WO2005100243A1

WO2005100243A1 - A process for the removal of thiocyanate from effluent

Info

Publication number: WO2005100243A1
Application number: PCT/AU2005/000543
Authority: WO
Inventors: Allan Brown
Original assignee: SUURI KULTA AB
Current assignee: SUURI KULTA AB
Priority date: 2004-04-15
Filing date: 2005-04-15
Publication date: 2005-10-27
Anticipated expiration: 2006-10-15

Abstract

A process for treating effluent containing thiocyanate so as to reduce concentration of the thiocyanate within the effluent, including the steps of: acidifying the effluent to a pH less than 3.0; placing the acidified effluent in the presence of ferric iron; adsorbing and/or absorbing the thiocyanate with activated carbon. The thiocyanate can be converted to tri-thiocyanate in the presence of the ferric iron. The tri-thiocyanate then being adsorbed and/or absorbed by the carbon. The process allows for regeneration of the carbon at raised temperatures.

Description

A PROCESS FOR THE REMOVAL OF THIOCYANATE FROM EFFLUENT FIELD OF THE INVENTION The invention relates to the removal of thiocyanate from effluent, such as that resulting from gold production using cyanide, for instance gold mine tailing slurries, to reduce concentrations of thiocyanate to environmentally acceptable levels. While the invention is particularly applicable to treatment of slurry tailings remaining after precious metal ore extraction, the instant process is equally suitable for treating thiocyanate-containing effluents from other sources including, but without limitation, effluent wash solutions from heap leaching, effluents from electroplating processes and effluents from thiocyanate producing processes. BACKGROUND OF THE INVENTION Cyanide is the most effective lixivant for gold and in spite of its toxicity to animal and aquatic life it is probably the most environmentally acceptable. Gold dissolves in cyanide according to the equation Au+2CN^"→Au(CN)2^" Stoichiometry indicates that 1kg of gold requires 0.497kg of cyanide to dissolve it. Industrial practice shows that the cyanide addition to effectively dissolve gold is 100-500 times the theoretical requirement i.e. a simple 5 gpt non refractory gold ore will often require a cyanide addition of 0.5kg/tonne to achieve a satisfactory gold recovery. Refractory gold ores can be even worse e.g. bacterial oxidation product can assay approximately 125 gpt Au which in theory would only require 62 grams per tonne of sodium cyanide for complete dissolution. The actual requirement can range from 15-30 kgl/tonne or 250-500 times the theoretical requirement. There are a number of reasons for this. a) Cyanide is relatively cheap, USD1.0 per kg compared to gold USD10-12,000.00 per kg so the incentive is to optimize gold recovery not save cyanide. b) Whilst the Au(CN)2^" complex is reasonably stable, the presence of excess cyanide in leach liquor will assist the stability of the complex and thus gold recovery. c) Cyanide reacts with a large number of elements and compounds, this is the major reason for the high cyanide consumption with most ores. Typical reactions would include: CN+S -> SCN xCN+Cu —> Cu(CN)x x 2-4 4CN+Ni → Ni(CN)4 6CN+C0 -» Co(CN)6 4CN+Zn —» Zn(CN)4 6CN+Fe -> Fe(CN)6 The amount of cyanide consumed by cyanicides will be dependent on the quantity of cyanicides present and this can have a major effect on project economics. The reaction of cyanide with sulphur and polysulphides in some products to form thiocyanates can also consume very large amounts of cyanide. NaCN+S → NaSCN Stoichometry shows that 1 kg of reactive sulphur will consume 1.53 kg of sodium cyanide to form thiocyanates or 1 kg of sodium cyanide will react with sulphur to form 1.65 kg of NaSCN or 1.18 kg of SCN. Whilst the cost of cyanide consumed by either heavy metals or sulphur compounds can be substantial, the major problem occurs when the cyanide and/or thiocyanate in the effluent liquor has to be detoxed prior to discharge. Free cyanide and many cyanide complexes are extremely toxic to animal and aquatic life. For example the toxic dose of cyanide for a human being is 50-1 OOmg, fish are even more sensitive and some will be affected in water containing as little as 50 micrograms/litre. There are a number of assaying methods for cyanide in solution each method assay not only the free cyanide but also the cyanide complexes. Total cyanide is determined by reducing the pH to 1.0 and assaying the CN by distillation, this technique breaks down all cyanide complexes some of which are non toxic. WAD (weak acid dissociable) cyanide is determined by reducing the pH to 5 and assaying the cyanide by distillation. Virtually all the complexes assayed as WAD cyanide are toxic so the WAD cyanide assay is the one used when establishing environmental criteria. WAD cyanide is usually defined as: CN^HCN+Zn and Ni bound CN-fO.75Cu(CN)4+0.67Cu(CN)3+0.5Cu(CN)2 +0.33Ag(CN)3. It should be noted that the above techniques do not assay for thiocyanate or cyanate. WAD cyanide complexes are relatively easy to destroy and there are a number of commercial processes that will reduce the WAD cyanide level of effluent liquors to environmentally acceptable levels including : i) Sulphur Dioxide; ii) Hydrogen Peroxide; iii) Caro's Acid (Peroxymonosulphuric Acid); iv) Alkaline Chlorination; v) Acidification, Volatilization Re-neutralization (AVR) Processes; vi) Biological Degradation; vii) Natural Degradation. Thiocyanates are formed when cyanide reacts with sulphur or sulphur compounds. The thiocyanate level of effluent liquors may vary widely from virtually 0 to 10 000 mg/l. Whilst extensive work has been carried out to quantify the toxicity of cyanide and cyanide compounds to virtually every living thing, the amount of work on thiocyanate toxicity is very limited. An empirical guide was that thiocyanates had only 1 % of the toxicity of cyanide, however some data contradicts this rule. For example it is quoted that trout are affected by cyanide at concentrations as low as 40 micrograms per litre whereas the toxicity of thiocyanates is quoted in the range 200 - 400 mg/l. It is not certain whether the cyanide limits are unreasonably low or the thiocyanate levels are too high, and may in fact be a combination of both. Caro's Acid reacts with thiocyanate forming cyanate and sulphate, further reactions oxidize cyanates to carbonates and ammonia. The quoted Caro's Acid requirement is 4 moles of Caro's Acid per mole of thiocyanate The relevant molecular weights are: SCN - 58 4H₂S0₅ - 456 Thus for each unit of thiocyanate to be oxidized 7.86 units of Caro's Acid will be required, this assumes of course that the Caro's Acid reacts only with thiocyanate but in fact Caro's Acid is an extremely powerful oxidizing agent and will react with many compounds present in tailings. Caro's Acid is prepared by mixing hydrogen peroxide with sulphuric acid, the approximate cost is USD400 per tonne or USD0.4 per kg. The table below sets out the Caro's Acid requirement and cost to detox liquors containing a range of thiocyanate levels.

