In the process of using FKN environmentally friendly gold-dissolving agent, the key to improving the heap leaching recovery rate is to enhance the permeability of the ore pile so that the leachate can fully contact and react with the free gold in the ore. How to increase the oxygen content during the gold leaching process is also an important condition for increasing the leaching rate. Therefore, in order to improve the technical indicators during the heap leaching process, especially for the treatment of refractory gold ores, such as fine-grained and muddy ores, the heap leaching process must be improved. During the leaching process, the process modifications include ore granulation, wetting agent addition and oxygen leaching to achieve the purpose of increasing the gold recovery rate.
1. Granulation and heap leaching
The core issue of heap leaching is how to ensure that FKN gold leaching liquid fully contacts and effectively reacts with the valuable components in the ore. It is more difficult for ores containing fine ore and clay. Around this core, a lot of research work has been carried out and breakthroughs have been made in the past ten years. Holmes and Naruer proposed the TL method and obtained a US patent (US Pat. No. 4017309). This method was further improved by the Chilean SMP Company and used in industrial production at the Lo Aguire copper mine, thus finding a way to overcome the inherent shortcomings of heap leaching. An effective way.
The full name of the TL method should be granulation-pretreatment-thin layer heap leaching method. Its essence is: ① improve the permeability of the ore itself and the ore pile through granulation; ② add FKN environmentally friendly gold-dissolving agent during the granulation process to make it Contact the ore in advance and react in advance to speed up the leaching speed; ③ Heap leaching in thin layers to ensure uniform liquid distribution and facilitate oxygenation.
The comprehensive result is that the permeability of leaching is improved, which effectively promotes the reaction kinetics process and internal and external diffusion processes, greatly improves the leaching recovery rate, shortens the heap leaching cycle, and reduces leaching agent consumption. This is the key technical problem that the heap leaching method needs to solve.
1) Granulation process
For heap leach production to be successful, the heap leach material must have good permeability so that the FKN environmentally friendly gold-soluble solution can pass through the ore pile evenly. Therefore, some more difficult gold ores can also be successfully treated by granulating heap leaching. .
Before granulation, most ores and materials containing precious metals need to be crushed to 25.4mm or finer in order to expose the precious metals contained in the ores and increase the overall yield of precious metals.
During the granulation process, clay and fine ore in the ore adhere to the coarse particles, forming a layer of fine particles. This kind of mineral particles has sufficient wet strength and is rarely damaged when re-wetted after curing. Since granulation can produce raw materials with good porous and permeability, it can overcome the main problems caused by ore particle size segregation during pile building, such as the migration of fine particles and the channeling of the solution during leaching.
Three important granulation parameters were determined through research, namely: ① The amount of binder added to the dry feed; ② The amount of water added to the binder and ore mixture; ③ The amount of water required to form the calcium silicate bond. Curing time. By reasonably determining these three parameters, crushed ores with poor permeability, large amounts of clay minerals, fine-grained tailings and other materials can be successfully pretreated.
Practice and research have shown that most crushed ores (-25mm) can be effectively leached through the following processes. First, mix 4kg of Portland cement into each ton of material, and then moisten it with water or FKN gold extraction solution until the final moisture content is about 12%. The wetted mixture is mechanically rolled to achieve the purpose of agglomeration, and then solidified for more than 8 hours, and then conventional stacking is carried out. dip treatment.
The cement added during the granulation process provides a protective alkali for cyanide leaching. After building the pile, start using conventional techniques
When leaching contains a small amount of fine particles, just adding solution will cause the fine particles to adhere to the coarse particles. But when the content of fine particles, i.e. -200 mesh, exceeds 10%, a binder needs to be added for granulation. Produces loosely bonded (not dense) aggregate-like polymers. These polymers have high permeability and stable mechanical strength during leaching. The test result is that Portland cement is the best, and the addition amount is generally 0.9-4.4kg per ton of ore; lime follows. But both binders provide the required protective alkali, with lime being the better one.
The binder must be added to relatively dry ore, mixed with the ore and then moistened. The binder should be added in the crushing circuit and mixed with the ore as it is crushed. Moreover, it can also absorb excess moisture in the ore, making it easy for the ore to pass through the crushing equipment without sticking to the screen and clogging the discharge chute. The binder is added to the crushing circuit, and it can be closely mixed with the ore, especially the lime that fills the entire ore pile to ensure alkalinity.
The mixture of ore, binder and solution must be cured for 8 hours. Ores containing a large amount of fine particles require a large amount of binder, and the curing time takes about 2-3 days. If only the solution is used to make the dough, no curing time is required and no special curing equipment is required.
