Gold Gravity Separation Process
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Gold gravity separation process is one of the common methods used in gold ore dressing. It achieves the separation by relying on the different proportion of material and different sedimentation velocity in the medium. In the alluvial gold mine, the gold mostly exists as the natural gold, which has large density difference with the gangue, so the gravity separation process is efficient and economic.
There are many kinds of gravity separation equipment used in the gravity separation method, such as jig, shaking table,chute gravity separation, spiral chute, centrifugal separator and wind separator,etc. There are three common gold gravity separation methods: jigging for gold extraction, shaking table for gold extraction and spiral chute for gold extraction.
1. jigging for gold extraction
The main equipment is jig. The jigging process means mixes the mineral particles and complete multi-layer according to their specific gravity in the vertical variable medium flow. The minerals with small specific gravity are located in the upper layer and the minerals with higher specific gravity are located in the lower layer.
Application: jigging for gold extraction is suitable for coarse mineral particles (any particle size except for fine materials). For the metal minerals, the upper limit of particle size is 50 mm,the lower limit is 0.2-0.007 mm. For the alluvial gold mine, and the lower limit of particle size can reach 0.04 mm under the premise of monomer dissociation and specific gravity difference is larger than 1.25.
Advantage: Easy to operate, large processing capacity; finished product can be obtained through single separation.
2. shaking table for gold extraction
The main equipment is shaking table, which is a separator used in the horizontal medium flow. The transmission mechanism propels the table surface to carry on longitudinal reciprocating movement, differential motion of slipstream and table surface. The ore with different grains is discharged from different zone of table surface under the layering effect that is perpendicular to the table surface and the separation effect that is parallel to the table surface.
Application: Shaking table is suitable for separating those minerals with fine particle size. And it can be divided into coarse sand shaking table, fine sand shaking table and slime shaking table according to the different particle size of ores. Among them, coarse sand shaking table is suitable for ore particles with particle size less than 0.5 mm,fine sand shaking table is suitable for ore particles with particle size range of 0.5-0.074 mm, and slime shaking table is suitable for ore particles with particle size of 0.074-0.037 mm.
Advantages: The range of feeding grain size is generally within 3~0.019 mm, stable and reliable gold extraction process, the distribution of ore belt is clearly visible, and the rich ore ratio is higher than other gold extraction methods, easy to manage, and the required ore can be separated at one time.
3. spiral chute for gold extraction
The main equipment is spiral chute, which is a separation equipment that makes the use of ramp flow. With the water,mineral, mineral gravity, the friction force between the ore grains and the chute bottom, the ore grains settle in the different areas of the chute according to the proportion. The particles with small proportion are taken away by the water, and the particles with large proportion are left, so the separation process is completed. The working of spiral chute is an intermittent operation.When the concentrate at the chute bottom is deposited to a moving degree, the feeding should be stopped to clear the concentrate， then start it again.
Application: Spiral chute is suitable for the fine materials with low silt content, and the range of particle size is 0.6 mm- 0.03 mm.
Advantages: simple structure, large processing capacity, low comprehensive cost.
The above three gold gravity separation methods are only used for monomer dissociation gold ore (alluvial gold mine). For lode gold, joint separation process is often adopted rather than the single gravity separation, such as gravity separation – flotation process (the gravity separation process is used to assist gold extraction). In the grinding and classification circuits, using the jig, spiral chute and shaking table to recycle the coarse particles and create better conditions for the subsequent flotation cyanidation process, which is beneficial to improve the final gold index and gold recovery rate.
In gold ore dressing process, the selection of gold extraction method should be determined according to the properties of the gold ore. It is suggested to conduct the mineral processing test before selecting the gold extraction process, analyze the properties of the gold ore, and finally determine the gold extraction process according to the result of mineral processing test.
Classic Gold Extraction Projects
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1. Guinea 8000tpd gold gravity separation-heap leaching- all-slime cyanidation project
Xinhai solution: grinding and classifying -gravity separation - all-slime cyanidation - desorption electrolysis
2. Tanzania 1200 tpd gold all-slime cyanidation project
Xinhai solution: three stage one closed circuit crushing - one stage grinding with single closed circuit - all-slime cyanidation - dry tailings stacking
3. Malaysia 700tpd gold CIL project
Xinhai solution: two stage closed grinding and classification. After the trash screen, the screen underflow flows to thickener for thickening, the thickener underflow is pumped to the leaching operation. The gold charcoal product is obtained by the leaching, adsorption, carbon extraction operation, then transported to the original factory for processing. The leaching tailings is treated with the pressure filter for dewatering, and the dry tailings are transported to the tailings storage.