The cost of degrading even modest levels of thiocyanate in effluents is extremely high. One way of quantifying the cost is to relate the cost of Caro's Acid to initial cyanide addition or cost. 1 kg of sodium cyanide will react with sulphur to form 1.18 kg of thiocyanate ion. 1.18 kg of thiocyanate will require 9.27 kg of Caro's Acid for total detox. Assuming a cost of USD400 per tonne for Caro's Acid the cost related to cyanide consumption is as follows.

i.e. assuming a cost of USD1.0 per kg for cyanide the cost of Caro's Acid for detox is USD3.70 per USD1.0 spent on cyanide. The normally quoted cost of cyanide detox is $1 per $1 spent on cyanide. Bacterial oxidation technology suppliers and others have developed processes using bacteria to detox tailings containing cyanides and thiocyanates. The reactions occur according to the general equation: SCN + H₂0 + 0₂ - C0₂ + S0₄ + NH₄ The process has been tested on a pilot plant scale. It is claimed that bacterial degradation rates of up to 250 mg/l/hour can be achieved. If this rate could be achieved on a commercial scale it would be economically quite attractive, to detox a solution containing 5,000 mg/l SCN would take approximately 20 hours. The process takes place at neutral pH so only mild steel reactors would be required with a volume of 20m³/m³/hour to be treated. The reaction vessels need to be aerated and the process kinetics are optimum at temperatures in the range 25-30°C. The only reagent required for this process is potassium phosphate which is relatively cheap and harmless. Bacterial degradation of thiocyanate shows a lot of promise but some work will be required to establish the operating criteria for a commercial scale plant and also the range of solutions that can be treated. Potential problems such as bacterial toxins will also need to be identified. One disadvantage of bacterial degradation is that the process works best on relatively clear liquor so that after cyanidation some form of liquid : solid separation is desirable. This is not a problem if the CIL tailing can be pumped directly to a tailings storage facility and only the clear decant liquor treated by bacterial degradation. If the tailing must be treated prior to deposition then a liquid : solid separation stage such as filtration or countercurrent decantation will be required which will increase the capital and operating cost of bacterial degradation. STATEMENT OF THE INVENTION It is therefore an object of the invention to economically remove a substantial portion of thiocyanate from an effluent stream, as compared to processes of the prior art. In a first aspect, the invention provides a process for treating effluent containing thiocyanate so as to reduce concentration of the thiocyanate within the effluent, including the steps of: acidifying the effluent to a pH less than 3.0; placing the acidified effluent in the presence of ferric iron; adsorbing and/or absorbing the thiocyanate with activated carbon. Designated the ROLB process by the Applicant, after some research it was first postulated that the mechanism of the ROLB process is in acidic conditions and in the presence of carbon, the thiocyanate can dissociate as follows : SCN^" = CN^" + S The cyanide formed by the weak dissociation of the thiocyanate reacts with the ferric iron to form an intermediary compound. A large number of tests were carried out to try to optimize the conditions particularly the effectiveness of the carbon catalyst since the carbon would be the major cost of the ROLB process. It was evident that the carbon requirement was significantly higher than that which would be required if it was acting as a true catalyst. Assays of solution after the ROLB process showed that a substantial proportion, preferably all, of the ferric iron may be reduced to ferrous and little or no iron may be removed from solution. A study of one embodiment of the invention was carried out and this work (in summary) indicated that, at low pH (circa 2.0) or less and in the presence of