Gold removal process by zinc replacement

The traditional cyanide gold extraction process mainly includes three processes of leaching, washing and displacement (precipitation).

1 Leaching - The process in which solid gold in ore is dissolved in an oxygenated cyanide solution.

2 Washing - In order to recover the gold-containing solution after leaching, the surface of the ore and the dissolved gold between the ore particles are washed with water to achieve a solid-liquid separation process.

3 replacement - the process of reducing, precipitating and recovering gold from a gold-containing solution with metallic zinc .

Since the 20th century, the recovery of gold from cyanide pulp has been carried out by washing the slurry first, followed by clarification and degassing of the noble liquid. Gold is precipitated from the clarified noble liquid, and the zinc replacement method has been used. The "carbon slurry method" that has been developed since the 1960s to add activated carbon to the slurry has developed rapidly. With the study of ion exchanger applications, the use of ion exchange resins to adsorb gold from cyanide or cyanide pulp has important practical value. Some research has also been done on solvent extraction and gold extraction from cyanide solutions. When sulfuric acid is added to the cyanide gold-containing liquid, isoamyl alcohol can be used to extract gold, and the extraction rate increases as the sulfuric acid concentration increases. If the extraction is carried out in a 2 mol/L sulfuric acid solution, gold can be separated from impurities such as arsenic and iron . Gold extraction was extracted from satisfactory isocyanate index of saline solution used oxoalkoxy phosphate; sodium sulfite reextracted also obtained good results and the like.

Cyanide immersion gold

The process of dissolving gold from the ore with an oxygenated cyanide solution is called cyanide leaching. At present, it has been perfected in terms of process, equipment, management or operation. As mentioned earlier, the dissolution of gold in an oxygen-containing cyanide solution is essentially an electrochemical corrosion process.

The main agents used in the leaching process are cyanide and protective base.

1) Cyanide

The cyanide used in industrial leaching of gold by cyanidation mainly includes potassium cyanide (KCN), sodium cyanide (NaCN), calcium cyanide [Ca(CN)2] and ammonium cyanide (NH4CN). Their relative solubility to gold is shown in Table 1.

The cyanide commonly used in the production is sodium cyanide, which is a highly toxic white powder, and commercial sodium cyanide is generally pressed into a spherical or massive form.

Industrially, cyanide melt is also used as a leaching agent. It is a mixture of calcium cyanide, salt and coke which are melted in an electric furnace. In addition to containing 40% to 45% of Ca(CN)2 and NaCN, it also contains some impurities harmful to the cyanidation process, such as soluble sulfides, carbon and some insoluble impurities. It is characterized by low price, but the amount is large, about 2 to 2.5 times that of sodium cyanide. In order to eliminate the effects of harmful impurities, pretreatment should be carried out when using cyanide melt. Through-air processing method is strongly stirred or added to the solution an appropriate amount of lead salts.

In theory, it takes only 0.5 g of sodium cyanide to dissolve 1 g of Au, but in actual production, the consumption of cyanide is 20 to 200 times the theoretical amount, or even higher. The amount of consumption depends mainly on the amount of other components in the ore that can react with cyanide.

2) Protection of alkali

The protection of the base is mainly to maintain the stability of the cyanide solution and reduce the hydrolysis loss of cyanide. The addition of the base in the cyanide leaching is maintained in the leaching tank or in the grinding process of the cyanide ore. When the ore composition is complex and contains some minerals such as pyrrhotite which are harmful to the cyanidation process, the protective alkali is added during the grinding process to facilitate the oxidation or precipitation of these harmful minerals.

The protective base can be potassium hydroxide and sodium hydroxide, but more commonly inexpensive lime (calcium hydroxide). Should telluride gold ore processing ore strong base such as required degree, or with sodium hydroxide as well.

The amount of the protective base to be added should be an appropriate amount, and the pH of the slurry is generally maintained at 10 to 11. At this time, the CaO mass fraction in the slurry is about 0.01% to 0.02%. Too low is not conducive to the prevention of cyanide hydrolysis. Excessively high, although it can promote the flocculation of negatively charged silicon mud, is conducive to slurry precipitation and liquid purification, but has a significant adverse effect on the leaching rate of gold.

When lime is used as a protective base, it is preferably added in the form of lime milk to facilitate process control.

