WO2007006058A1

WO2007006058A1 - Process for the manufacture of sodium sulphide compounds

Info

Publication number: WO2007006058A1
Application number: PCT/ZA2006/000086
Authority: WO
Inventors: Gary V. Rorke; Ritva M. Muhlbauer; Elmar L. Muller
Original assignee: Bhp Billiton Sa Limited
Priority date: 2005-07-06
Filing date: 2006-07-06
Publication date: 2007-01-11
Also published as: ZA200704164B

Abstract

Sulphate contaminated water is subjected to sulphate reduction and then to ion separation to produce a dischargeable water product and a NaSH product of commercial value.

Description

PROCESS FOR THE MANUFACTURE OF SODIUM SULPHIDE COMPOUNDS

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the treatment of sulphate contaminated water and to the simultaneous manufacture of sodium sulphide compounds therefrom.

[0002] Sulphate contaminated water most often arises as a result of mining activity and is known as acid mine drainage (AMD). AMD is a global environmental issue for both closed and operational mines. In general acid mine drainage occurs as a result of mining activity where sulphide minerals have been exposed to oxygen and water. This oxidation process is accelerated by the activity of iron and sulphide oxidising microorganisms. These waters are generally acidic and highly enriched with sulphate and a range of heavy metals. Ground water seepage or annual precipitation either flows through or fills the mine workings to the point of decant, at which stage the metals, any acid formed and sulphate ions are washed into the local drainage system.

[0003] Current legislation worldwide places a limit of about 400-500mg/L sulphate on groundwater and 2000mg/L sulphate on industrial effluents. Most state and federal laws in the United States as recommended by the USEPA prescribe a discharge limit for TDS ranging from 250 to 2000mg/L, with a drinking guideline of 250mg/L, which would imply that the maximum allowable sulphate concentration in water would be less than 250mg/L However, regulatory agencies are becoming increasingly concerned over elevated sulphate concentrations in effluents and the impact that high salinity discharges have on the environment. This concern is likely to result in the application of more stringent sulphate regulations globally, which will require the treatment of sulphate. In the South African context, discharge limits for sulphate are much lower at about 200mg/L and cannot generally be achieved through neutralisation alone.

[0004] Several technologies are available for the reduction of sulphate levels in

AMD streams.

Chemical Precipitation Technologies

Limestone Treatment

[0005] AMD is treated either by adding limestone to stirred tanks or by passing AMD through drains filled with limestone rocks. This neutralises the acidity of the solution and precipitates some heavy metal ions such as ferric from solution. This method is not effective for an AMD stream containing significant levels of sulphate, although it is effective in reducing acidity.

Limestone/Lime Treatment (1)

[0006] AMD is treated by neutralising with limestone. The pH is further raised by the addition of lime. This removes additional sulphate and precipitates additional heavy metal ions which are not precipitated by the limestone alone. The solution pH is then re-adjusted to near neutral by sparging carbon dioxide through the solution causing calcium carbonate to precipitate. Again, this technology is not effective for the reduction of high sulphate concentrations in mine water to acceptable discharge levels although it is good for reducing any acidity, and sufficiently reducing calcium sulphate concentrations to produce non-scaling process water. vmi r ^■

[0007] These processes take the limestone/lime treatment further. They operate in a similar way as the limestone/lime treatment but additives such as aluminium hydroxide are also added. The aluminium is then re-precipitated as aluminium sulphate further reducing sulphate levels in the final plant solution.

[0008] A similar process using barium sulphate precipitation has also been proposed ⁽³⁾.

[0009] These processes cannot reduce the sodium levels in solution and thus for a high sodium sulphate AMD cannot produce water with acceptable quality due to the presence of residual sodium.

Hvdrothermal Type Processes

[0010] In the hydrothermal type processes the AMD is usually pre-treated with limestone or lime and the resulting precipitates are removed. Additional calcium sulphate is then removed, as anhydrite, by heating the solution under pressure. The hydrothermal type processes rely on the decreased solubility of calcium sulphate at temperature and are thus ineffective for predominantly sodium sulphate AMD.

Ionic Separation Technologies

Reverse Osmosis and Nano-Filtration ⁽³⁾

[0011] The AMD is pre-treated with limestone/lime to remove metals that result in fouling of the reverse osmosis (RO) membranes. The resultant precipitates are removed. The solution is then filtered to remove fine particulates. The clarified solution is passed through a RO or nano-filtration step where up to 70% of the solution that passes through the membrane is recovered as a clean water permeate. The majority of the salts remain behind forming a concentrated salt solution called the brine. This solution is supersaturated with calcium sulphate and various methods are employed to prevent the precipitation of calcium sulphate in the RO or nano-filtration step. The final process products are purified water, metal hydroxide and gypsum sludge as well as residual brine.

[0012] The clean water permeate from a reverse osmosis process will be well within acceptable limits for discharge to the environment or drinking guidelines. Sodium sulphate has an extremely high saturation level compared to calcium sulphate and

the subsequent RO stage efficiencies are reduced. The result is a lower overall water recovery (<90%), and consequently a significantly higher brine production. The disposal of the brine in sufficiently lined sites is a high cost.