ferric iron thiocyanate is converted to the trithiocyanate radical H(SCN)₃ that can be readily absorbed by activated carbon. Subject to varying conditions, there are a number of other possible intermediate compounds that could also be formed then absorbed and stabilized by activated carbon. In a preferred embodiment the ferric iron may be added as ferric sulphate. Whilst the invention does not depend on the form of the ferric iron added to the effluent, it is nevertheless important to ensure optimal conditions for the removal of thiocyanate as compared to the addition of a ferrous iron. In another preferred embodiment the removal of thiocyanate in acidic conditions below pH 2 is important. In a more preferred embodiment, the pH may be equal to or below 1.5. In one embodiment of the present invention the acidity of the effluent may be reduced using sulphuric acid. As will be shown, the efficiency of the removal of the thiocyanate in an environment above pH2 is considerably reduced. In a more preferred embodiment, whilst activated carbon is an essential feature of the invention, its means of introduction to the effluent may increase the efficiency of adsorption of the thiocyanate. Thus in a more preferred embodiment the activated carbon may be introduced in a flow countercurrent to the process stream. Testwork has shown that the adsorption of thiocyanate by activated carbon may be directly related to the solution strength of the slurry and the amount of thiocyanate already adsorbed by the carbon. Thus greater efficiency may be achieved by contacting fresh activated carbon with the low thiocyanate slurry at the end of the process train then contacting the carbon with increasing thiocyanate concentration slurry as the carbon is moved to the head of the process train. In an alternative embodiment hydrogen peroxide may be used to further remove thiocyanate from the treated effluent. In a further embodiment, testwork has shown that heating the carbon to +400°C in an inert atmosphere removes all the adsorbed compounds and completely reactivates the carbon. Adsorbed tri-thiocyanate has been found to bond more strongly to the carbon than does thiocyanate. Thus, during water-washing of the loaded carbon, the acidic thiocyanate more readily washes off the carbon. EXAMPLES

Example #1 -Testwork using pure Ferric and Ferrous Ions A series of tests was carried out using pure ferric sulphate, pure ferrous sulphate and sulphuric acid instead of acid ferric liquor to demonstrate that the process was driven by pH and ferric sulphate and not some incidental feed compound in the bacterial oxidation liquor.

Tests 468 - 469 Ferric Sulphate

Test No. 468:

50 mis 10,000 mg/l SCN solution 7 mis Ferric sulphate solution containing 56 g/l ferric ion

Sulphuric Acid added to reduce pH to 1.8 Carbon addition 100 g/l Conditioning time - 5.5 hours

Test No. 469 50mls 10 000 mg/l SCN solution

8mls Ferric sulphate solution containing 56 g/l ferric ion

Sulphuric acid added to reduce pH to 1.3 Conditioning Time - 5.5 hours

Test 470 Ferrous Sulphate Test No. 470

50mls 10 000 mg/l SCN solution

8mls Ferrous sulphate solution 56 g/l ferrous ion

Sulphuric acid added to reduce pH to 1.5

Conditioning time 5.5 hours The results of the comparison tests are shown in Tables 1A and 1 B.

Comparing the above results with standard ROLB tests using acidic ferric liquor,

100 g/l fresh activated carbon and 6 hour contact time. The tests with ferric sulphate and sulphuric acid indicate that the process works as well if not better with pure compounds compared to results using acid ferric liquor. This supports the theory that at low pH in the presence of ferric iron the thiocyanate can be converted to a compound that can be adsorbed by activated carbon. The acid ferric liquor produced in the bacterial oxidation circuit does therefore not appear to contain other compounds that facilitate the ROLB process. Test 470 using ferrous sulphate gave a lesser removal of thiocyanate than those using ferric sulphate.

