SX), electro-winning or precipitation process.

A portion of coagulated particles from the PBC in the form of sludge from the sludge zone 44 is recycled from the PBC 36, via line 26, before or at the high energy mixing tank 22 at a rate of 5-30%, typically 10-20% of the PLS feed 20 flow, to provide a total solids concentration in the PLS stream in the high energy mixing tank of 500 to 10 OOOppm, typically 1000 to 5000ppm. When necessary, excess sludge accumulating in the underflow cone 42 may be removed occasionally.

The benefits and effectiveness of the process of the present invention can be utilised on all types of liquid/ solid separation devices in the hydrometallurgical industry, in particular those that require a high throughput rate of from 5 up to 100 m³/hr solution per m² of clarifier area of the device.

Other suitable separation devices include:

Gravity clarifier/s,

Polishing filter press/es,

Polishing candle filter/s,

Sand or multimedia pressure filter/s,

Lamella (inclined plate) clarifier/s,

Membrane filter/s, or

Floating media pressure filter/s.

The invention will now be described with reference to the following non- limiting Examples:

Example 1

Stirred jar tests were conducted with commercially available synthetic coagulants and flocculants used for treating hydrometallurgical process streams to ascertain if any of these could be used effectively in the treatment of a PLS and provide reduction of total suspended solids (TSS) or turbidity.

In the tests, 0-100 ppm of coagulant was added to 500ml samples of PLS and stirred on a laboratory flocculator. Details of these flocculants are provided in Table 1 below. From the results of the tests, it is clear that the tests however failed to provide any significant reduction in turbidity. In accordance with the present invention, however, it has surprisingly been found that whereas conventional coagulants were not successful, the addition of bentonite causes the coalescence of ultra-fine particles and the turbidity of the feed was reduced from 28 NTU to a supernatant turbidity of 3 NTU.

Table 1

Example 2

In accordance with the invention and with reference to the process described in above with reference to Figure 1:

Bentonite was made up as a 2% slurry, and added to the feed 14 as a coagulant 16 at a rate of 25 mg/l (of feed). The feed flow rate was 2 m³/hr of zinc PLS in a 500mm diameter PBC pilot plant (i.e. a feed flow rate of 10 ntVhr per m² of the PBC).

Flocculant was added at a rate of 2mg/l as a 0.02% solution.

Sludge 44 was recycled via the line 26 at a rate of 10-20% of the feed 20 flow, with the target of increasing the solids in the feed (in tank 22) to 1000 - 5000ppm.

Figure 2 shows the results of TSS tests on the PBC feed and PBC outlet using bentonite as a coagulant in the process of the present invention.

Figure 3 shows the results of turbidity tests on the PBC feed and PBC outlet using bentonite as a coagulant in the process of the , present invention.

Comparative Example 3

The process used in Example 2 was carried out, except bentonite was not added. The feed flow rate was 2 m³/hr in a 500mm diameter PBC pilot plant. Flocculant was added at a rate of 2mg/l as a 0.02% solution.

Figure 4 shows the results of TSS tests on the PBC feed and PBC outlet where bentonite is not used in the process.

Figure 5 shows the results of turbidity tests on the PBC feed and PBC outlet where bentonite is not used in the process.

Claims

1. Treatment of a hydrometallurgical stream, including the steps of: a) adding an absorbent particulate clay to the stream to coagulate ultrafine particles in the stream; b) adding a settling aid; and c) separating ultrafine particles coagulated with the clay from the stream.

2. The treatment as claimed in claim 1 , wherein the clay is a montmorrillonite clay.

3. The treatment as claimed in claim 2, wherein the clay is bentonite.

4. The treatment as claimed in claim 3, wherein the bentonite is added added at an addition rate of 10 - 500 mg/l.

5. The treatment as claimed in claim 4, wherein the bentonite is added added at an addition rate of 10 - 60 mg/l.

6. The treatment as claimed in claim 5, wherein the bentonite is added added at an addition rate of 10 to 50 mg/l.

7. The treatment as claimed in any one of the preceding claims, wherein the settling aid is preferably added in a two-step mixing process: a) a high energy mixing step where a portion of the settling aid is mixed into the PLS stream at a power input of 200-400 Watts per m³ of tank volume; followed by b) a low energy mixing step where a portion of the settling aid is mixed into the PLS stream at a power input of 20-60 Watts per m³ of tank volume.

