Nitric acid pre-oxidation method for gold extraction - dilute nitric acid, sodium chloride oxidation leaching gold

About 100 years ago, people abandoned the gold leaching with acid and chlorine oxidation method, but now the acid treatment process and arouse people's interest. The single-stage system for recovering gold and silver from sulphide ore reacts quickly, eliminating the need for large or expensive equipment or special pressurization equipment. It eliminates the calcination and subsequent cyanide leaching steps and is a direct recovery process. The simple method of gold has obtained several patents in some countries.
1) Development of the process The UK conducted an initial experimental study on the process of oxidizing and leaching gold from dilute nitric acid and sodium chloride. Since there is not a large amount of gold ore that is difficult to be disposed of in the UK, it has established contacts with Australian companies. The company has a charcoal plant and a small laboratory in Kalgoorlir, Western Australia. After testing various types of ores, concentrates and tailings, it is considered that this new acid treatment process is not superior to the traditional cyanidation process for easy-impregnated ores. However, the test results for many sulphide ores indicate that this new process is promising, so the treatment of this sulphide mineral is concentrated.
The effect of dilute nitric acid and sodium chloride on pure gold samples was initially investigated. In 10% sodium chloride solution and 35% nitric acid solution with a temperature of 95~100 °C, the small gold nuggets dissolve completely within 15 min, while the 25% nitric acid solution dissolves gold for about 30 min. The study shows that the corrosion rate of the sample surface is 0.6. Kg / (m 2 · min), these results can be used to determine the amount of residual nitric acid required to dissolve the exposed gold after sulfide attack.
The actual nitric acid oxidation of sulphides normally associated with gold is more complex than that indicated by the chemical reaction formula. The reaction of metal sulphide with nitric acid produces sulphate, NO and water. However, side reactions occur in the presence of sodium chloride, such as the formation of nitro ions and nitrosyl chloride which readily dissolves gold. In this highly oxidized medium, other ore components such as arsenic and antimony are also oxidized to complicate the problem. All of this means that each ore must be analyzed immediately after it has been broken and ground, at least several times a day, to ensure that the ore properties do not change significantly.
Therefore, the actual amount of nitric acid used in the reaction cannot be exactly equal to the stoichiometric amount of sulfide present in the original ore or concentrate. Excess nitric acid must be added, but this does not mean a waste of nitric acid.
2) The mineral powder used in the laboratory experiment is a particle size of 0.050 mm, 150 g per batch. The glass bottle is equipped with a stirrer and a condenser, and is equipped with a thermometer socket and a fine grinding ore inlet. The glass bottle is heated by a resistance wire having a plurality of adsorption devices, and the first container has an air inlet for oxidizing the oxidized nitrous oxide to NO 2 .
The sodium chloride and nitric acid are first heated, then the stirrer begins to stir and the finely ground ore is gradually added as the temperature increases.
When high sulphide ores and concentrates are used, an exothermic reaction usually begins between 35 and 40 °C. The heating is stopped when the temperature inside the bottle rises to 95~100 °C.
The gold chloride is extracted in an acidic solution, and after testing, when treated with carbon, gold is deposited as a metal element from the solution. The results of the US Bureau of Mines test show that the amount of gold adsorbed from the chloride solution is 519 370 g per ton of carbon, while the amount of silver adsorbed in the nitrate solution is 115 070 g per ton of carbon, and the adsorption of carbon from the cyanide solution The amount of gold is 62 200 g/t and the amount of adsorbed silver is 31 100 g/t. A study conducted by Imperial University in London, UK, illustrates the problem of separating gold salts from other metals in acidic solutions.
It should be noted, fine pyrite from an acid solution to extract gold quickly, so it is important to sulphides completely removed.
3) The intermediate test plant intermediate test plant is a set of two sets of serpentine tubes with black polyethylene, one set is 100 m long and the other set is 250 m long. The pipe burst pressure is 2.8 MPa (working pressure is between 1.05 and 1.4 MPa). The pipe diameter is 25 mm and the pipe wall thickness is 6.3 mm. The tube was coiled with a radius of 0.9 m and inserted into a Ñ„2 100 mm groove. Connected with a titanium tube. The heat exchanger is a steam heated titanium coil. The system has two diaphragm pumps.
The test was carried out with 1 kg concentrate (grinding concentration 30%~60% solids, pumping speed 50~100 L/h) ground to 0.050 mm. The mass fraction of nitric acid varies between 7% and 15%, and the recovery of gold from mixed concentrates of oxides and sulfides is 95%. For comparison, several tests were carried out with alkaline cyanide at 45 ° C with a gold recovery of about 80%.
During the test, the slurry was filtered through a three-way valve to a large Buchner ceramic filter to separate the filtered process residue from the gold-containing acid solution and then to wash the filter. The filtrate was pumped into two adsorption columns of ф 75 mm × 18 000 mm containing activated carbon glass tubes to deposit gold on the carbon. Since the filtrate contains iron sulphate, there may be arsenate or citrate, so it is not recycled. On this scale, no excess nitrate is recovered by submerged combustion, but the filtrate is tested in production to determine the nitrate content. Returned to the factory at the same time, treated pulp returned via the condenser below the first container to the tee, the NO escape into the first scrubber into the air while the NO is oxidized to NO 2, in order to dissolve. Gold is recovered by burning charcoal, which is an economical method because it can adsorb a very high amount of gold. If silver is present, excess gold ingot (a gold, silver alloy) will precipitate. It is also possible to recover gold by electrolysis or other methods, but burning charcoal is the simplest method, and it is also possible to recover residual nitrate adsorbed on charcoal.
The Cambridge Institute of Mining in the United Kingdom has confirmed that the recovery rate of nitric acid is over 90% and the recovery rate of gold is high. The results of some mineral tests listed by the Cambridge School of Mines are shown in Table 1.

