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Zinc powder replacement is used to deal with the pregnant solution from gold cyanidation. The principle is replacing gold or silver with zinc powder from leaching extract. It can also use zinc wire instead of zinc powder in industry, which has similar operations with zinc powder replacement technology.
Use the table of contents below to navigate through the guide:
Zinc powder replacement includes four operations such as: washing (solid-liquid separation), purification, deoxidation, and replacement.
1. Washing: Separate the leaching solution with solid after cyanidation.
2. Purification: Weed out the suspended solids in leaching solution and keep the pregnant solution clear until into replacement operation. DeoxidationRemove dissolved oxygen from the pregnant solution (leaching solution).
3. Replacement: Replace precious metal with zinc powder from pregnant solution (leaching solution), and receive gold or silver clay.
The main factors that can influence zinc powder replacement technology includes: concentration of cyanidation and alkali, concentration of oxide, the role of lead salts, temperature and impurities in solution, the purification of pregnant solution(leaching solution).
1. Concentration of cyanidation and alkali
When cyanidation is in low concentration, zinc will react into zinc hydroxide and zinc cyanide on the surface, which prevents the replacement of gold. If the concentration of cyanidation and alkali is too high, the speed of zinc dissolution is increased which is good for the replacement of gold but also increase the consumption of zinc and the production cost. So the concentration of cyanidation and alkali must be controlled during production.
2. Concentration of oxideThe dissolved oxide in the solution is harmful to zinc replacement. It will influence the effect of zinc replacement. So deoxidation must be done before replacement and keep the leakproofness of deoxidation and replacement system. The reaction could not carry on if this part of system leak air.
3. Effect of lead salt
In the displacement process, the main function of lead is to form "zinc-lead electric couple" with zinc, then dissolve gold. The common lead salt used in the production includes lead nitrate and lead acetate. The lead can accelerate the displacement of gold ore, but the excessive lead can cover the surface of zinc and slow down the displacement rate. If too much lead enters the gold slime, the grade of gold will be decreased. Therefore, the amount of lead salt used in the production should be appropriate.
4. Temperature and impurity in solution
The increasing temperature will accelerate the rate of displacement reaction. If the temperature is below 15 ℃, the displacement ratio will be affected. The impurities in the solution mainly include copper, mercury, nickel and soluble sulfide, etc. The copper can make the zinc powder form a layer on the surface, which decreases the contact area; the mercury complex is easy to react with zinc, then form an alloy, which makes zinc brittle; the soluble sulfide will react with zinc and lead, and produce zinc sulfide and lead sulfide on the surface of zinc and lead.
5. Cleanliness of pregnant solution (leaching liquid)
The cleanliness of pregnant solution (leaching liquid) is one of the important conditions that affect the zinc powder displacement. The suspended solids and oils that exceed the requirements are not allowed in the pregnant solution (leaching liquid). The oils that mix in the pregnant solution will form an oil film covering the surface of zinc, then affect the displacement effect. The slime will block the filter cloth of the displacement equipment and affect the displacement operation, even affect the quality of gold slime.
In the production, the gold can be recovered by zinc powder (silk) replacement and precipitation, that is cyano-zinc displacement process (CCD process and CCF process). Besides, the zinc powder displacement can also be used to process the pregnant solution (leaching liquid) after pool leaching or heap leaching. Generally speaking, the gold ore with high silver can be treated by zinc powder displacement method, which is beneficial to the recovery of silver. In addition, zinc powder displacement process can also be used to treat the gold concentrate that needs to be upgraded.
It is necessary to consider more problems than those ideal conditions in the laboratory during the actual production. Xinhai Mining suggests that each mine owner should decide whether adopt zinc powder displacement process or not under the mineral processing test results, plant size, plant conditions, investment cost, then the balance between the technical index and economic index can be achieved.