4. Tanzania 50000t/a all-slime cyanidation project
Xinhai solution: two stage crushing with single closed circuit – two stage closed grinding and classifying – roughing –cleaning - all-slime cyanidation - dry tailings stacking
5. African 700 tpd rock gold ore all-slime cyanidation project
Xinhai solution: two stage crushing with single closed circuit - two stage closed grinding and classifying - all-slime cyanidation - dry tailings stacking
6. Asia 500tpd gold gravity separation project
Xinhai solution: three stage crushing with single closed circuit - stage grinding - stage gravity separation - regrinding and re-separation of coarse concentrate
7. Asia 300tpd gold flotation project
Xinhai solution: stage grinding - stage gravity separation - stage flotation process
Gold Cyanidation Process
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Gold cyanidation process is a kind of main modern gold extraction method that adopting cyanide as the leaching liquid to separate gold from ore or concentrate.
There is no consistent mechanism that can explain the effect of cyanide on gold dissolution. Most people believe that the gold ore can generate be dissolved by forming a complex of gold in the presence of oxygen in cyanide solution. The surface of gold dissolves gradually from the surface to the inside in cyanide solution. The concentration of oxygen in the solution is related to the rate of dissolution of gold. The cyanidation time depends on the nature of the material, the cyanidation method and the cyanidation condition. Generally stirring cyanide leaching is often more than 24 hours, sometimes as long as more than 40 hours, cyanide gold selection needs 72 hours, leaching leaching leaching leachate to take more than 5 days.
Gold extraction methods from cyanide leaching solution：
Gold cyanidation process can be divided into agitating cyanidation process and leaching cyanidation process. Agitating cyanidation process is applied to treating the tailings after gravity separation, amalgamative tailings and flotation gold-bearing concentrate, or applied to full slime cyanidation. Leaching cyanidation process is used to treat flotation tailings and low-graded gold ore.
Among them, agitating cyanidation process mainly includes two types of gold extraction process. One is cyanide-zinc replacement process (CCD method and CCF method), which uses zinc powder (silk) replace and recover gold after the continuous counter-current washing. Another is the all-slime cyanidation (CIP process and CIL process),which adopts activated carbon to absorb and recover gold from cyanide pulp directly without filtration and washing.
One. Agitating Cyanidation Process
1. Cyanide-zinc replacement Process (CCD method and CCF method)
Cyanide-zinc replacement process (CCD method and CCF method) is to use zinc powder (silk) as the reducing agent to replace gold and silver from the leaching liquid, which mainly includes material preparation, washing (solid-liquid separation), purification, deoxidation, replacement,acid pickling and smelting.
2Au(CN)-2 +Zn=2Au+ 2Zn(CN)-2
• washing (solid-liquid separation)
Effect: Separate the solid leaching residue from gold-bearing solution.
Process: Multistage countercurrent washing
Equipment: thickener, washing thickener,press filter
Effect Factors: 3-5 washing levels, and the washing ratio is 4
Effect: Remove the fine suspended solids that affect the grade of gold mud from the pregnant solution
Process: vacuum filtration, filter pressing
Equipment: plate-and-frame press filter, tubular filter
Index: Suspended solids is less than 5g/m3.
Effect: Remove the oxygen from the pregnant solution, cause the
Process: Vacuum deoxidation
Equipment: Deoxygenating tower
Index: Vacuum degree is 0.009 ~ 0.096 MPa,and the oxygen content of pregnant solution is less than -0.5 mg/L
• Add zinc powder
Effect: Add zinc powder and lead nitrate to the pregnant solution.
Equipment: Disc feeder
Parameters of zinc powder: Particle size: -325 mesh (45 um); The content of pure zinc is larger than 94 percent.
Dosage: Zinc powder is 15-70 g/m3,lead nitrate is 10 percent of zinc powder.
Effect: Replacement of gold and filtration of gold mud
Equipment: plate-and-frame press filter
Index: Grade of barren solution is less than 0.02 g/m3; Gold-bearing grade of gold mud is 8-15%; Moisture of gold mud is 30-40%.
Application: the ratio of Ag and Au is 5:1;easy to achieve solid - liquid separation; widely used in the cyanidation offlotation concentrate.
2. All-slime cyanidation process
All-slime cyanidation process (CIP process and CIL process) is to add the activated carbon into the cyanide slurry, and the dissolved gold is absorbed on the activated carbon, and then the gold is extracted from the activated carbon. Here are seven main steps of all-slime cyanidation process: preparation of leaching material, agitation and leaching, countercurrent carbon adsorption, desorption of gold-loaded carbon, electro deposition and electrolysis, smelting, carbon regeneration.