8. The treatment as claimed in claim 7, wherein the high energy mixing step is at a power input of 250-350 Watts per m³ of tank volume.

9. The treatment as claimed in claim 7 or 8, wherein the low energy mixing step is at a power input of 40-50 Watts per m³ of tank volume.

10. The treatment as claimed in any one of claims 7 to 9, wherein the stream has a retention time of 2 to 20 seconds in the high energy mixing step.

11. The treatment as claimed in claim 10, wherein the stream has a retention time of 2 to 10 seconds in the high energy mixing step.

12. The treatment as claimed in any one of claims 7 to 10, wherein the stream has a retention time of 30 to 120 seconds in the low energy mixing step.

13. The treatment as claimed in claim 12, wherein the stream has a retention time of 40 to 60 seconds in the low energy mixing step.

14. The treatment as claimed in any one of the preceding claims, wherein the settling aid is a flocculant.

15. The treatment as claimed in claim 14, wherein the flocculant is a non-ionic long chain polymer having a size from 5 to 20 Daltons.

16. The treatment as claimed in claim 15, wherein the flocculant is a non-ionic polyacrylamide having a size of from 10 to 12 Daltons.

17. The treatment as claimed in any one of the preceding claims, wherein the settling aid is added at an addition rate of 1 - 100 mg/l.

18. The treatment as claimed in claim 17, wherein the settling aid is added at an addition rate of, usually 1 - 10 mg/l.

19. The treatment as claimed in claim 18, wherein the settling aid is added at an addition rate of 1-5 mg/l.

20. The treatment as claimed in any one of the preceding claims, wherein the separation step 3) takes place in a separation device that has a high throughput rate of 5 to 100 m³/hr of solution per m² of device.

21. The treatment as claimed in claim 20, wherein the separation device is selected from:

® Gravity clarifier/s, a Polishing filter press/es,

• Polishing candle filter/s,

• Sand or multimedia pressure filter/s, s Lamella (inclined plate) clarifier/s, β Membrane filter/s, s Floating media pressure filter/s, or

22. The treatment as claimed in claim 21 , wherein the device is a pinned bed clarifier.

23. The treatment as claimed in claim 22, wherein the pinned bed clarifier has a throughput rate of 5 to 30 m³/hr of solution per m² of clarifier area.

24. The treatment as claimed in claim 23, wherein the pinned bed clarifier has a throughput rate of 10 to 15 nrVhr of solution per m² of clarifier area.

25. The treatment as claimed in any one of the preceding claims, wherein a portion of coagulated particles separated from the stream in the separation step 3) is re-circulated to at or before step 2) of the process.

26. The treatment as claimed in any one of the preceding claims wherein, in step 3), coagulated particles form a sludge bed.

27. The treatment as claimed in claim 26, wherein the stream is injected into the sludge bed.

28. The treatment as claimed in any one of the preceding claims, wherein the stream is an acid stream having a pH of less than 7.

29. The treatment as claimed in claim 28, wherein the stream has a pH of 2 or less.

30. The treatment as claimed in any one of the preceding claims, wherein the stream is a pregnant leach solution (PLS) from a leaching process.

31. The treatment as claimed in any one of the preceding claims wherein, after the separation step 3), the clarified stream is obtained.

32. The treatment as claimed in claim 31, wherein the clarified stream is subjected to a metal recovery or purification process.

33. The treatment as claimed in claim 32, wherein the clarified stream is subjected to a solvent extraction (SX), electro-winning or precipitation process.