Fire assay method for determination of gold in samples

Weigh a certain amount of the tested gold alloy material, add silver to the sample in a quantitative manner, wrap it in lead foil and blow it in a high-temperature molten state, lead and base metals are oxidized and separated from gold and silver, the gold and silver particles are weighed after gold is separated with nitric acid, and the gold amount is calculated after correction with the pure gold standard sample measured together.

1 Principle of the method

Weigh a certain amount of the gold alloy material to be tested, add silver to the test material in a quantitative manner, wrap it in lead foil and blow it in a high-temperature molten state. Lead and base metals are oxidized and separated from gold and silver. The gold and silver particles are separated by nitric acid and weighed. The gold content is calculated after correction with the pure gold standard sample measured together.

2 Reagents and materials

1. Nitric acid (ρ=1.42g/ml), high-grade purity

2. Nitric acid (1+1), high-grade purity

3. Nitric acid (2+1), high-grade purity

4. Lead foil: pure lead (99.99%), processed into a square sheet with a side length of about 51mm and a thickness of about 0.1mm.

5. Pure silver (99.99%)

6. Pure gold standard: electrolytically refined pure gold with a gold content of 99.95% to 99.99%.

3 Instruments and utensils

1. Box-type high-temperature electric furnace (with temperature control device)

2. Microanalytical balance: maximum weighing 20g, sensitivity 0.01g.

3. Tablet mill: small, rolling thickness can reach 0.1mm.

4. Ash dish

① Ash dish: made of animal ashes, preferably cattle and sheep ashes. Burn the animal bones into ashes and grind them into ash powder with a particle size of less than 0.175mm. Add 10% to 15% water and press them into ash dishes on an ash dish machine. Use them after natural drying. Ash dish size: diameter 30mm, height 23mm, concave depth 10mm.

② Magnesium oxide ash dish: Mix calcined magnesia powder (particle size 0.147mm) with 525 silicate cement at 85:15 and add a small amount of water to press into shape. Use them after air drying for one month. Magnesium oxide ash dish size: diameter 40mm, height 25mm, inner diameter 30mm, concave depth 15mm.

5. Gold basket: Made of stainless steel sheet or platinum sheet with a thickness of 0.5mm to 1.0mm.

4 Analysis steps

1. Prediction of gold and silver content

⑴ Weigh two portions of 0.5g of the sample, accurate to 0.00001g, one of which is wrapped with lead foil, and the other is added with 2 to 2.5 times of pure silver according to the estimated gold content, and then wrapped with lead foil. Blow the two samples at 920±10℃ (bone ash dish) or 960±10℃ (magnesium oxide ash dish) in a high-temperature furnace at the same time.

⑵ Calculate the predicted value of the gold and silver content of the sample from the weight of the gold and silver particles after the sample without pure silver is blown.

⑶ Tap the gold and silver particles of the sample with pure silver after the sample is blown with a hammer on both sides to make the particles oblate, brush off the attachments on the bottom, and anneal at about 800℃ in a high-temperature electric furnace for 5min. Take it out and cool it, then roll it into a thin sheet with a thickness of 0.15±0.02mm, anneal it at 750℃ in a high-temperature electric furnace for 3min, and roll it into a hollow roll after taking it out.

⑷ Place the alloy roll in nitric acid (1+1) heated to 90℃ and separate the gold for 30 minutes. Pour out the nitric acid solution, add preheated nitric acid (2+1), and continue heating to separate the gold for 30 minutes.

⑸ Pour out the nitric acid solution, wash with hot water 5 times, move the gold roll (or broken gold) into a porcelain crucible, dry it, burn it in a high-temperature electric furnace at 800℃ for 5 minutes, take it out and cool it, weigh it, and calculate the predicted value of the gold content of the sample. Calculate the predicted value of the silver content of the sample based on the alloy content ⑵ and the predicted value of the gold content ⑸ of the sample.

2. Test material

⑴ Test material

① According to the predicted values ​​of gold and silver content, weigh two samples according to Table 1 and put them into lead foil respectively, accurate to 0.00001g.

② Accurately add pure silver to each sample so that the gold-silver ratio is 1:2.5, and add lead foil to wrap it into a sphere according to the numbers given in Table 1.