Table 1 Some results of experiments conducted at the Cambridge Institute of Mining with the HMC process of nitric acid and sodium chloride
Types of
Leach rate /%
Pyrite concentrate
Pyrite tailings
Flotation concentrate
Flotation concentrate
Re-election concentrate (锑)
Arsenic pyrite
Arsenic pyrite tailings

The large amount of ore processed at Redhill's HMC Process Center Institute is from Wales (the gold mining area is part of the Snowdonia National Park and is banned). Nature of ore: Among the ore bodies that are found in gravel mounds, the most common mineral is quartz . Chlorite package body generally contain Fe 2 0 3, also contains white mica. Yellow copper ore, pyrrhotite gold content is also very rich. Arsenic pyrite is symbiotic with a collection of gold and magnetization. The ratio of Fe to As+S is almost always 1:2. A typical sample contains 3.3% diamond, 0.1% nickel , and 31.43% arsenic.
Except for the telluride -gold aggregate, there is no other gold, and the aggregate is the most difficult to treat. It is believed that no one has conducted a suitable chemical leaching study until the recovery rate of gold and silver in the HMC process center reaches 90%.
Among the mineral aggregates are arsenic ore (CoAsS), chalcopyrite (CuSFe 4 S 5 ), silver bismuth (3Cu 2 SSb 2 S 3 ), and concentrated red silver (3Ag 2 SSb 2 S 3 ). Attoite (PbTe), bismuth silver ore (AgTe), arborvitae gold ore (Pb 5 AuTeSb 3 S 5~8 .).
4) The feasibility of the serpentine process is based on a 5t/h plant (340 days in 1 year and 34,000 tons in 1 year). If it is based on the standard equipment of the intermediate test plant, it is assumed that there is a suitable channel. The peaceful and complete site includes a 9t steam boiler workshop for generators, air compressors and heaters with an investment cost of $900,000. The nitric acid recovery system and simple immersion combustion equipment are based on UK prices. If you calculate a new field plant (starting from mining, crushing, grinding equipment , plant and processing of 5t / h gold and silver recycling workshop), the investment does not exceed 2 million US dollars.
According to the data of the intermediate factory, the approximate cost of designing 5t/h is shown in Table 2. If it includes mining, grinding, transportation and diesel fuel, it is estimated that these costs in Australia are 5-6 dollars.

Table 2 approximate cost of the serpentine tube system

Processing capacity (5t/h)
Per ton of ore cost / USD
Nitric acid (20% acid loss 10%)
Salt (natural sodium chloride)
Indifferent activity ($1,500·t -1 )
Water (50m 3 )
Fuel charcoal produces steam
Fine grinding ore or concentrate purchase price
other fee:
Workers and managers (including miscellaneous expenses)
Borrowing and discounting (5a)

Therefore, for a small plant of 5t / h, the plant can handle the ore or concentrate with high pyrite content and the total cost can be 48~53 US dollars / t. If the ore is more complex, that is, the content of arsenic pyrite is high, then to increase the cost of materials, arsenic can be fixed by lime treatment, which also involves the production of excess sulfuric acid and the conversion of iron sulfate to iron hydroxide (or iron oxide). problem. However, it must be stated that no problems with sulfur production were encountered during laboratory tests.
Therefore, if 90% of the gold in the concentrate containing 10g/t of gold is leached, according to the current price of gold ingots of 350 US dollars / ounce, it can guarantee a higher profit in this small snake tube factory.
Finally, successful experience in treating non-sulfide or low-sulfide ores and tanks with alkaline cyanidation in plastic pipes should not be overlooked.

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