TABLE 1 A

Example #2 - Testwork using pure Sulphuric Acid alone Three tests were carried out to establish the thiocyanate removal by activated carbon in the presence of sulphuric acid with no ferric iron. In each case the GIL tailing was treated with sulphuric acid to reduce the pH to approximately 1.5 before the addition of carbon. The carbon was cycled from one test to the next. The results are shown in Table 2A. These tests show that some removal of thiocyanate is possible in the presence of sulphuric acid without any ferric iron present but the efficiency of removal is significantly reduced. TABLE 2 A

Example #3 - Removal of Thiocyanate in Non-Acid Conditions One test was carried out to determine the quantity of thiocyanate removed from GIL tailings by activated carbon without addition of acid ferric liquor. In 18 hours 100 g/l of activated carbon removed approximately 1700 mg/l of thiocyanate from a solution containing 10 000 mg/l SCN. This was significantly less than any result in the presence of acid ferric liquor. In addition the carbon sample was not washed with fresh water so the amount of thiocyanate removed with washing was not established. It is concluded that without the acid ferric liquor the efficiency of the thiocyanate removal by activated carbon is very low and thus not a viable commercial process.

Example #4 - Introduction of Activated Carbon Countercurrent to Effluent Feed Earlier testwork had shown that activated carbon may absorb up to 20% of its weight of HSCN₃ however the absorption rate may reduce as the carbon becomes loaded. Also, the loading kinetics may increase with increasing thiocyanate concentration in liquor. This work indicated that an effective process may be for the activated carbon to be fed counter current to the GIL tailings treated with acid ferric liquor. To test the theory a batch counter current test was designed to simulate a continuous process with the activated carbon moving counter currently to the CIL tails. To simulate a continuous process four solutions were used as follows. Solutions 1-3 were from earlier testwork where the thiocyanate had been partially removed. The assays of these solutions were. Solution SCN mg/l 1 1080 2 2890 3 5750 Solution 4 was prepared by adding 150 mls/l of acid ferric liquor to CIL tailings liquor assaying 10 000 mg/l SCN giving a solution containing 8696 mg/l SCN. Carbon was fed sequentially to solutions 1 , 2, 3 and 4 simulating counter current flow. The residence time in each stage was three hours which in a batch test was probably excessive but in a continuous mode would be reasonable residence time to avoid short circuiting etc. Initially four carbon concentrations were used 50, 70, 100 and 140 g/l. After the final cycle tests with 100 and 140 g/l carbon were discontinued since the total amount of thiocyanate removed from the four solutions was significantly higher than the initial solution namely 11 ,158 mg/l and 12,911 mg/l removed compared to the starting point of 8,696 mg/l. The carbon from the 50 and 70 g/l tests was regenerated by heating to 600°C for 30 minutes in an inert atmosphere and the procedure repeated with fresh solution. The 50 and 70 g/l carbon samples were regenerated and recycled four times giving a total of five cycles at each carbon concentration. The results of the tests are set out in Tables 4a to 4d. The following points are noted. ♦ At high carbon concentration the quantity of thiocyanate removed from the four solutions is greater than the amount in the starting point solution. ♦ A carbon concentration of 70 g/l appears to be slightly in excess of that required to remove 8 696 mg/i of SCN. ♦ The loading kinetics are significantly better in counter current operation than in either batch or concurrent operation. Fresh (or regenerated) carbon is required to remove thiocyanate from low concentration solutions, whilst loaded carbon can remove some thiocyanate from high concentration solutions. The final thiocyanate levels in the tests with 70 g/l carbon were excellent ranging from 23-112 mg/l SCN (the average for five tests was 58 mg/l), at levels less than 100 mg/l SCN the CIL liquor could probably be discharged or if not (the thiocyanate could be) easily removed using bacterial degradation or a small amount of hydrogen peroxide. The testwork demonstrates that the carbon could be regenerated by heating to 600°C and that there is no loss of absorption efficiency after regeneration, in fact regenerated carbon appears to be more efficient. The carbon samples were weighed after each regeneration stage and there was little or no weight loss or gain indicating that there was no build-up of precipitates in the carbon and that carbon losses during regeneration are small.