2. Solid-liquid separation

After the ore is leached by cyanidation, a slurry consisting of a gold-containing solution and tailings is produced. In order to separate the gold-containing solution from the solid tailings, washing and filtration are required. The commonly used separation process includes: concentrating and filtering cyanide pulp, and then washing the filter residue on the filter with de-gold depleted liquid or water, and then rejecting or re-treating the gold-containing solid, ie tailings, and the gold-containing solution. Used for displacement precipitation of gold. In the solid-liquid separation, the washing water is added, and the washing water is generally used as a lean liquid or clean water discharged from the displacement operation. When the treated ore contains less harmful cyanide impurities, all the lean liquid can be returned to the leaching operation. At this time, clean water is generally used as the washing water, which can improve the washing efficiency and make the cyanide tailings. The concentration of sodium cyanide in the solution is reduced, the loss of sodium cyanide is reduced, and the sewage treatment operation is simplified. When there are more harmful cyanide impurities in the treated ore, the lean liquid generally does not return to the leaching process, and some of the poor liquid is used as the washing water; at this time, if the clean water is used as the washing water, although the washing efficiency is improved, The increase in lean liquid emissions increases the amount of gold lost in lean liquids, reduces total replacement rates, increases cyanide consumption, and increases wastewater treatment costs and costs.

At present, there are various washing methods, and the washing methods for separating the gold-containing solution and the tailings from the slurry include a decantation washing method, a filtration washing method, and a fluidized washing method. In the production practice, what kind of washing method and washing equipment are selected is the key to improving the washing efficiency and reducing the production cost.

1) Decanting washing method

The decantation washing method is widely used in North America, and it can be classified into a batch decantation washing method and a continuous decanting washing method.

1 Intermittent decantation washing method. Intermittent decantation washing is usually used in combination with intermittent agitation cyanation. One of its operation methods is that after the cyanide slurry is clarified in the clarification tank, the upper gold-containing clarified liquid is extracted by a siphon with a float to be exchanged for gold, and the remaining thick slurry is withdrawn into the stirring leaching tank and the NaCN dilute solution is used for leaching again. . The second method is to feed the cyanide slurry into a thickener, and the gold solution produced by the overflow is sent to the replacement gold. The thick slurry in the thickener is pumped to the stirring leaching tank and the NaCN dilute solution is used for leaching again. The secondary leached slurry is then sent to a clarification tank or a thickener for further processing. This is repeated several times until the gold in the lotion reaches a trace amount.

The gold-containing solution produced by the second leaching operation usually contains less gold and can be used for one leaching of the next batch of raw materials. The third leaching solution is used for secondary leaching of the next batch of raw materials, and these solutions are continuously used. The precipitated gold is sent until the specified concentration is reached.

Thick washing is the process of washing the leached pulp with a thickener. The leaching slurry or the slurry to be washed is washed with a large amount of washing water while being given to the thickener, and the solid particles are self-settled in the thickener. The concentrated pulp is discharged to the discharge port with the bottom flow (or discharged to the next thickener for washing again), and the dissolved gold in the upper supernatant is recovered with the overflow process of overflowing into the human gold, or as a higher level. Washing water.

At present, there are many types of thickeners used for washing in cyanide plants at home and abroad. If the number of layers of thickeners can be divided into single layers and multiple layers; if the transmission mode is used, it can be divided into central transmission type and peripheral transmission type. In recent years, a new type of thickener, namely a high-efficiency thickener, has been introduced and copied in China. Regardless of dewatering or washing, the effect of a high-efficiency thickener is 2 to 3 times higher than that of a single-layer thickener of the same specification. If the flocculant is added, the effect is more than 5 times higher.

No matter what type of thickener is used, as long as it is used for washing, it is rarely used in a single layer, and is generally a multi-stage countercurrent washing composed of a plurality of single-layer series or multi-layer thickeners. Figure 1 is a flow chart of a three-stage countercurrent wash consisting of three single-layer thickeners.

The intermittent decantation washing method has few industrial applications because of the long working time, the large amount of solution used, and the large equipment footprint.

2 continuous decantation washing method. The continuous decantation washing method is one of the methods widely used at home and abroad. It is based on the principle that the slurry and washing liquid are reversely moving, and is called a continuous countercurrent decantation washing method abroad (Fig. 2). In this method, the slurry and the washing (lean) liquid are fed into the thickener from the opposite direction and flowed into the first-stage thickener to realize the washing and solid-liquid separation of the slurry. Therefore, the thickener is the main equipment for continuous countercurrent decanting operation. To this end, the largest concentrator that has been used abroad has a diameter of 150 to 180 m. The thickeners used are single-layered and multi-layered. The continuous countercurrent decanting washing process and solution balance of a three-stage single-layer thickener in a gold processing plant in China for daily treatment of 100 tons of ore are shown in Fig. 3.

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