2) Granulation
① Through granulation, ores containing a large amount of fine particles and clay can be processed by low-cost heap leaching process, and those large low-grade gold ore bodies can be mined and utilized.
②Increase the total recovery rate of gold. Because fine grinding increases the exposure of free gold without causing segregation. Otherwise, the materials without granulation will stratify and segregate during the stacking process, resulting in local concentration of fine-grained materials and hindering the penetration of the immersion liquid.
③Increase the penetration speed and shorten the leaching time. This is more economically meaningful for mines that can expand production capacity without significantly increasing infrastructure investment for preparing impregnation pads.
④ The porous property of the ore mass allows the ore pile to "breathe", providing necessary oxygen for gold dissolution and accelerating the reaction between gold and cyanide solution. This provides the possibility to increase the height of the ore pile, which relatively reduces the pad production cost per ton of ore to be processed, and the land can also be used more effectively.
⑤ The ore pile composed of ore clusters has high porosity, so it can effectively wash the cyanide remaining in the leached ore pile. Stable pellets minimize dust contamination problems when the leach pile is discarded.
⑥ The noble liquid obtained by heap leaching contains high precious metal content, and the recovery rate of gold can be improved by substitution or activated carbon adsorption.
However, crushing systems and agglomeration systems are more expensive and should not be installed in industrial production unless absolutely necessary. Before starting industrial production, the technical and economic conditions for using granulated heap leach feedstock must first be determined.
3) Granulation equipment
①Cylinder granulator. The most commonly used granulating equipment is a cylinder granulator, and most cylinder granulators
The rotation speed is only 20%~60% of the critical speed. The ratio of barrel length to diameter is 5:2.5. This type of equipment is superior to other types of equipment because of its large capacity. The granulator has three operating parameters: first, the rotation speed of the cylinder, which can be adjusted through sprockets, pulleys or a variable speed drive; second, the inclination angle of the cylinder should be maintained between 1°-4°; third, the material is The appropriate residence time in the cylinder granulator is 1-4 minutes. The conveyor should be at right angles to the length of the drum to prevent spillage or accumulation of material. A rubber scraper is installed along the length of the barrel to prevent materials from sticking to the barrel wall.
② Disc granulator. This granulator has several operational control parameters that can be varied to suit the material being processed. The slope of the disk is generally at an angle of 40°-65° with the horizontal line, the depth is 0.46~0.91m, and the rotation speed of the disk is 30~50r/min, depending on the diameter and slope of the disk, as well as the ore particle size and disk diameter. A 6.1m diameter disc granulator can process 90t of ore per hour. Approximately 95kW of power is required to drive such a large-scale granulator.
The cylindrical granulator can be used outdoors, while the disc granulator can be set up indoors to avoid problems such as sand and dust.
③Piling and granulating. As shown in Figure 4, this is the simplest granulation method and is suitable for processing finer materials.
The discharge end of the conveyor belt connected to the crushing equipment is about 4.6~6.1m above the ground. This way the ore pile will be of sufficient height to allow the material to roll down the slope of the pile and be mixed; the ore flow down the conveyor belt can be moistened from the front or rear. The liquid supply pipe or rubber pipe must be connected to the frame of the conveyor belt. Spraying produces coarser droplets and does not require a wetting device. Below the jet there is one or more heavier mixing rods suspended in the falling ore stream. Its function is to make the outer layer of wet ore and the inner layer of dry ore
Use a front-end loader to load the material from the ore pile onto a truck and send it to be piled again or directly piled, so that the ore is mixed again; the material can also be unloaded into an ore storage bin and built by a stacker.
④Belt conveyor granulation. Spray water onto the -9.5mm ore below the screen. All products removed from the screen are sent to a common conveyor belt. The material is then mixed through four conveyor belts with heavy metal rods suspended in the falling ore stream.
The number of conveyor belts for mixing depends on the amount of fines in the ore. -100 mesh of 5% hard siliceous ore requires only two or three conveyor belts, but if the proportion reaches 10% or 15%, four or five conveyor belts are needed. If the proportion exceeds 15%, it needs to be processed by a cylinder or disc granulator. .
Each transfer point requires a nozzle and mixing rod. The transmission speed of the conveyor belt is 76~91m/min. The one with a width of 0.61m can process 136t of ore per hour, and the one with a width of 0.76m can process 227t of ore per hour. Note that the spray liquid cannot directly wet the conveyor belt. All solutions should be sprayed onto the ore. The inclination angle of the conveyor belt is 1500. The vertical space distance between the pulley at the discharging end and the receiving end of the next conveyor belt is about 1.8m. This allows the nozzles and mixing rods to be installed, giving the ore a chance to mix. As long as the ore does not slide down the belt, the slope can be steeper.
