In situ leaching refers to the use of a leachant in or near the geological location of a deposit, without the ore being transported, by leaching through a borehole. If it is a direct leaching of an ore body buried in the ground, it can also be called solution mining. According to the effect on the structure of the ore body, it can be divided into three cases: no change in structure; change of structure (hydraulic crushing, blasting); low-grade residual part of the mine (surrounding rock, retaining wall, sinking area of â€‹â€‹the pit, some people think It should also include waste rock heap leaching). The last type of literature is called in place leaching. Only when the ore is a structure with excellent permeability such as sandstone , it is possible to directly perform leaching without changing the structure. The ore body is usually required to have a porosity of 20% to 30% and a permeability coefficient of 0.5 to 5 m/d. The advantage of in situ leaching for the complex geological structure of deep ore body, eliminating the need to open up, ore mining and handling processes, the construction period is much shorter, but also save a lot of funds. Solution mining does not destroy the earth's surface, nor does it need to transport large quantities of ore, which has less impact on the environment and ecology  .
The disadvantage of in-situ leaching is that the recovery rate of valuable metals depends largely on the geological structure of the ore body. Some parts of the leaching liquid cannot be deep, and some minerals are gangue wrapped, and the leaching rate is lower than usual. The result of the leaching of the ore. In addition, due to the limitation of the leachant, the recovery of valuable metals other than copper , especially precious metals, is lower. Solution mining poses a threat to groundwater. Two steps can be taken to reduce this threat. One is to choose a leachant to reduce the pollution of groundwater by chemical reaction products. The second is to flow the leachate in the target area to prevent leakage or mixing with external water  .
The usual leachant is sulfuric acid, but in recent years it has also been suggested to use ammonia and ammonium salts as the leachant. Ammonia leaching of sulfide ore is faster than acid, especially when the gangue is alkaline, ammonia may have better economic benefits than acid. Leaching studies of chalcopyrite with ammonia have shown that ammonia concentration, oxygen partial pressure, and temperature are three key factors influencing the rate of leaching.
When other valuable metals coexist with copper, the use of leachants should also consider the recovery of these metals. Sulfuric acid is a leachant that recovers uranium while leaching copper. Kennecott's Bingham Canyon in the United States and Anamax Twin Buttes in Anamax are recovering uranium while producing copper. For copper ore containing gold and silver , there is also a scheme of considering cyanide leaching, replacing the precious metal with copper powder, and then accumulating copper.
There have been many years of industrial practice experience in in-situ immersion of ground outcrops or shallow buried ore bodies. Old Rliable, Inc. of Arizona, USA, used explosives from the ground to break up ore bodies, and the ore was broken to a diameter of 20 to 30 cm. The leachate is drenched from the top and then withdrawn from the recovery well or from the bottom of the ore. Due to the uneven implementation of the blasting, the design index was not met and the leaching recovery rate was only 20%. Even so, it is still profitable. The Occidental mining company drilled wells to 305m for hydraulic fracture experiments, resulting in a crushing effect only at 305-366m.
In order to reduce on-site experiments and amplification costs, underground leaching must be simulated in the laboratory. In order to clarify the leaching mechanism of large ore, X-ray computed tomography (CT) has recently been used to observe changes in the leaching process of various parts of the core. It can thus be clearly seen that the dissolution of copper begins near the original crack and then forms a channel. There are short circuits in some places, and some copper minerals are not in contact with the leachate. They added a polymer compound to the leachate to block the short circuit and improve the leaching efficiency.
In-situ leaching main type
As the leaching is carried out underground, it will be seriously affected by the groundwater level. According to the relative height of the ore body and water level, it can be divided into several cases, which are illustrated by the following figure  . The arrows in the figure indicate the flow direction of the liquid.
The ore body close to the ground is above the groundwater level, such as the remnant of the mined mine, which is similar to the waste rock heap leaching. In the second case, the ore body is below the groundwater level, but it is relatively shallow. After the blasting, the water level is controlled at the lowest point. Solution circulation, infiltration or flooding can be used, and detailed hydrogeological data of the mining area is required to determine which leaching process to use. An oxidizing agent should be added when the content of sulfide ore is high.
The force crushing experiment resulted in a crushing effect only at 305-366 m.
The third case is a very deep ore body (hundreds of meters). First check the flow direction to reduce the pollution of groundwater, and use blasting or hydraulic crushing. In this case, a strong natural water pressure can increase the solubility of oxygen, so it is possible to directly take oxygen as an oxidant for leaching.