⑵ Standard test material

According to the gold content of the test material given in Table 1, weigh 4 portions of pure gold standard samples, accurate to 0.00001g, and the following operations are the same as (IV.2.(1).②). Take the average value of the test results of the 4 portions of standard test materials as the measured mass of the gold roll of the standard test material.

3. Determination method and steps

⑴ Ash blowing

① Preheat the ash dish in a high-temperature electric furnace at about 950℃ for 20min, then put the test material and the standard test material into the ash dish in a reasonable order so that each test material can be close to the standard test material, and close the furnace door.

② After all the test materials are melted, open the furnace door slightly for ventilation, and ash blow at 920±10℃ (bone ash dish) or 960±10℃ (magnesium oxide ash dish). When a colored film appears on the surface of the molten gold, close the furnace door. Keep the temperature for 2min and then turn off the power. When the furnace temperature drops to 720℃, take out the ash dish and cool it down.

⑵ Annealing and sheet grinding

① Use tweezers to take the gold and silver particles out of the ash dish, tap the two sides with a hammer to make it flat and round, brush off the attachments on the bottom, hammer the surface of the particles to a thickness of about 2mm, put them in a porcelain boat, and anneal them at 800℃ for 5min.

② After taking out the porcelain boat and cooling it, grind the gold and silver particles into thin sheets with a thickness of 0.15±0.02mm, and anneal them at 750℃ for 3min.

③ Take out the annealed gold and silver particles, cool them, roll them into hollow rolls, and put them into the gold separation basket one by one.

⑶ Gold separation

① First gold separation: Put the gold separation basket into nitric acid (1+1) preheated to 90-95℃ for gold separation, and the gold separation time is as shown in Table 1. Take out the gold separation basket and wash it with hot water 3 times.

② Second gold separation: Put the washed gold separation basket into nitric acid (2+1) preheated to 110℃ for gold separation, and the gold separation time is as shown in Table 1. Take out the gold basket and wash it with hot water for 5 to 7 times.

⑷ Burning and weighing

Take out the gold roll from the gold basket, put it into the porcelain crucible in turn and dry it on the hot plate, then burn it in a high-temperature electric furnace at 800℃ for 5 minutes, take it out and cool it down, and weigh the mass of the gold roll on the micro-analytical balance in turn.

Table 1

Gold content of alloy gold% Test material Gold content g Lead foil amount g Gold separation order Gold separation time min

30.0～45.00.3013120

225

45.0～55.00.4013125

230

55.0～99.90.509～13130

240

6 Allowable error

Table 2%

Gold content

Allowable difference

30.00～50.00

0.05

>50.00～80.00

0.04

>80.00～99.00

0.03

>99.00～99.90

0.02

7 Precautions:

1. The reagents and samples used in the gold test should be fully mixed.

2. The temperature should be slowly raised in the first 20 minutes of melting, otherwise the lead will settle too quickly, affecting the capture effect.

3. The lead obtained should be 25-40 grams.

4. The temperature should not exceed 900℃ during ash blowing, otherwise it will cause loss of precious metals.

Operational procedures for determination of CN- content in sewage

1 Sampling

Accurately transfer 10-50 ml of the test solution into a 150 ml beaker depending on the CN- content in the sewage.

2 Titration

Add 1-2 drops of 0.02% silver indicator to the test solution and titrate with AgNO3 (T=0.05306) standard solution of appropriate concentration until the solution changes from yellow to orange-red. (with black as background)

3 Result calculation:

[[CN-]mg/l=T.V×1000/50ml=1000T.Vstandard (mg/l)/Vto be tested

Where, T-mg/ml, 1mg=0.05306.

V-ml.

4 Applicable standards and monitoring and control:

1. The company recycles the tail water of mineral processing and adheres to the zero emission standard; the quality inspection adopts AA-680 atomic absorption determination method, and the determination parameters are controlled by national standards and atomic absorption standard curves.

2. Strictly implement national water quality control standards, control CN- concentration ≤0.5g/m3, and implement the "Taijin Company's Domestic Water Quality Discharge and Monitoring Management Regulations" of the Safety and Environmental Protection Department.

3. Any excessive emissions detected should be reported to the ore dressing plant and the Safety and Environmental Protection Department in a timely manner for rectification and safety hazard assessment; if the situation is serious, report to the company's safety committee and initiate the company's emergency response plan procedures.