Fixer for recycling silver
The method for recovering silver from the fixing solution includes a precipitation method, a displacement method, a hypochlorite method, a sodium borohydride method, a sodium dithionite method, and an electrolysis method.
In the waste fixing solution, silver is often present in the form of a complex such as Ag(S2O3)23-, and the concentration of silver is 0.5 to 9 g/L.
First, the precipitation method
Sodium sulfide is added to the fixing solution to react Ag(S2O3)23-plasma with S2- to form silver sulfide precipitate and solution separation:
2Ag(S2O3)23-+S2-=Ag2S↓+4S2O32-
There are several methods for recovering silver from a silver sulfide black precipitate:
The nitric acid dissolution method dissolves the silver sulfide with dilute nitric acid (acid:water = 1:2 to 3) to produce a silver nitrate solution and elemental sulfur, and filters to obtain a silver nitrate solution.
Ag2S+4HNO3=2AgNO3+S0+2H2O+2NO2↑
Method for firing a silver smelting sulfide in 700 ~ 800 ℃ roasting, sulfur oxidized to sulfur dioxide into the gas phase, silver oxide and silver is generated subsequently decomposed to silver metal.
Ag2S+1.5O2=Ag2O+SO2↑
2Ag2O=4Ag+O2↑
Substitution method in hydrochloric acid solution at room temperature, the silver may be displaced from the silver sulfide, iron scrap:
Ag2S+Fe+2HCl=2Ag+FeCl2+H2S↑
Sulfide precipitation is simple and easy, silver recovery is complete, suitable for small-scale use, but there is an excess of sodium sulfide in the post-silver solution, and the fixer cannot be regenerated.
Second, the replacement method
Available iron powder, zinc powder, aluminum powder as a reducing agent, silver fixing solution reduced to metallic silver. This method is efficient and simple, but the main disadvantage of the metal replacement method is that the replacement metal dissolves into the solution, making the fixing solution difficult to regenerate. For example, iron replacement is the addition of iron or iron filings or iron powder to the acidic fixing solution, and the silver is replaced by a reduction precipitate:
2Ag(S2O3)23-+Fe=2Ag↓+Fe2++4S2O32-
The replacement product contains silver powder, iron powder, silver sulfide, etc., and is black. After further purification, crude silver powder can be obtained. In an acidic medium, S2O32- in the fixing solution decomposes to produce elemental sulfur and sulfur dioxide.
Third, hypochlorite method
Hypochlorite decomposes the silver-thiosulfate complex in the spent fixer, while silver forms a silver chloride precipitate and separates from the solution.
8ClO-+Ag(S2O3)23-+4OH-=AgCl↓+4SO42-+2H2O+7Cl-
For example, when a fixing solution containing 6 g/L of silver is treated, it is treated with a mixed solution containing 10% to 15% NaClO and 1 to 1.5 mol/L NaOH. The sodium hypochlorite method consumes a large amount of sulfurized sulfate and requires a large amount of chemicals.
Fourth, sodium dithionite method
This method is a simple and effective method for extracting silver from waste fixing liquid. First adjust the pH of the solution to near neutrality, adjust with glacial acetic acid and sodium hydroxide, or use ammonia water.
Then, solid or liquid sodium dithionite (Na2S2O4) was added to the waste fixing solution, heated to 60 ° C and stirred vigorously, and silver was precipitated from the solution. However, if the pH is too low, even sodium disulfite or sodium thiosulfate will decompose to produce elemental sulfur, and the same phenomenon occurs when the temperature exceeds 60 °C. The method is simple in process and high in efficiency; the fixing solution can also be recycled.
Five, sodium borohydride method
Sodium borohydride (NaBH4) is a strong reducing agent that was widely used in the chemical analysis field in the early days and was later applied to the separation and extraction process of precious metals. Some factories in foreign countries have replaced the traditional zinc powder, iron powder replacement method and sodium sulfide precipitation method with this method, and have shown their advantages when dealing with small batches and low concentration waste liquids. The silver in the spent fixing solution is recovered by sodium borohydride, mostly under the condition of pH=6-7, and the amount of sodium borohydride added is determined according to the silver content in the solution. Generally: Ag: NaBH4 = 1: 0.45 (mass ratio). The following reaction occurred:
8Ag(S2O3)23-+NaBH4+8OH-=NaBO2+6H2O+16S2O32-+8Ag
Sodium borohydride and sodium hydroxide can be firstly formulated into a solution containing NaBH4 12%, NaOH 40%, diluted 10 times when used, and slowly added to the waste liquid according to the silver content in the fixing solution to form a black precipitate of silver.
6. Electrolysis
The electrolytic recovery of silver in the silver-containing waste liquid and the fixing liquid exhibits many advantages both technically and economically. Many studies, improvements, and developments have been made in various forms of electrolyzers, electrolyzers, or silver machines.
According to the structure of the equipment, in summary, the electrolytic silvering machine can be divided into two categories: a slotted electrolytic silvering machine and a closed-cell circulating electrolytic silvering machine. Among them, the slotted type silvering machine has been basically eliminated due to low efficiency, large floor space, harmful gas production and environmental pollution, and it is generally used to conduct silver extraction by a closed mechanical stirring electrolytic silver machine.