Margma's in-situ experience
The US Margma company has used different forms of land immersion for production for decades, and has accumulated a lot of experience, which is described in the following aspects  .
A geological structure
In geological exploration, in addition to determining the size, shape and grade of the ore body, it is necessary to ascertain the direction of the texture in the ore body, to find out which part of the ore body the leachate can not infiltrate, and what part can produce a solution conduit. These factors can affect the penetration of the solution. Particular attention needs to be paid to configurations that can cause solution flow to deviate from the target ore body. They believe that if there is a break near the ore body, they cannot use in-situ leaching.
The most difficult thing to master in situ leaching is the groundwater geology, which is not only complicated because of the geological structure of the mining area, but also will continue to change during the implementation of the leaching process. It is difficult to obtain the required data without sufficient on-site measurements and tests. A multi-well water injection test is usually used with a plug test to determine the difference in properties in each direction. The physical properties of the ore body such as pore volume, permeability and homogeneity control the flow direction of the fluid, the flow rate and the volume of liquid stored.
The main hydrological data required are the water head, flow rate and flow pattern. The streamline is the route along which the liquid flows along the gap in the ore. Mastering the distribution of the streamline is useful for understanding the flow path of the leachate. These factors are related to the consumption of leachate and its flow rate through the ore body and the recovery efficiency of the collection well. Obviously, when setting up wells and collecting wells for leachate, try to direct the leachate to the area where the target mineral is enriched. A mathematical model of the nature of the material and geological data produced from the data determined on site can be used to guide the design of the well site and subsequent industrial production. The correct mathematical model should reflect the flow field throughout the ore body.
The quantitative index of the permeability of rock and soil is expressed by the permeability coefficient, also known as hydraulic conductivity, which can be obtained by Darcy's law:
Where q is the unit seepage flow, also known as the permeation rate, m/d;
Kâ€”â€”permeability coefficient, m/d;
Iâ€”â€”Hydraulic slope, dimensionless.
It can be seen that when I is 2, 9=K, indicating that the permeability coefficient is numerically equal to the seepage flow per unit area when the hydraulic gradient is 1. The larger the permeability coefficient of the rock and soil, the stronger the water permeability, and the weaker the opposite. This law can be applied to seepage and three-dimensional flow problems in heterogeneous, anisotropic media.
Design of C well and well site
The parameters required for the design of the well and wellsite include the leaching fluid pumping and collection speed, flow field, pore volume, and production efficiency. Optimizing the aforementioned parameters is the task of wellsite design. The wellsite design consisting of well depth, caliper, distribution, structural materials and equipment is to conform to the anisotropy of the ore body so as to cover the entire ore body as much as possible. Arrangement of wells along the easy-penetration direction may cause solution short-circuits and should therefore be aligned in the vertical direction. The liquid level in the liquid well is higher than that of the ore body to ensure contact between the solution and the ore body. The injection pressure of the solution should be less than the pressure of hydraulic fracture to avoid loss of solution. These conditions can be estimated not only in advance, but also directly after the start of in-situ leaching.
Wellsite design To achieve the goal of most efficient metal recovery, four factors need to be considered:
(1) duration of operation;
(2) Recovery rate of the target mineral;
(3) The concentration of copper in the produced liquid;
(4) The time during which copper appears in the solution, commonly referred to as the break through time.
The shorter the time from penetration to reaching the maximum concentration, the more advantageous. A good wellsite design allows the leachate to penetrate the target mineral and the space around each well to the maximum extent. It is believed that the mineralized rock mass should be at least 30m thick to be effectively immersed in the ground, otherwise it will easily lead to solution loss. [next]
San Gonuel Mine
Magma's San Manuel mine began mining underground sulphide mines in 1955. It then withdrew from the well and dismantled its support. The surface began to sink in the 1960s. The upper part is oxidized ore. In 1985, open-pit mining began and copper was produced by an extraction-electrowinning process. However, the distribution of oxidized ore is irregular, and a large amount of ore cannot be mined.
In 1988, in-situ leaching was used to inject a solution from a group of well-distributed wells, and the leachate was collected from the original working surface of 725 m deep underground and sent to the extraction. However, due to factors such as mining and geological structures that have been carried out, the flow of the leachate is difficult to control. Therefore, in 1989, a well-to-well in-situ leaching method had to be redesigned.
