Table of Contents
Efficient Merrill Crowe precipitation of gold and silver is dependent upon the proper control of certain physical and chemical properties of the solution. The most important of these properties are listed below:
Suspended solids: such as ore slime and precipitates of calcium carbonate, with hydrates of aluminum, magnesium and iron, present in the pregnant solution before clarification. These combined solids should be completely removed by efficient clarification.
Suspended precipitates: as above defined, which may continue to form in the solution after clarification. This formation occurs slowly and is almost completely prevented by clarifying and precipitating simultaneously.
Scale forming compounds: mainly the carbonates and sulphates of lime.
Oxygen and carbon dioxide gases dissolved in the solution: these prevent efficient precipitation of the precious metals. The solution must be oxygen free.
Cyanide strength: a small amount of free cyanide is necessary to catalyze the zinc:gold reaction but as little as 50 ppm of free cyanide should normally be sufficient. This is only the case if the solution is relatively free from base metals like copper.
Alkalinity: normally the solution pH is already above 10 as a result of leaching. pH should be maintained at this minimum level.
Copper: excess copper will prevent the zinc:gold reaction as it preferentially precipitates on the zinc surface. The free cyanide strength should be maintained at a level of 3 times the copper concentration to ensure the copper does not precipitate preferentially.
Typical dosages
Zinc – The stoichiometric ratio of zinc to gold is 1 to 0.3 wt/wt but typically additions are 3 to 4 times stoichiometric. When silver is present higher dosages are often employed in the range of 5-6 to 0.6 Ag wt/wt.
Cyanide – small amounts of free cyanide should be available in the range of 50 ppm. When copper is present free cyanide should be at least 3 times the copper concentration. The quantity of copper will need to be measured. A silver nitrate titration will indicate free cyanide available when it is actually associated with copper. The cyanide has a greater affinity for the silver in the titration and easily breaks the copper bond indicating free cyanide that is actually bound.
Lead Nitrate – typically added in the range of up to 0.2:1 Pb(NO3)2 : Zn wt/wt is added As long as a standard salt is used the dosage can be left unchanged but if comparisons are being done the lead acetate dosage should be decreased by 1.8% to maintain the same lead concentration as the nitrate. Lead nitrate greatly aids in the precipitation process and also inhibits the formation of zinc hydroxide on the zinc surface. The lead also allows for the formation of lead sulfide in preference to zinc sulfide, which is insoluble in cyanide.
General Information
- Clarification is a very important factor in obtaining efficient precipitation
- Lower cyanide concentrations are favorable to the precipitation of copper on to the zinc surface. This will make the zinc surface inactive to the precipitation of gold or silver. This can increase the quantity of zinc remaining in the final precipitate.
- Low cyanide concentration can also allow the formation of calcium zincate which can coat zinc surface interfering with precipitation.
Purpose: The purpose of this test procedure is to allow for the calibration of the reagent dosage for a Merrill Crowe system and to test various reagent schemes.
Equipment: The equipment required can vary depending on what the laboratory has available. The preferred testing method is to use a vacuum system to remove oxygen but heat can also be employed.
- Test solution – actual plant leach pregnant solution or a simulated solution representing the plant solution. A minimum of 500 ml should be used per test.
- Reagents
Gold standard solution – to allow the variation of gold grade to be tested
Copper sulphate pentahydrate – to test the variability of copper in the process
Sodium cyanide – to test the impact of cyanide on the precipitation process
Sodium Hydroxide – pH adjustment
Lead salts – either as acetate or nitrate
Sodium Hydrosulphite – alternative oxygen scavenger
Silver nitrate – to titrate the free cyanide
Rhodanine indicator – to titrate the free cyanide
Zinc dust – to be used as the precipitation reagent
Sulfuric acid – to be used for protective alkalinity titration. - Equipment
Buchner Flask (vacuum flask) – 1 liter
500 ml beakers
Volumetric flask – 500 ml
Titration equipment for free cyanide
Stirring Hot plate
Stir bars
Vacuum pump
Dissolved oxygen meter (optional)
Rubber stoppers
Funnels
Filter paper – Whatman 1 – 11 um or similar
Procedure: There are four basic procedures to remove the oxygen from the pregnant solution: heat, nitrogen sparging, chemicals, or vacuum. The preferred method is to use a vacuum system but the other methods have been shown to work. Near 0 deg C there is approximately 15 ppm of dissolved oxygen present, 50 deg C 5 ppm and at 100 deg C near zero ppm dissolved oxygen.