In combination with domestic reality, China has developed a closed-cell high-efficiency silver feeder. Its outer casing is a plastic cylinder and has a sealing cover. The inner lining has a thickness of 0.5 to 1 mm stainless steel as a cathode, and a hollow graphite cylinder stands in the center of the cylinder as an anode. The anode-anode distance is 35 to 40 mm, and the solution is closed in the machine. The fixing liquid enters between the cathode and the anode at a high speed from the lower part of the cylinder in the tangential direction, spirally rises rapidly, flows out from the upper part in the tangential direction, and enters the liquid storage tank. The waste fixer is continuously circulated at high speed with a pump, and a higher current density can be used.
Electrolytic technical conditions are as follows: tank voltage 2 ~ 2.2V; liquid temperature: 20 ~ 35 ° C; current density 175 ~ 193A / m2; circulating fluid volume: 510L; electrolysis time: containing silver 3 ~ 4g / L, 3 ~ 4h, Containing silver 5~6g/L, it takes 5~6h; after electrolysis, the liquid contains silver: the stock solution contains 2.5~9.3g/L of silver, and the post liquid contains 0.5~0.7g/L of silver. When the tail liquid is not regenerated, it contains silver. Dropped to 0.15g / L); silver grade: 90% ~ 93%, after casting can reach 98% ~ 99%; silver recovery rate of about 95%.
Tungsten carbide roller has characteristics of good wear resistance, high temperature red hardness, thermal fatigue resistance and thermal conductivity and high strength , have been widely used in high-speed wire rod, bar, rebar, seamless steel tubes, etc. Domestic production of tungsten carbide roller materials mostly WC- Co, WC- Co- Ni- Cr two series, and the content of Co, Co- Ni - Cr is in the range of 6wt% ~ 30wt%. From the use of perspective, tungsten carbide rollers has good mechanical properties, its flexural strength up to 2200 MPa or more, shock toughness up (4 ~ 6) × 10^6 J/ m^2, Rockwell hardness (HRA) is up to 78 to 90, widely in the high-speed wire rod rolling process, which is much higher than single-slot chilled cast steel or high speed steel rolls.Tungsten carbide is made of Tungsten Carbide Powder and binder phase (such as drilling, nickel, etc.), and then pressing and sintering, regardless of the conditions under cold rolled or hot rolled has excellent wear resistance, tungsten carbide rollers has been widely used in pre-finishing mill and finishing of high-speed wire rod currently. On the performance of tungsten carbide roller in hot-rolling wire rod , the material must meet the following requirements:
1. Sufficient anti-fracture strength;
2. Good abrasion resistance;
3. The smooth surface finish;
4. Excellent corrosion resistance, thermal fatigue, thermal cracking performance.
Tungsten carbide roller rings can working in bad conditions , small profile rolling (especially rebar rolling) process conditions is harsher than the high-speed wire rod, and therefore corresponds to the profile rolling ,tungsten carbide rollers recommend using high binder phase carbide.
Tungsten carbide roller material design
Pre-finishing all vehicles roller should ensure its high toughness, strength, rigidity and thermal conductivity, followed before considering its wear resistance. When designing each vehicles roller, pre-finishing materials should choose carbide grades of Co, Co- Ni- Cr binder content is high (greater than or equal to 25wt%) , requiring an average WC grain size of coarse (5μm ~ 6μm), to obtain a higher shock toughness, proper strength and hardness. For the finishing of the roller movements, particularly the last two rollers of the finish rolling, which suffered load is small, and high relative velocity of the material to be pressed (80 m / min ~ 120 m / min). In this case, the wear resistance of the roller to be the most important requirements, and must ensure the strength , timpact toughness and hardness of a reasonable match, so the binder of Co / Ni content ratio and the average grain WC control of particle size and other factors must have greater control in front of different pre-finishing rolling roll.
Tungsten carbide roller category
According to the structure of tungsten carbide rollers, it can be divided into solid tungsten carbide roller and composite tungsten carbide roller. Solid tungsten carbide rollers have been widely used in pre-finishing and finishing stands high speed wire rod mill (including fixed reducing the rack, pinch roller rack). Composite tungsten carbide roller is made of cemented carbide and other materials, and it can be divided into tungsten carbide composite roll rings and solid tungsten carbide Composite Roller. Tungsten carbide composite roll rings mounted on the roller shaft; solid tungsten carbide composite roller will be directly cast in the roll axis to form a whole, a large load is applied to the rolling mill.
Tungsten carbide roller production process control
Tungsten carbide rollers produced by powder metallurgy method, the key to its process control is the chemical composition of the material and the mixture was prepared, pressed molding, sintering and deep processing and other preparation process parameters.
1. Preparation of starting material (WC focus quality): As the WC raw material and quality control of the use of different levels of quality may fluctuate, resulting in adverse effects on microstructure.
2. Preparation of the mixture: Mixture preparation is the key to the production process of the roller, the roller of failure modes - trachoma, mainly generated by this procedure.
3. Pressing: roller pressing is an important process of the roller mill.
4. Sintering: roller sintering is to determine the final quality of the roller production processes, use of advanced low-pressure sintering technology, HIP sintering technology can greatly improve the performance of roller.
5. Deep processing: deep processing rollers have a greater impact on the quality and accuracy of the roller surface.
Carbide Roller,Carbide Ring,Mill of Rolling Line,Steel Wire Rod,Mill Rolling Finish Stand,Mill Roller Groove Type
Luoyang Golden Egret Geotools Co., Ltd , https://www.xtchvof.com