The wells in the well site are distributed in a group of seven wells, the liquid collection wells are in the middle, and the six injection wells are distributed in a flat hexagon. Each group is connected by a common angle, as shown in the following figure. The injection well is 12m away. Three wells were drilled in the well field test. The diameter of the injection well was 3.8cm and the diameter of the collection well was 15.25cm. The wells are lined with PVC pipes. Because there is a break in the middle of the ore body in the test area, in order to avoid the break, the injection well only holes in the upper and lower parts of the PVC pipe, and the wells with a depth of well to 150m are only in the well. A hole was made in the lower part of the pipe wall. The submerged submersible pump is installed in the well for pumping, and the flow rate is monitored by an electromagnetic flowmeter. The well level is measured hourly by a data acquisition system in the collection well and is adjusted by the injection rate of a nearby injection well. Initially controlled at 36m, then gradually reduced to a depth of 60m, which will result in a decrease in the production of the collection well. The injection speed was 1147 L/min, the liquid collection well flow rate was 1000 L/min, and about 13.5% of the solution was lost. Part of it is infiltrated into the deep formation by gravity, and some of it may flow into the surrounding well. At the same time, it cannot be ruled out that part of the liquid collected in the collection well is from groundwater.
In the well field test, the hydraulic conductivity (m2/d), hydraulic conductivity (m/d), permeability (m/d) and storage coefficient were measured at different periods. These parameters of the three groups of wells are quite different, and after leaching, the hydraulic conductivity, hydraulic conductivity and permeability increase, apparently due to the dissolution of minerals. However, close to the fluid collection well permeability declines and some of which may be a solution of ingredients (such as gypsum, jarosite) caused precipitation.
The relationship between the concentration of injected acid and the concentration of copper produced during the leaching test is shown in the figure below. The average composition of the injection liquid of the three groups of wells was (g/L): Cu-0.20 (final 0.15), Fe-15.03, and acid-24.2. The liquid collection well produced (g/L): Cu-1.39 (final 1.27), Fe-15.08, acid-24.2, and also contains Mg and Al. [next]
The acid concentration was increased to 31.56 g/L during the 20 days from the start of the leaching for 220 days, and the test for reducing the acid addition was carried out from the 392th day, and the average concentration was lowered to 20.3 g/L. Acid consumption is proportional to the acid concentration in the injection. The copper concentration curve shows that the direct phase drops rapidly, and then a stable phase is performed, which does not increase due to the increased acid concentration of the implant. There was no significant decrease in the concentration of the injected acid (the drop at 415 days was caused by the well pump, not the leaching). The ratio of the amount of acid consumed to the amount of copper leached was 7 or more for most of the time, and decreased to about 3 after reducing the amount of acid used only for 392 days. Tests have shown that a raffinate containing 20 g/L of acid is sufficient as a leachate, and an excessively high acid concentration increases the reaction of the gangue and promotes the dissolution of magnesium and aluminum . Early column immersion experiments also showed this trend. The 551-day test recovered 1000 tons of copper and consumed 5,900 tons of acid.
After the end of the test, the rock samples drilled in representative areas before and after leaching were used for comparative analysis, and the geometric and median system models were used to calculate the change of the grade, and the recovery was calculated. The calculation result of the geometric method is that the acid-soluble copper recovery rate is 63.9%, and the median system method is 58.4%, which are quite close, but the latter has a relatively large relative error. Interestingly, the ratio of acid-soluble copper to total copper before and after leaching decreased by only 7%, indicating that "acid-insoluble copper" was also partially leached. Although the calcium concentration in the leachate is always around 0.5g/L due to the solubility of calcium sulfate, the average content in the ore body also changes significantly, always around 1.2%. However, rock sample observed calcium apatite minerals, montmorillonite is changed, the bottom header rock sample wells filled with some or all of calcium sulfate. It has also been observed that copper silicate minerals are covered and may have an impact on leaching.
1 Hiskey JB, Intern Symp. Hydrommetallurgy'94, IMM, SCI. Cambridge, England, 11 to 15 July 1994, Chapman & Hall, London, 43~67
2 Bell SL, Glenn D, Welch P, Bennett PG, Hydrometallurgy, 1995, 39, 11~23
3 Wadsworth ME, Intern. Conf. Hydrometallurgy, 1983, Atlanta USA, 1
4 Richard B, Dan R, Proc. Copper'95-Cobbe95, vol III, Electrorefinning and Hydrometallurgy of Copper, eds. Copper WC, Dresinger DB, Dutrizac JE et al. Nov. 26~29, 1995, Sandiago, Chile, TMS , 363~375
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