CIP process: cyanidation leaching first and then add the activated carbon to the slurry pump for adsorbing the gold;
CIL process: Add the activated carbon to the leaching tank, leaching and adsorption simultaneously.
In the CIP process, the leaching and adsorption are two independent operations. In the adsorption operation, the leaching process has been basically completed, and the size, number and operation conditions of the adsorption tanks are determined by the adsorption parameters.In the CIL process, the leaching and adsorption operations take place simultaneously. In general, the leaching operation takes longer than the adsorption operation, so the size of the tank, aeration and dosing are determined by the leaching parameters. Since the adsorption speed is a function of the concentration of dissolved gold in the solution, so 1-2 levels of pre-leaching are often added before leaching and adsorption in order to improve the concentration of dissolved gold in the front adsorption tank and increase the leaching time.
CIP process VS CIL process:
1. CIP process is longer than the CIL process, but the CIL process is featured with a large amount of carbon and low carbon concentration, so the amount of slurry transporting is several times(generally 4 times) of CIP process;
2. The amount of overstocked metal in the CIP process is relatively large, and the distribution of the amount of overstocked metal in the two processes is different. The distribution of overstocked metal on activated carbon is basically the same as that in solution in CIP process. But in CIL process, the metal mainly accumulates on the activated carbon, and the amount of gold in the solution also varies.
3. CIL process has a higher gold content than CIP process, which is determined by the process structure. The CIL process means absorb and absorb simultaneously. The new dissolved gold is continuously added into the solution, so the gold in the solution is constantly supplemented and the gold content in the solution is relatively high. The CIP process is a single adsorption, the adding amount of dissolved gold in the solution is very small, so the grade of gold is lower.
Two. Leaching cyanidation process
Leaching cyanidation method is also one of the gold cyanidation leaching processes. The cyanide solution is permeated through the ore layer, so the gold is leached from the ore. Leaching cyanidation method is suitable for placer deposits and soft porous material.
Leaching cyanidation method can be divided to pool leaching process and heap leaching process.
1. Pool leaching process
The main percolation leaching equipment is leachate tank, which includes tank, iron tank or cement tank. The bottom of the pool is horizontal and turn as slightly sloping, round, rectangular or square.The tank is fitted with a false bottom made of perforated acid-proof board,which is covered with a filter cloth. And the grid that made of wood or corrosion-resistant metal strips covers on the filter cloth. When leaching,loading the ore in the pool, and adding leaching agent at the top of the pool,so the leaching liquid flows out from the false bottom part, where is used for filtering and supporting the ore.
The leaching time of leachate tank is related to the dissolution rate of the solvent to mineral, and the penetrationrate of the solvent in the material layer. The infiltration velocity mainly depends on the loading height, material porosity, silt content, the viscosity of leaching agent and material characteristics.
2. Heap leaching process
Heap leaching process means transfer the raw ore to the prepared yard, or directly stock on the waste rock or low-grade ore, then use cyanide leaching liquid to spray or infiltrate, and the ore reacts with the cyanide leaching liquid. After several recycling of leaching liquid and repeat spray, the leaching liquid can be collected. Then, treating leaching liquid by the activated carbon or metal zinc method and the barren solution returns to heap leaching operation for reuse.
Generally, heap leaching site is the gentle slope hills (natural grade 5-15 ˚) that nears the mining site. Removing weed sand lose material firstly, and then consolidate it as the foundation with 5 ˚gradient. The sides are high and the middle is low, which makes leach liquid flow into the stationary tank easily. The yard is covered with geomembrane to prevent leakage. Around the storage yard, a 0.4-meter ridge of soil shall be built as a ditch to prevent the rainwater from flowing into the yard. Besides,build a 0.3-meter-thick of lean ore before stacking the ore.
Advantages: Heap leaching process has low production cost and it can be put into production quickly. In addition, the scale of heap leaching is more flexible, even more than tens of thousands of tons. Heap leaching or granulated heap leaching can be adopted after the ore is crushed to a certain size.
Three. Resin-in-pulp process
Resin-in-pulp process uses the ion-exchange resin to absorb the Gold from the ore slurry.
R-OH- +Au(CN)-2 = R- Au(CN)-2 +OH-
Compared with the CIP process, the resin-in-pulp process is featured with large adsorption capacity, good wear resistance, without organic pollution, loose desorption condition, and easy to regenerate. However,its adsorption selectivity is poor.
Application: raw ore and concentrate.