TABLE 4 A

TABLE 4B

TABLE 4C

TABLE 4D

Example #5 - Use of Hydrogen Peroxide to Remove Residual Quantities of Thiocyanate Three tests were carried out to assess the viability and economics of using hydrogen peroxide to remove the remaining thiocyanate from CIL tailing that had been treated using the ROLB process. A ROLB product solution was prepared from a mix of several products and this was then treated with hydrogen peroxide at three addition rates. As shown in Tables 5A and 5B the removal of thiocyanate using hydrogen peroxide is reasonably consistent. Assuming a cost of USD300 per tonne for hydrogen peroxide and assuming hydrogen peroxide removes 40 mg/l per gram per litre of hydrogen peroxide added, then to remove 100 mg/l of thiocyanate from one cubic metre of liquor would require 2.5 kg of hydrogen peroxide costing USD0.75 per cubic metre. As such hydrogen peroxide could only be economically used to remove relatively small amounts of thiocyanate and as a short term technique.

TABLE 5A

TABLE 5B

Example #6 - Carbon Concentration The cost of carbon regeneration and attrition losses make up the major part of the operating cost of the ROLB process and as such most of the testwork has been directed to optimizing (i.e. minimizing) the carbon requirement. Testwork identified the following: 1. The ultimate thiocyanate adsorbsion capacity of activated carbon is approximately 200 mg per gram, of carbon. This was determined by contacting carbon samples sequentially with acidified high thiocyanate solutions. 2. If a carbon sample is left in contact with the same solution even for a long period the adsorbsion is only 55-65 mg/g. So to achieve optimum loading the carbon must be sequentially contacted with high concentration thiocyanate solutions. 3. Whilst the adsorbsion limit is close to 200 mg/g carbon the kinetics drop after a loading of 150 mg/g carbon is achieved. 4. Thus for the purpose of estimating carbon requirement in a commercial operation a figure of 150 mg/g will be used. 5. Thus to remove the thiocyanate from solution containing 10 000 mg/l SCN a carbon concentration of 67 g/l will be theoretically required. In the counter current cycle tests a carbon addition of 70 g/l removed most of the thiocyanate so the above assumption appears valid.

EXAMPLE #7 - ACID/FERRIC IRON LIQUOR ADDITION. A number of tests were carried out to establish the effect on thiocyanate removal of variations in the addition of acid ferric liquor. Since this liquor is produced in the BIOX circuit in quantities far greater than would be required in the

ROLB process the work was aimed at optimizing the process and in particular the effect on carbon requirement rather than any attempt to minimize the acid ferric liquor addition. Three tests were carried out to specifically identify the effect of acid ferric liquor on thiocyanate removal. Test conditions were as follows. ♦ CIL tailing liquor 10 000mg/l SCN ♦ Carbon addition 100 g/l. ♦ Contact time 6 hours. ♦ Acid ferric liquor additions, 140 mls/l 200 mls/l 280 mls/l TABLE 6 A

This table shows that there is some benefit in increasing the addition of acid ferric liquor, the effect is probably related to the pH rather than the ferric concentration. The initial pH in the above tests is recorded below.

Test No. pH 339 1.97 340 1.77 341 1.58

Tests 400 and 401 demonstrate the effect of maintaining the pH below 2.0 using acid ferric liquor.

TABLE 6B

* all thiocyanate assays corrected for dilution The pH recorded during the tests were as follows. pH

An additional 80 mls/l of acid ferric liquor was added to test No 401 to keep the pH below 2.0. CARBON REGENERATION TEST WORK. The ROLB process may be most effective when high quality activated carbon is used to adsorb the thiocyanate species. Without regeneration and reuse the cost of carbon may be such that the process may be uneconomic with even relatively low concentrations of thiocyanate to be removed. An early test was carried out in which loaded carbon was heated to 800°C for 2 hours in an inert atmosphere. When this carbon was reused it was shown to be as effective as fresh carbon in adsorbing thiocyanates from acidified solutions. A number of techniques were then tested to regenerate loaded carbon without the capital and operating cost of a carbon regeneration kiln. Techniques included. Treatment with cold water. Treatment with boiling water. Treatment with slaked lime solution Ca(OH)₂. Treatment with sodium hydroxide solution. ♦ Treatment with sulphuric acid. Heated to 95°C. The results of these tests are shown in the attached table. From this it appears that the techniques may only be partially effective compared to thermal regeneration. The cost of boiling water etc is also high so even if this technique was successful it would be unlikely to have a significant cost benefit compared to thermal regeneration. Once the ROLB process mechanism had been identified as one of adsorption of thiocyanates and related compounds washing or eluting the carbon became unattractive since the eluate would contain thiocyanates in some form which would have to be destroyed prior to discharge of the eluate, i.e. the thiocyanate would just go round in circles. Following the thiocyanate adsorption kinetic testwork a series of tests were carried out to establish the optimum regeneration temperature. Samples of loaded carbon from test #622 that had adsorbed 173 mg SCN/gram carbon were heated to 300°C, 400°C, 500°C and 600°C for 30 minutes in an inert atmosphere. These carbon samples were then sequentially contacted with thiocyanate solutions (to which acid ferric liquor had been added) and the adsorption kinetics established. The results are set out in the attached table and graphs below. The results for a fresh carbon sample are included for comparison. The results show that provided the regeneration temperature is 400°C or above the carbon is fully regenerated even though it had been heavily loaded with SCN. The temperature required to regenerate carbon is one in excess of the decomposition temperature of trithiocyanate and related compounds. At this stage for design and future testwork a regeneration temperature of 600°C has been selected, this should be sufficient to remove not only the thiocyanates but also any organic material that may be adsorbed on the carbon.