⑤Incline vibrating step chute granulation. There are 12 steps about 7.6cm high in this granulation equipment. The ore rolls down from the steps and is mixed. Each step is at an angle of approximately 35° to the horizontal. The vibration amplitude and frequency of the trough can be varied. Water is added through the water spray rod at the top of the vibrating tank, and the ore is granulated along the steps or sliding down the chute. It is important for this type of granulation equipment to maintain a constant ore feed rate. The residence time of materials on the tank is less than l0s. If a binder is required, it must be mixed with the dry ore in a separate device and then sent to the step chute granulator. ⑥Reverse belt granulator. Figure 6 shows a steeply inclined conveyor belt granulator that feeds ore at the upper end. The speed of the conveyor belt attempts to move the ore to the top of the conveyor belt, but the steep slope of the belt causes the ore to slide down.
The granulated ore mass has a short leaching cycle and a high leaching rate. Using the granulation process to treat ores and tailings containing a lot of mud and poor permeability can achieve better recovery rates and economic benefits.
2. Moisturizing effect
The fundamentals of using wetting agents to increase precious metal recovery from heap leaching are getting a lot of attention. In theory, adding special surfactants, i.e. wetting agents, to the leaching solutions can reduce their surface tension, allowing the ore and FKN gold leaching agent to come into more complete contact. Thereby improving permeability and recovery rate. Improving permeability can shorten leaching time, reduce cyanide and pumping costs, reduce the dry area of the heap leach field, and enhance penetration of individual mineral particles. Finally, the purpose of improving metal recovery rate is achieved. The wetting agent is a material that enhances leaching. It can not only increase the leaching speed and final recovery rate, but also ensure that the wetting agent will not have adverse effects on the adsorption and desorption of precious metals.
1) Wetting agent type
General chemical reagents used in heap leaching that have the ability to increase the wetting efficiency of the leach agent are called surfactants. These agents may have a variety of chemical derivatives, charge characteristics, and molecular masses. The classification of surfactants is listed in the table
1.Surfactant classification
1.Cationic type
A)Carboxylic acid
1) Soap, fatty acid, rosin acid, naphthenic acid
2) Others
B) Sulfate ester
1) Alkyl sulfate
2) Sulfate oil
3) Sulfate esters, ether and amines
2.Anionic type
A) Simple amine salt
B) Quaternary ammonium salt
C) Aminoamide and imidazoline
D) Amine oxide
E) Others
3. Hermaphrodite
A) Alkyl group plus carboxyl group or sulfo group or sulfate group
2) Liquid surface characteristics
The cohesion of water is key to the surfactant process. Since water molecules are dipolar, Figure 7 depicts water molecules in the form of a triangle. Oxygen atoms occupy one corner, and hydrogen atoms occupy the other two corners. Although the molecule is electrically neutral, a negative charge field surrounds the oxygen atoms and a positive charge field surrounds the hydrogen atoms. These regions of the water molecule are further apart, affecting the properties of the water. The dipolar nature of water and the negative charges at the highest points attract the positively charged regions at the bottom, illustrating the tendency of water to stick to itself. This tendency of each water molecule creates water's surface tension against nearby molecules.
This phenomenon reduces the wettability of water and reduces the contact area between the leachate and the ore evenly. Surface tension reduction results in more complete wetting of all solid surfaces. A good surfactant can reduce the surface tension of water from 0.72N/m to 0.3N/m at its relatively low concentration (100μg/g or less).
In terms of the ability of surfactants to reduce surface tension, a characteristic is, to a large extent, the presence of hydrophilic and hydrophobic groups on the same molecule. These parts of the wetting agent have different properties when they are dissolved in water. The hydrophilic end of the surfactant has the affinity to attract itself to water and is easily soluble in water. The hydrophobic end of the surfactant has no affinity for water and is oriented on the surface of the water.
If low concentrations of surfactants are added to water, they tend to accumulate on the water surface. The hydrophobic ends of the molecules are pushed out and line up on the water surface. As more molecules gather on the surface, its surface tension is reduced until the surface is completely covered. The concentration at which the surface tension no longer decreases is called the critical colloidal ion concentration. When no additional surfactant concentration is added to the surface, excess activator must remain in solution.
This reduction in surface tension also helps increase metal recovery during leaching, since more liquid must be brought into intimate contact with the ore's many irregular surfaces to dissolve the gold.
3) The role of wetting agent
Wetting agents actually act as ideal surfactants in heap leaching. The role of wetting agents in improving the recovery rate of immersion gold is still in the experimental research stage. S.R.Josph and H.N.James proposed a plan to use wetting agents to improve the permeability and recovery rate of ore piles in New Mexico.