Capacitance ⁽⁴⁾

[0013] This technology operates on the principle of capacitive or electrostatic deionization to remove dissolved ions from solution and has been shown to remove sulphate although high salt concentrations are more problematic to desalinate. Once units are loaded to capacity with salts, the ions are flushed thereby forming a brine.

[0014] This technology which has not yet been commercially implemented, is likely to be employed in a similar fashion as reverse osmosis and will suffer the same inefficiency with sodium sulphate waters. Ion Exchange ^(3|5)

[0015] An ion exchange resin could be used to remove ions from solution and concentrate them in a smaller stream, such as in the GYPSIX process ⁽³⁾. This technology is likely to be employed in a similar fashion as reverse osmosis and will suffer the same inefficiency with sodium sulphate waters.

Biological Sulphate Reduction Technologies

Biological Sulphate Reduction (Paoues ⁽⁶⁾, Biosure ⁽⁷⁾, lmpi ⁽⁸⁾)

[0016] These processes may have additional limestone pre-treatment if required. Common to all is an anaerobic biological sulphate reduction stage. The AMD is fed along with an organic carbon source (alcohol, wood chips, sewerage etc) or a mixture of organic carbon and hydrogen to an anaerobic biological reactor. The sulphate in solution is reduced to hydrogen sulphide using the organic carbon or hydrogen as an electron donor. Some of the hydride sulphide will be used to precipitate select metals ions in AMD as sulphides for removal. Most of the hydrogen sulphide is oxidised to elemental sulphur and removed, as in the Paques process, or precipitated with iron.

[0017] These processes cannot reduce the sodium levels in solution and thus for a high sodium sulphate AMD they cannot produce water with acceptable quality because of the residual sodium concentration. Wetlands

[0018] Anaerobic conditions needed for sulphate reduction are difficult to achieve in wetlands. Also, for a high sodium sulphate AMD these processes cannot produce water with acceptable quality because of the residual sodium concentration.

[0019] Generally the aforementioned technologies are effective for treating water containing solubilised calcium sulphate but are not as effective for treating water containing sodium sulphate.

SUMMARY OF INVENTION

[0020] The invention is concerned with a process which aims to address, at least partly, the aforementioned problems while at the same time generating a brine stream that has commercial value.

[0021] The invention provides a process for the manufacture of a sodium sulphide compound from a sulphate-containing aqueous solution which includes the steps of subjecting the solution to a sulphate reduction stage and then to an ion separation stage thereby to produce a concentrated sodium sulphide solution.

[0022] The aqueous solution may be contaminated with the sulphate i.e. be acid mine drainage.

[0023] The ion separation stage may be implemented using a technique selected

from reverse osmosis, ion exchange and electrical deionization, or any combination thereof. [0024] Further concentration of the brine may be effected through evaporative processes. Production of a solid product can be achieved by evaporative

crystallisation.

BRIEF DESCRIPTION OF THE DRAWING

[0025] The invention is further described by way of example with reference to the accompanying drawing which is a flow chart of one way of implementing the process of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0026] Ion separation technologies are capable of producing water from sodium sulphate AMD that is acceptable for disposal. However these technologies produce large volumes of brine and the cost of disposal thereof can be substantial, particularly since AMD seepage can extend for an indefinite period of time.

[0027] The invention aims to reduce the amount of brine that has to be disposed of by generating a brine stream that has commercial value. In order to achieve this the sodium sulphate is converted to sodium hydrogen sulphide (NaSH) which is a byproduct of commercial value.

[0028] Sodium hydrogen sulphide is used, for example, in wood pulping, copper mineral flotation, the chemical and dye industries, leather processing and so on.

[0029] The accompanying flow sheet illustrates how acid mine drainage 10, which is rich in sodium and sulphate, can be treated in accordance with the principles of the

invention. [0030] In a first step 12 the AMD is sent to a pre-treatment tank where an alkali in the form of lime or limestone 14 is added, if required. This also precipitates some metal hydroxides. A solution 16 which contains soluble hydrogen sulphide and which is derived from a downstream sulphur-reducing step (described hereinafter) is circulated to the stage 12 to precipitate heavy metals. This is in accordance with the following reactions:

M^a+ + a(0H^") → M(OH)₃ Reaction 1

M^2b+ + b(HS^") — MS_b + bH⁺ Reaction 2

[0031] Slurry from the pre-treatment tank is subjected to a liquid/solid separation step 18 in which a combination of thickening and filtration techniques is used to dewater the metal sulphide and hydroxide precipitates 20. Dewatering agents 22 are added to the separator as may be required. The dewatered precipitates 20 can be sent to a disposal facility.