ADSORPTION KINETICS OF REGENERATED CARBON SAMPLES TABLE 7B

Cumulative SCN Removed mg/gram Carbon

Fresh 100 g/l

Heated to 300 degrees C Heated to 400 degrees C «- Heated to 400 degrees C «- Heated to 600 degrees C

THIOCYANATE ADSORBSION TESTS A series of tests was carried out to establish the adsorbsion kinetics of thiocyanate (trithiocyanate) on activated carbon and also to determine the ultimate loading of thiocyanate (trithiocyanate) on activated carbon. The test procedure was as follows: CIL tailings liquor 10000 mg/l SCN Acid Ferric liquor addition 150 ml/l Initial solution assay 8696 mg/l Carbon additions 100 g/l 100 g/l 75 g/l 50 g/l The carbon was contacted with the solution for a total of 3 hours with samples being taken for assay after 0.5, 1.0, 2.0 and 3.0 hours. The carbon was then removed and recycled to fresh solution without any regeneration. The carbon was used a total of five times (six in the case of the 100 g/l carbon addition). The results of these tests are tabulated and graphed below to show the carbon adsorbsion kinetics.

THIOCYANATE ADSORBSION KINETICS

Test 1 100 g/l Carbon Concentrate TABLE 8A

Test 2 100 g/l Carbon Concentration TABLE 8B

Test 3 75 g/l Carbon Concentration TABLE 8C

O 200 25

Test 450 g/l Carbon Concentration TABLE 8D

TABLE 8E

Cumulative SCN Removed mg/g Carbon

-Fresh 10Og/l -Fresh 100g/l Fresh 75g/l - Fresh 50g/l

3 ₄ Cycle No

1. Thiocyanate adsorbsion is most rapid in the first 1-2 hours, after 2 hours the adsorbsion kinetics are quite slow. 2. When partly loaded carbon is contacted with fresh high concentration thiocyanate solution further adsorbsion can occur. After 5 passes the loaded carbon continues to adsorb further thiocyanate however the quantity removed reduces with each pass as would be expected. The results obtained above were recalculated to show the loading of the carbon as milligrams SCN per gram of carbon. The results are set out in the attached table 8E and graph and these demonstrate the ability of the carbon to adsorb up to 200mg SCN per gram of carbon. It is noted that the loading of carbon at a concentration of 50 and 70 grams per litre is higher than that per 100 grams per litre due probably to the fact that the thiocyanate solution tenor is higher with the lower carbon additions. NEUTRALISATION OF ROLB LIQUORS. The liquor/slurry discharge from the countercurrent carbon contact section whilst detoxed with respect to thiocyanate and WAD cyanide contains ferric and ferrous iron, some base metals, soluble arsenic (if bacterial oxidation liquor is used) and has a pH in the range 2-3. This product would not be acceptable feed to a tailings storage facility and needs to be neutralised prior to discharge. Testwork has shown that if the acid ROLB product is treated with limestone (calcium carbonate) and the pH increased to plus 5.5 in an agitated and aerated environment then the iron is precipitated as a stable carbonate, the aeration oxidising any ferrous to ferric, any base metals are precipitated and arsenic is stabilized in the form of basic ferric arsenate. Inductively coupled plasma (ICP) scans of the neutralised ROLB liquor have shown that the liquor is benign and suitable for discharge to the environment if required. TEST SUMMARY Furthermore, in the absence of ferric iron, cold water washing of the loaded carbon can lead to removal of a significant level of thiocyanate from the acidic thiocyanate loaded carbon (-80% SCN removal from acidic thiocyanate loaded carbon). Whereas, in the case of a process carried out in the presence of ferric according to one or more forms of the present invention, removal of thiocyanate from the ferric thiocyanate loaded carbon has been seen to be significantly reduced (-10% SCN removal from the ferric thiocyanate loaded carbon). Thus the advantageous practicalities of wash-water disposal of the ferric thiocyanate loaded carbon become apparent. It was further found that, after dehydration at 110°C, the main mass loss from the carbon occurred at about 550°C for the carbons contact with acidic thiocyanate and about 460°C when contacted with ferric thiocyanate. The surprising result for ferric thiocyanate indicates that relatively mild carbon regeneration conditions are sufficient to remove the loaded thiocyanate in order to regenerate the carbon. It may be practical to periodically use higher temperatures to assist in the removal of adsorbed organics.

