Cinnabar is red, with high density, brittleness and good natural floatability. It can be selected by preselection, gravity separation and flotation or by a combination of several methods.
1. preconcentration
The hand selection of mercury ore is the traditional preselection method. Cinnabar is red or dark red, easy to identify with the surrounding crop and gangue minerals. At the same time, due to the brittleness of cinnabar, the ore is easy to break from the dense cinnabar embedment in the crushing process, and cinnabar is often exposed to the crushing surface of the ore, which is conducive to hand selection. Hand selection is usually carried out on the hand selection belt with a speed of 0.1 to 0.4 m/s. Large ores are hand-selected, and sometimes special hand-selected tables are set up before coarse crushing. Hand selection has two ways of picking out concentrate and picking out waste rock, the former is used for the case of low ore grade, the latter is used for the case of high ore grade. The ore usually needs to be coarsely crushed and graded before the beneficiation. The typical hand selection process in China is shown in the following figure. The hand-selected grade shown in the figure is 20 ~ 100 mm, and some mines divide this grade into several grades for hand-selected. The indexes selected by hand vary greatly in different ore grades and different periods. Generally, the rate of hand-selected waste rock is about 40-70% (for the amount of selected ore). The ore is often washed by washing to remove the slime on the ore surface before hand sorting. Due to the good natural floatability of cinnabar, fine cinnabar is easy to be lost with the washing water, so there must be corresponding facilities (such as sedimentation tank) to precipitate and recover fine cinnabar in the washing water.。
Hand-selected flow chart
Crushing and screening is another preselection method for mercury ore. Like all brittle minerals, cinnabar is enriched in fine grained ores as a result of selective crushing during the crushing process. Select the appropriate size of the screen for classification, discard the upper level of the screen, so as to achieve the purpose of preselection. At the Pedernales mercury mine in Mexico, the raw ore is crushed and screened, the +32 mm particle grade, which accounts for one-third of the raw ore, is discarded, and the -32 mm particle grade is returned to the rotary cellar for mercury smelting. The Sulphur Bank sorting plant in the United States used a 225mm grid to grade raw ore, discarded the + 225mm coarse block, and then entered the screening, washing hand sorting and gravity sorting of the -225mm particles.
2.重选
Cinnabar has a high density, which is conducive to the treatment by gravity separation. However, in production practice, a single gravity separation process is generally not used to treat mercury ore. The reasons are as following: First, the cinnabar is mostly disseminated with uneven coarse-fine or fine-grained intercalation, and it is difficult to recover the fine cinnabar fully and effectively in the re-separation process. Second, due to the brittleness of cinnabar and good natural floatability, the fine cinnabar produced in the grinding process is easy to float on the water surface and lose in the gravity separation. Therefore, the re-sorted tailings are often treated by flotation after re-grinding. From 1959 to 1963, a single gravity separation process was used to treat ores with an average grade of 0.186%(Hg), with an average recovery rate of 56%(Hg), an average grade of 0.087%(Hg) in tailings and an average grade of 3.4%(Hg) in concentrates in Xinhuang Mercury Mine. It was later changed to a single flotation process in 1964. Therefore, mercury concentrator often adopts the combined process of heavy and floating. Gravity separation is only used to recover a part of the coarse cinnabar that has been dissociated from the monomer, or to select a part of the cinnabar product.
The most common applications for gravity separation of mercury ores are shakers and jig machines. The shaker can obtain a high quality concentrate for the production of cinnabar products. The jig is mostly used to deal with the pre-screened particle grade of the ore before grinding, or set in the grinding classification circuit to recover the cinnabar of monomer dissociation in time to avoid over-crushing. At the same time, a part of the coarser cinnabar product can be recovered from the jig concentrate. In recent years, some selection plants are trying to replace the rough selection shaker jig. A jig and a natural mercury trap are used to recover most of the natural mercury in the raw ore at the Isomuka concentrator in Japan.
3.flotation
Flotation is the most common and effective method in mercury ore dressing. Flotation can recover not only cinnabar but also natural mercury, chloro-sulfide mercury ore and other mercury minerals effectively. Flotation is also used to treat polymetallic ores containing mercury.
The main target mineral in most mercury ores is cinnabar. Cinnabar has good natural floatability, so the flotation process and reagent system of single cinnabar type ore are relatively simple. The process commonly used is one or two stage grinding, and the grading overflow particle size is generally ~85%, -200 mesh, and the flotation particle size is coarser in individual sorting plants. The flotation process often uses a rough selection, one to two times of selection, two to three times of sweeping. Flotation agents, many heavy metal salts as the activator of cinnerite, such as copper sulfate, lead nitrate, lead acetate, mercury chloride and so on. Copper sulfate is the most widely used, and the general dosage is 100~300 grams/ton. There are also options that do not use activators. The collector of cinnabar is xanthate. Domestic factories generally use ethyl xanthate, the amount of 80~240 g/ton. Some factories have used xanthate and black medicine mixed drugs, and the selection index is similar to that of xanthate alone. The ore containing cinnabar and chloro-thionite (with a ratio of 7:3) was treated by the McDermitt concentrator in the United States with isopropyl xanthoxanthate as collector. Amyl xanthoxanthate was used as collector in the treatment of ores containing 70% natural mercury and 30% cinnerar at Isomuka concentrator in Japan. Some experimental data show that tannin, tannin extract, citric acid, carboxymethyl cellulose, starch and sodium silicate can be used as gangue inhibitors or slime dispersants in order to improve the concentrate grade of flotation of low grade mercury ores. However, it is not used much in production practice. The optimal pulp pH value of cinnabar flotation is 6 and 7.5~7.8, but in the pH value of 5~8.5, cinnabar can also maintain a high flotation recovery. Therefore, the general concentration plant does not use pH regulator, and the natural pH value of the pulp is roughly between 6 and 8.
For single cinnabar type ore, the flotation can obtain a good separation index. When the raw ore grade is 0.1~0.5%(Hg), the flotation recovery rate can reach 90~96%. When dealing with more complex ores, the recovery rate is slightly less than 90%. The grade of flotation concentrate varies greatly among different concentrators, with the majority of domestic concentrators generally ranging from 10 to 30%(Hg), the highest of foreign concentrators reaching 75%(Hg) and the lowest only 1.5%(Hg). In addition to the complexity of raw ore grade and mineral composition, the grade of concentrate is often related to the smelting method of concentrate.
Flotation plant equipment, the domestic selection plant mostly uses small and medium-sized grinding equipment and mechanical agitation flotation machine. Flotation column is used as roughing equipment in a few sorting plants, and its technical and economic index is similar to that of mechanical agitation flotation machine. Some large-scale mills abroad use autogenous mills and larger flotation machines.