[0032] The solution from the separator reports to an anaerobic biological sulphate reducing reactor 26 to which a suitable organic substrate, or a mix of an organic substrate and hydrogen (28), is supplied. The sulphate is reduced to hydrogen sulphide by microorganisms as indicated in the following reactions: H⁺ + SO₄ ^"2 → HS^" + H₂O Reaction 3

4C₃H₇OH + 9SO₄ ^"2 + 9H⁺ → 12CO₂+ 9HS^" + 16H₂O Reaction 4

[0033] As previously indicated some of the hydrogen sulphide-rich product stream

16 is recycled to the sulphide precipitation tank 12, if required. Most of the product stream reports to a reverse osmosis (RO) preconditioning tank 30 to which lime 32 is added to raise the pH and thereby remove heavy metals, if required, or to keep the hydrogen sulphide dissociated and solubilised as HS^". [0034] Slurry from the RO preconditioning tank 30 reports to a liquid/solid separator

34 in which a combination of thickening and filtration techniques is used to dewater any gypsum and hydroxide precipitate 36. Dewatering agents 38 can be added to the pre-treatment tank 34 as may be required.

[0035] Solution from the separation step 34 is treated with acid as may be necessary in a pH control step 40 and an antiscalant 42 is added to the solution where required. This is followed by a fine polishing or nano-filtration step 44 to remove particulates prior to treatment in a multistage RO stage 46 .

[0036] Slurry from the stage 46 goes to a liquid/solid separator 54 in which a combination of thickening and filtration techniques is used to dewater any gypsum precipitate 56. Dewatering agents are added, as may be required, at this stage.

[0037] A residual brine solution 58, which contains significant concentrations of sodium and sulphide ions, may be further concentrated by evaporation or crystallisation 60 to produce a final product 62.

[0038] The aforementioned process offers the following benefits:

• prevention of the release of water of unacceptable quality into the environment;

• production of a dischargeable or potentially potable water product;

• minimisation or even elimination of the amount of hazardous sites for brine disposal;

• production of a sodium sulphide or sodium hydrogen sulphide saleable product to offset operating cost; and • possible production of a saleable potable water product to offset operating cost.

[0039] The aforementioned process can be varied in a number of ways:

Stripping of H^S and use of residual sodium

[004O]A possible process route is that the hydrogen sulphide is stripped out as a gas following the sulphate reduction stage. The residual sodium ion solution is purified and concentrated using any suitable combination of ion separation technologies. The H2S gas is then sparged into the concentrated sodium ion solution to form a NaSH product.

Stripping of HoS and use of another sodium source

[0041]A possible process route is that the hydrogen sulphide is stripped out as a gas following the sulphate reduction stage. The H₂S gas is then sparged into a solution containing concentrated sodium ions to form a NaSH product.

Alternate ion separation technology

[0042]The process flow diagram described essentially consists of a sulphate reduction stage followed by an ion separation stage. The example uses reverse osmosis. Other technologies, e.g. ion exchange and capacitance, or a combination thereof, could be employed in place of, or in addition to, reverse osmosis depending on the types of other impurities that are present in the AMD or that are created by the type of organic supplied to the sulphate reduction stage. Use of hvdrothermal anhydrite precipitation

[0043]Hydrothermal anhydrite precipitation could be incorporated to reduce unwanted calcium sulphate.

[0044] The described embodiment produces a concentrated sodium hydrogen sulphide solution. As indicated with reference to step 60 it is possible to create a crystalline product. Another possibility is to alter the pH during the control step 40 to create a sodium sulphide product.

References

1. Geldenhuys, AJ, Maree.JP, de Beer,M and Hlabela.P (2001 ) An integrated limestone/lime process for partial sulphate removal. Conference on Environmentally Responsible Mining in South Africa, Sept, 2001 CSIR, Pretoria, South Africa.

2. SAVMIN Technology Report, Coaltech 2020.

3. R J Bowell (2000) Sulphate and salt minerals: the problem of treating mine waste, Mining Environmental Management, May 2000.

4. Key Structure Holdings, (Pty) Ltd a CSIR licensee. 5. Feng, D, Aldrich.C and Tan,H (2000) Treatment of acid mine water by use of heavy metal precipitation and ion exchange. Minerals Engineering vol 13(6) pp 623-642. 6. R.J. M. van Lier, C.J.N. Buisman and N. L. Piret Thiopaq® Technology:

Versatilhigh-rate biotechnology for the mining and metallurgical industries. 7. Helene Ie Roux (2005) Goldmine embraces biotech water treatment, Mining

Weekly, Feb 25 -March 3,2005. 8. W Pulles, P Rose, L Coetser and R Heath (2003) Development of Integrated

Passive Water Treatment Systems for the Treatment of Mine Waters, Sixth

International Conference ACID ROCK DRAINAGE "Application and Sustainability of Technologies". 14-17 July 2003 Cairns, Queensland,

Australia

Claims

1. A process for the manufacture of a sodium sulphide compound from a sulphate-containing aqueous solution which includes the steps of subjecting the solution to a sulphate reduction stage and then to an ion separation stage thereby to produce a concentrated sodium sulphide solution.

2. A process according to claim 1 wherein the aqueous solution is acid mine drainage.

3. A process according to claim 1 or 2 wherein the ion separation stage is implemented using a technique selected from reverse osmosis, ion exchange and electrical deionization.

4. A process according to any one of claims 1 to 3 wherein the sodium sulphide solution is concentrated by an evaporative process.