CONCLUSIONS DRAWN FROM TEST RESULTS In the absence of ferric iron, relatively low adsorption of thiocyanate was observed (-50 mg/g at 25 g/L carbon). Carbon molar loadings based on solution assays were found to be consistent with the adsorption of HSCN on to carbon from acidic thiocyanate solution. However, in the presence of ferric iron, surprisingly high adsorptions of the thiocyanate occurred (-300 mg/g at 25 g/L carbon). Carbon molar loadings and visible spectroscopy results were found to be consistent with the adsorption of H(SCN)₃ on to carbon from ferric thiocyanate solution at lower loadings with some conversion to polymeric (SCN)_X species at higher loadings, as well as oxidation products including sulphates. CONTINUOUS PILOT TEST OF ROLB PROCESS A five day continuous pilot test was conducted to demonstrate the efficiency of the process according to one or more embodiments of the present invention for the removal of thiocyanate from cyanide leach tailings. Samples of cyanide leach tailings and acid ferric liquor were obtained from an operating mine, the assay of the two products was as follows:

With reference to Figure 1 , the pilot test plant circuit consisted of seven reaction vessels connected in series and included the following: 1 x 15 litre feed mixing reactor 4 x 15 lire ROLB reactors 2 x 30 litre neutralisation reactors A schematic diagram of the circuit is set out below in the attached as Figure 1. The feed to the circuit consisted of equal amounts of carbon in leach (CIL) tailing and acid ferric liquor to give a combined flow rate of 5 litres per hour, the nominal residence time per 15 litre stage was 3 hours. The pH in the feed reactor was maintained at 1.85-1.90 by the controlled addition of 25% (w/w) sulphuric acid. Carbon was added to the four ROLB reactors at a range of concentrations to assess the process efficiency at various carbon concentrations. The carbon was advanced in counter current flow to the flurry at three hourly intervals with the total carbon being advanced to the preceding ROLB reactor. Thus total carbon contact time with the slurry was 12 hours. Fresh or regenerated carbon was added to ROLB Reactor 4 at each carbon advancement. The loaded carbon recovered from ROLB Reactor 1 was lightly washed prior to regeneration in a rotary furnace at 600°C in an inert atmosphere (nitrogen and/or steam) and returned to the circuit at an appropriate stage. The discharge from ROLB Reactor 4 was neutralised in 2 x 30 litre stages where 50% limestone slurry was pumped to Stage 2 at a rate to maintain a pH of 5.5. The subsequent discharge from Stage 2 had a pH of approximately 605. Both neutralisation stages were aerated at a flow rate of approximately 5 litres/min. All stages in the circuit were operated at ambient temperature of approximately 25°C. Monitoring of the process included. • Hourly pH and redox measurement in all reactors. • Three hourly control samples for liquor assays. • Shift composite samples for selected liquor and carbon analysis. • Profile samples taken during steady state conditions.

Loaded Carbon Testwork A sample of loaded carbon was regenerated in a rotary furnace in an inert atmosphere under the following conditions. Temperature 600°C. Time 30 minutes. Atmosphere Nitrogen. The off gas was passed sequentially through 1 M NaOH and 1 M H2S04 solutions.

Results The pilot plant was operated at a range of carbon concentrations to test the efficiency of the process - a summary of the result is set out below.

ROLB PILOT PLANT CIRCUIT PROFILE ASSAYS

The results obtained with a range of carbon show that: • At a carbon concentrate of 50g/l the thiocyanate level was reduced from approximately 2,500mg/l to approximately 10mg/l after neutralisation.

• At a carbon concentrate of 30g/l the thiocyanate level was reduced to approximately 30mg/l.

• At a carbon concentrate of 25g/l the thiocyanate level was reduced to approximately 35mg/l. After each cycle the carbon was regenerated by heating to 600°C for 30 minutes in an inert atmosphere and returned to the process. The testwork showed that heating regenerated the carbon and no loss of adsorption efficiency was noted.