The testing will examine the impact of reagent dosages on a filtered stock pregnant solution as well as examining the impact of varying copper and gold concentrations.
Once a baseline has been established the results will be tested on an unfiltered sample plant sample to provide details on the impact of clarification.
Standard Solutions
Prepare standard solutions for gold, copper, cyanide, and lead nitrate/acetate. As well prepare a standard silver nitrate titration solution and rhodamine indicator. The procedure for cyanide titration will be provided as a separate document.
Gold Standard – A standard AAS solution of 1000 ppm should be utilized – this contains 1 mg of Au per ml.
Copper Sulphate – a 1000 ppm Cu solution should be prepared.
In a 1 liter volumetric flask add 3.929 grams of CuSO4*5H2O (25.451% Cu). Add 1000 ml of distilled or deionized water to the flask – fill to mark. Cap and mix thoroughly until dissolved. This contains 1.0 mg/ml of Copper.
Sodium Hydroxide – a 10% wt/wt solution should be prepared.
In a 1 liter volumetric flask add 100 grams of NaOH. Add 1000 ml of distilled or deionized water to the flask – fill to mark. Cap and mix thoroughly until dissolved.
Cyanide – a 1000 ppm free CN solution should be prepared.
In a 1 liter volumetric flask add 1.881 grams of NaCN (53.150% CN). Add 750 ml of distilled or deionized water to the flask. Adjust the pH of the solution to 10.5 with a 10% wt/wt NaOH solution. Fill to mark. Cap and mix thoroughly until dissolved. This contains 1.0 mg/ml of CN-.
Lead Acetate – a 1000 ppm Pb solution should be prepared.
In a 1 liter volumetric flask add 1.570 grams of Pb(CH3COO)2 (63.697% Pb). Add 1000 ml of distilled or deionized water to the flask – fill to mark. Cap and mix thoroughly until dissolved. This contains 1.0 mg/ml of Pb.
Lead Nitrate – a 1000 ppm Pb solution should be prepared.
In a 1 liter volumetric flask add 1.599 grams of Pb(NO3)2 (62.535% Pb). Add 1000 ml of distilled or deionized water to the flask – fill to mark. Cap and mix thoroughly until dissolved. This contains 1.0 mg/ml of Pb.
Pregnant Solution Sample
A suitable sample of unfiltered pregnant solution should be acquired from the plant. Tests will be conducted on this solution in the filtered and unfiltered state to show the influence of clarification. Approximately 20 liters of solution should be collected. Multiple samples can be collected with varying copper and gold grades to test the impact of these components or the solution can be adjusted with copper and gold reagents to test variations.
Filtration
Filter the majority of the pregnant solution setting aside a few liters for unfiltered testing.
Assays
Conduct AAS assays on the filtered pregnant solution including Au, Ag, Cu. Titrate a sample of the solution to determine free cyanide and protective alkalinity. Based on this analysis calculate the dosage of CN, Zn, Pb required using the base case dosages outlined above or from the excel table provided.
Deaeration
It is necessary to deaerate the pregnant solution sample. This can be conducted using vacuum (preferred), heating, nitrogen sparging or chemical addition.
Heat Method
- Measure out 275 ml of pregnant solution.
- Adjust the free cyanide concentration by adding the appropriate amount of cyanide stock solution. 50 ppm minimum or 3 times the copper concentration.
- Weight the solution.
- Bring the pregnant solution to a boil on a stirring hot plate (stir gently) in a beaker covering with a watch glass. Allow the solution to boil for 1 minutes.
- Keep the beaker covered and once boiling is complete reweigh the sample.
- Use this new weight for all reagent calculations.
- The dissolved oxygen can be checked with a DO meter or by using the Winkler Method. Develop a standard heating time based on these results.