Tests were conducted at the Ortiz Mine in both a laboratory column leach and a production heap leach field. The test work is to identify the following issues:
①Does adding surfactant increase the recovery rate and permeability? ②If the recovery rate is increased, what is the mechanism? ③What dosage is needed to achieve the above requirements? ④ Can adding surfactant generate enough revenue to compensate for the cost of chemical agents? ⑤What is the best way to apply chemicals? ③Should chemicals be added at the beginning of leaching or throughout the entire spraying process?
As can be seen from the experimental results, the results of the two experiments are promising. Compared with the two adjacent leach yards, the recovery rates of these two dumps (yard numbers E-22 and G-22) increased by 7.7% and 9.5% respectively using Drew739 and Nalco 2DA-375. The 2DA-375 was later replaced with the 85DA-059 and two more trials were conducted. 85DA-059 is a mixture of fatty acids and alkoxides. This second round of trials was less promising, with a 1.0% increase in recovery using Nalco85DA-059 and an actual decrease of 2.3% with Drew739. At the end of the year, two final trials were conducted, this time using Nalco 85DA-059. This time the recovery rates increased by 4.7% and 5.0% respectively. The overall recovery rate increased by an average of 4.3% across these 6 trials. The dosage was maintained at 16 μg/g for all six trials.
Table 2 Qrtiz mine field test results
Yard number
Wetting agent raw mineral
bit recovery rate/%
Front pile Back pile Average Test pile Difference
E-22 Drew739 1.18 72 68 70.7 77.7 7.7
G-22 Nalco2DA-375 2.02 68 76.1 72.1 81.6 9.5
E-23 Drew739 1.71 75 86.2 80.6 78.3 -2.3
H-23 Nalco 85DA-059 1.87 73 82.7 77.9 78.9 1
E-25 Nalco 85DA-059 2.21 79.1 69.4 74.3 79 4.7
A-26 Nalco 85DA-059 2.02 68.2 73 70.6 75.6 5
The Ortiz mine produces an average of 50,000 ounces of gold per year. For a mine of this size, a 4.3% recovery rate increase would increase annual gold production by more than 2,750 ounces ($1.3 million at a gold price of $480/ounce). The additional surfactant cost per ton of ore is less than 2 cents. The Ortiz mine processes 900,000 tons of ore annually. The annual cost increase for a mine of this size is less than $20,000, so the profit margin on investment is quite attractive.
Like most agents, wetting agents react differently with different ores. At the Ortiz mine, using one of three surfactants resulted in significant recovery improvements of up to 4.3%. However, in other mining areas, such as the Mesquite mine, there is no significant improvement, which indicates that there is still much work to be done on the mechanism of wetting agent improving recovery. In summary, trials at the Ortiz mine have shown that the gold gains from using wetting agents on reactive ores are substantial.
3. Oxygenated leaching
According to a report by C.M. Kenney, the results of the atmosphere column leaching test completed by Hazen for Kamyr Company showed that oxygen has a good effect on heap leaching of low-grade gold ore (1.3g/t) in California, and the ore particles were produced with a particle size of 25.4mm. , six column immersion tests were conducted in a column with a diameter of 14cm and a height of 1.8m. Spray oxygen into 4 columns, 2 of which are capped; the remaining 2 columns are not capped and do not spray oxygen. The cyanide solution is passed through the leach column for 34 days, followed by washing with water for 2 days.
Although trials and further studies are needed to determine the effects on other ores and the economics of adding oxygen to specific heap leaches, the trends demonstrated in the current study are encouraging.
The above description shows that in the experiment, it was found that the gold leaching speed using oxygen-enriched air is much higher than that using air. In the rolling bottle test, oxygen and air were used for 24 hours, and oxygen-enriched air was used for the leaching test. The gold leaching rate reached 93.3%. Therefore, oxygen increases the rate of gold dissolution but does not affect the final recovery.
In the column leaching test in a Ф14m column, the use of oxygen also increased the gold leaching rate. After leaching for 36 days, the leaching rates of the four columns using oxygen ranged from 89.4% to 92.3%, with an average of 91.1%, while the gold leaching rates of the two ordinary column leaching tests using air were only between 78.2% and 85.4%. After 15 days of leaching, the average gold leaching rate in the oxygenated leaching column was 86%, while the mining
The average gold leaching rate for a conventional column using air was only 76%. The consumption of FKN environmentally friendly gold-dissolving agent in all tests was less than 0.23kg/t. Compared with ordinary columns, the sodium cyanide consumption of oxygenated columns was slightly lower.
In order to solve the problem of oxygenation operation in heap leaching production, Kenney proposed an oxygenation heap leaching device.