• Elution tests were carried out on samples of loaded carbon using hot water and dilute caustic soda solutions, elution did remove some of the compounds loading the carbon, however earlier test had shown that whilst elution alone would remove some pollutants from the carbon would not completely regenerate the carbon. It is likely that elution in conjunction with thermal regeneration will be required for complete carbon regeneration.

Weak acid dissociable (WAD) cyanide analysis shows that the ROLB process reduced the WAD cyanide concentration from approximately 500mg/l in the feed to approximately 0.26mg/l in final ROLB neutralisation product. This confirmed the determinations carried out in the earlier batch test program.

Testwork was carried out on the ROLB product tailings to ensure that they would meet any environmental requirements and to ensure that the products are stable. The following results were obtained.

TCLP Tests on ROLB Product Solids

The above shows that the solids produced by the ROLB process remained stable when tested using the standard USEPA method, the metal discharge levels were also below the USEPA limits. A further stability test was carried out by leaving ROLB liquor in contact with ROLB solids for a period of 30 days and measuring the change in concentration of a range of elements. The results are shown in the table below.

The results show that the tailings remained stable except for some variations in arsenic concentration. All elements were below USEPA discharge limits.

CARBON REGENERATION DATA Test Parameters Regeneration conditions Regen Temperature °C 600 Soak Time Mins 30 Furnace Type Rotary Atmosphere N₂ Scrub Liquid 1 [NaOH] 1 M Scrub Liquor 2 [H₂S0₄] 1 M Results

ASSAY LABORATORY REPORT TCLP - US Standard Method

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ASSAY LABORATORY REPORT TCLP - US Standard Method

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Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A process for treating effluent containing thiocyanate so as to reduce concentration of the thiocyanate within the effluent, including the steps of: acidifying the effluent to a pH less than 3.0; placing the acidified effluent in the presence of ferric iron; adsorbing and/or absorbing the thiocyanate with activated carbon.

2. A process as claimed in claim 1 , further including the steps of; dissociating a proportion of the thiocyanate present in the effluent to cyanide and one or more sulphur compounds; reacting the cyanide with the ferric iron to form stable and insoluble ferric cyanides.

3. A process as claimed in claim 2, wherein weak acid dissociable cyanide present during the process is complexed and stabilised by reaction with the ferric iron.

4. A process as claimed in claim 1 or 2, wherein a substantial proportion of the ferric iron is reduced to ferrous iron during the process.

5. A process as claimed in claim 1 , wherein at least a proportion of the thiocyanate is formed into tri-thiocyanate in the presence of the ferric iron, and the tri-thiocyanate is adsorbed and/or absorbed by the carbon.

6. A process as claimed in claim 5, wherein substantially all of the thiocyanate is formed into tri-thiocyanate.

7. A process as claimed in any one of the preceding claims, wherein the effluent is acidified to about pH 2.0 or less.

8. A process as claimed in claim 7, wherein the effluent is acidified at about pH 1.5 or less.

9. A process as claimed in any one of the preceding claims, wherein the ferric iron is ferric sulphate.

10. A process as claimed in any one of the preceding claims, wherein the step of acidifying the effluent includes using sulphuric acid.

11. A process as claimed in any one of the preceding claims, wherein the process has a process stream and further includes the step of adding the activated carbon in a flow counter-current to the process stream.

12. A process as claimed in claim 11 , wherein the activated carbon is first contacted with low concentration thiocyanate in a downstream portion of the process stream and, as introduction of the activated carbon is moved upstream, contacting the activated carbon with increasing thiocyanate concentration.

13. A process as claimed in any one of the receding claims, further including the step of using hydrogen peroxide to further remove thiocyanate remaining in the treated effluent.

14. A process as claimed in any one of the preceding claims, further including the step of heating the activated carbon used in the process in an inert atmosphere to reactivate the carbon.

15. A process as claimed in claim 14, wherein the carbon is heated to at least 400°C.

16. A process as claimed in claim 15, wherein the carbon is heated to at least 600°C.

17. A process as claimed in any one of claims 14 to 16, wherein the inert atmosphere includes nitrogen and/or steam.

18. A process as claimed in any one of the preceding claims, further including the steps of; raising the pH of the treated effluent to 5.5 or greater using calcium carbonate; aerating and agitating the treated effluent; whereby a substantial proportion of the iron is precipitated as a stable carbonate and at least a proportion of base metals present are precipitated and at least a proportion of any arsenic present is stabilized as a ferric arsenate.