- A cooled test sample should be re-titrated for CN to ensure that the cyanide was not destroyed by heating.
- Add zinc and lead dosages and allow to mix gently for 5 minutes.
- Turn off stirrer, and take a 25 ml sample.
- Filter the sample and immediately analyze the filtrate for Au, Ag, Cu by AAS.
Chemical Method
- Measure out 250 ml of pregnant solution.
- Adjust the free cyanide concentration by adding the appropriate amount of cyanide stock solution. 50 ppm minimum or 3 times the copper concentration.
- Weight the solution.
- To the solution add 37.5 mg of sodium hydrosulfite and gently mix until dissolved. Try not to over stir the sample and reintroduce oxygen.
- The dissolved oxygen can be checked with a DO meter or by using the Winkler Method. Develop a chemical dosage based on these results.
- A test sample should be re-titrated for CN to ensure that the cyanide was not destroyed by the reagent.
- Add zinc and lead dosages and allow to mix gently for 5 minutes.
- Turn off stirrer, and take a 25 ml sample.
- Filter the sample and immediately analyze the filtrate for Au, Ag, Cu by AAS.
Vacuum Method
- Measure out 250 ml of pregnant solution.
- Place the sample in a Buchner funnel.
- Adjust the free cyanide concentration by adding the appropriate amount of cyanide stock solution. 50 ppm minimum or 3 times the copper concentration.
- Place a rubber stopper in top and attach the vacuum hose to side arm.
- Gently stir the solution while applying vacuum to deaerate the sample.
- The dissolved oxygen can be checked with a DO meter or by using the Winkler Method. Develop a standard time based on these results. 15 minutes should be sufficient.
- Add zinc and lead dosages and allow to mix gently for 5 minutes.
- Turn off stirrer, and take a 25 ml sample.
- Filter the sample immediately and analyze the filtrate for Au, Ag, Cu by AAS.
Nitrogen Sparge Method
- Measure out 250 ml of pregnant solution.
- Place the sample in a Buchner flask.
- Adjust the free cyanide concentration by adding the appropriate amount of cyanide stock solution. 50 ppm minimum or 3 times the copper concentration.
- Sparge the sample with nitrogen gas for 30 minutes.
- Gently stir the solution while sparging to deaerate the sample.
- The dissolved oxygen can be checked with a DO meter or by using the Winkler Method. Develop a standard time based on these results. 30 minutes should be sufficient.
- Add zinc and lead dosages, place rubber stopper in flask and allow to mix gently for 5 minutes.
- Turn off stirrer, and take a 25 ml sample.
- Filter the sample immediately and analyze the filtrate for Au, Ag, Cu by AAS.
Variable Analysis
A series of tests should be conducted varying the reagents to define the optimum dosages for each pregnant solution.
- Repeat the tests above altering the zinc, lead and cyanide to develop a matrix of results. Once the optimum reagent dosage is defined repeat the test on unfiltered pregnant solution to show the impact of clarification.
Baseline Testing
- For a given pregnant solution sample test the following
Adjust the free cyanide to 3 times the copper concentration
Vary the zinc dosage from 3 to 10 to 1 Au
Maintain a constant lead to zinc ratio of 0.2 to 1 wt/wt
Based on the results adjust the zinc dosage further if required
Cyanide Testing
- Using the same pregnant solution sample test the following
Use the optimum zinc dosage from the previous test
Maintain a constant lead to zinc ratio of 0.2 to 1 wt/wt
Vary the free cyanide to from 0 to 5 times the copper concentration
Cyanide Testing
- Using the same pregnant solution sample test the following
Use the optimum zinc dosage from the first test
Use the optimum cyanide dosage from the first test
Vary the lead to zinc ratio from 0 to 1 wt/wt
Pregnant Solution Variations
- The pregnant solution should be adjusted to define the impact on gold and copper grades and the above testing repeated.
Adjust the gold grade keeping the copper grade constant
Adjust the copper grade keeping the gold grade constant
Determine cyanide, zinc and lead dosages for each solution
Additional tests can be conducted to compare the impact of pH, deaeration time, reaction time, filtration.
Below is sample table of test variables for baseline testing using typical pregnant solution assays from Touchstone.
