During the cyanidation operation and during the storage of the cyanide-containing solution, it is necessary to maintain a certain concentration of free alkali in the solution, which is referred to in the literature as "protecting the base". It neutralizes the mineral acid produced during the process, prevents the decomposition loss of cyanide and produces HCN gas, and obtains the pH required for the gold to dissolve normally. Lime is a common base used in most factories. It is cheap and contributes to the agglomeration of solid materials in the slurry, which accelerates the concentration and filtration of the slurry. First, the amount of lime and alkalinity The lime concentration limit in the solution is about 0.15% CaO. The concentration during normal operation ranges from 0.002% to 0.012% CaO. At this time, the corresponding pH is 9-12. The step number plant is operated under the condition of “negative alkalinity†(ie, the solution is close to slightly acidic), and the concentration of free cyanide ions is titrated after the addition of phenolphthalein. The operation of some plants is carried out under high alkalinity conditions, which contributes to the decomposition of the telluride . Most plants operate under high alkali conditions in order to reduce the loss of cyanide. It is advantageous to use a solution of low pH if some of the sulfides in the ore are more susceptible to oxygen in high pH solutions. In order to quickly dissolve gold in certain minerals, the pH of the solution should be maintained at least at a level of 9 (Figure 1). Of course, in order to rapidly decompose several tellurides in the ore, it is more necessary to operate under high pH conditions. Figure 1 The dissolution rate of several natural gold and pure gold in different pH solutions In short, the CaO content of the solution must be determined or the pH of the slurry determined based on the specific conditions of the ore. Further, when water containing a magnesium salt, the concentration of CaO in the solution should be maintained at 0.002% or less, to avoid excess lime solution generating the magnesium salt Mg (OH) 2 precipitates. In addition, high alkalinity may contribute to the settling speed of the slurry in the concentrating equipment . Second, the determination of alkalinity The correct method for determining the alkalinity of the slurry is, in principle, the addition of an acid titration to the sample to a pH of about 10 as the end point. However, in order to eliminate the normal interference of cyanide, the classical method is to take the sample with sulfuric acid or oxalic acid standard droplets to check pH 8.3 with phenolphthalein test paper as the end point. The response is: CaO+H 2 SO 4 CaO+(COOH) 2 H 2 O The important thing here is to eliminate the interference of cyanide on titration. Since many cyanides, especially zinc cyanide complexes, interfere with the measurement at pH 8-9, the results are inaccurate, so the cyanide should first be decomposed to form HCN for removal. In pure lime liquor, there is a specific ratio between the percent concentration of CaO and the pH. However, in the practice of cyanidation, the ratio between the percentage of CaO and the pH is likely to change greatly due to the constant change of the composition of the solution. However, the measurement can be used to find out the changes between the two to meet the needs of the operation process. Third, the consumption of lime Lime consumption reported in the literature range from Concentrate Calcine 15kgCaO per ton of ore 0.25 ~ 5kgCaO, per ton of the pyrite. The amount of CaO per unit weight of slaked lime [Ca(OH) 2 ] and quicklime (CaO) is different, and should be distinguished when it is used in production and reading. In practice, CaO entering the solution increases the consumption of lime due to the precipitation of calcium sulphate, which is often difficult to avoid in many plants. The main reasons for the increase in lime consumption include: (1) CO 2 brought in from the air when inflating into the mixer; (2) Acidic sulfides formed by oxidation of other sulfides in the pyrite field; (3) using acidic substances brought in water and certain metal (magnesium, aluminum ) ions; (4) The reaction of certain specific substances in the ore with alkali. Fourth, pH control The pH of the control solution is achieved by controlling the lime concentration of the solution. In most factories, the solution sample is taken every 1~2h, and the lime concentration is controlled by manually adjusting the lime addition amount of the lime feeder according to the measurement result. It is feasible to automatically measure and automatically control the pH of the slurry using a glass electrode, but in the high pH range, the measurement results are often unsatisfactory. Folding mask machine, also known as Lijin C-type mask machine, is a fully automatic machine used for the production of folding mask bodies. It uses ultrasonic technology to bond 3 to 5 layers of PP non-woven fabric, activated carbon and filter materials, and cut out Folding the mask body can process 3M 9001, 9002 and other mask bodies. According to the different raw materials used, the masks produced can meet different standards such as FFP1, FFP2, N95, etc. The ear straps are elastic non-woven fabrics, which make the wearer`s ears comfortable and stress-free. 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CaSO4+H2O Ca(COO) 2 +2H 2 O