Study On Recycling Of Low - Carbon Grade Waste Cathode By Flotation Method

Spent pot-lining or spent pot liner (SPL) is a kind of toxic solid waste discharged from Aluminum smelting plants and can contaminate environment while piled in the open air. The valuable ingredients in SPL, namely carbon, cryolite, sodium fluoride and calcium fluoride, can be recycled as renewable resources. As an effective approach, flotation can comprehensively recover values in SPL while meeting environmental requirements. However, with low carbon grade SPL (<40%), floatation method can only get bad floatation index and faulty floatation products, and require a further treatment. These refining methods include acid leaching and/or basic leaching for floatation concentrate and calcining for floatation tailings. The further treatments will extremely raise the cost and make floatation method lose its competitive advantages.To get better floatation index, the floatation process should be improved to solve these problems. This article has been devoted to enhancing floatation index of refractory SPL with low carbon grade (36.1%) by optimizing floatation conditions, improving floatation reagents and flow sheet. In addition, experiments have also been conducted to treat waste water produced by the floatation experiments to meet the national standard for waste water discharging. It should be emphasized that the treated waste water can be recycled.Primarily, X-ray diffraction, ultimate analysis, optical microscope analysis and leaching-toxicity analysis have been utilized to reveal composition, microstructure and leaching toxicity of SPL. The results show that main ingredients in SPL are carbon and electrolyte (including cryolite, sodium fluoride and alumina etc.) and the interfaces between carbon and electrolyte particles are distinct, which suggests separation of the particles by mechanical disruption is possible. Carbon in SPL is highly graphitized carbon, which is strongly hydrophobic so that it can be purified by froth floatation. Soluble fluoride ion and cyanide ion concentration in SPL are as high as 2602.7mg/L and 24.6mg/L, respectively. The concentrations are far higher than national standard. Thermodynamics calculations show that toxic materials in SPL, such as sodium cyanide and sodium fluoride, will not let out toxic gases while reacting with neutral and basic water. Thus, floatation process conducted with basic pulp is safe.Besides, Particle Image Velocimetry (PIV) technology was applied to analyze flow field of XFD1.5L floatation machine, which is main equipment for the floatation experiments. Flow field structure has been detected. Variation tendency of fluid velocity along with increasing of stirring rate has been analyzed. Average bubble diameter, number density and bubble velocity related to flow field have been measured. All the above researches provide a basis for theoretical calculation of SPL floatation dynamics. The results show that stirring tank can be divided into three zones, where the district between Y=0mm and Y=25mm is "stirring zone", the district between Y=25mm and Y= 120mm is "separating zone" and the district between Y=120mm and Y=150mm is "froth zone". To remove the "dead zone", the structure of "arc bottom" is suggested.Afterwards, floatation dynamics research has been done under varied grinding fineness, stirring rate and pulp density to fit dynamics functions. And optimum floatation conditions have been fixed, which are grinding fineness of-0.074mm particles taking 90% by mass fraction, stirring rate of 1700rpm and pulp density of 25%. Floatation dynamics research shows that the floatation process of SPL meets first-order kinetic equation. The function relationships between maximum recovery and stirring rate, aeration rate and pulp density are single peak functions, respectively. When stirring rate increases together with aeration rate, maximum recovery shows two dimensional parabolic function with the two independent variables as follow: z=-310.79+0.489x-477.54y-1.31×10-42+828.52y2Where z is maximum recovery, x is stirring rate, y is aeration rate.And maximum recovery shows parabolic function with pulp density as follow: z3=34.06+2.7C-0.039C2Where z3 is maximum recovery, C is pulp density.In addition, experiments have been done on using mixed oil and oil emulsion as collector, adding collector by stage, varied depressor dosage and adding method, the effect of foaming agent, dispersing agent and pH regulator on floatation index. With gasoline mass fraction of 75%, relative to gasoline or kerosene, the mixed oil of gasoline and kerosene is a better collector which can get floatation concentrate with either higher carbon grade or recovery. Compared to kerosene, kerosene emulsion can significantly improve carbon grade and recovery of concentrate. For kerosene emulsion, emulgator plays an important role, which can not only lower oil-water interface tension to make stable emulsified oil but also improve carbon recovery of concentrate. For the same dosage of collector, relative to adding collector at one time, adding it by several times can improve floatation index. And the bigger the number of adding times is, the better the floatation index is. Foaming agent can increase floatation rate and concentrate carbon grade. The optimum dosage sodium silicate is 500g/t. By adding sodium silicate in the mill, it can be more effectively used. By adding dispersing agents and pH regulators concentrate carbon degree can be slightly raised, while carbon recovery sharply decreases. Therefore, dispersing agents and pH regulators are not suggested to be added.Finally, through floatation-process improving, four roughing-double cleaning closed circuit floatation process was adopt. And a concentrate with carbon grade of 82.1%, carbon recovery of 98.1%and a tailing with carbon content of 1.21%can be obtained by applying this floatation process. That means the problem of floatation produces with bad quality has been settled. Both of concentrate and tailing with F- and CN- content lower than national standard are no more toxic solid wastes and can be safely used as renewable industrial resources. The floatation waste water, first treated by lime and bleaching powder sedimentation and second Polymeric Ferric Sulfate (PFS) sedimentation, can meet national waste water discharge standard with F- and CN- content being decreased to 4.1mg/L and 0.03mg/L. And the treated waste water can be recycled.As a result, floatation concentrate, tailing and sediment of first stage sedimentation have been obtained. The floatation concentrate, which contains carbon with mass fraction higher than 80 percent and a small quantity of cryorite, calcium fluoride and β-alumina, can be used as fuel, reducing agent and raw material for carbon electrode making. The floatation tailing, which mainly contains cryorite, calcium fluoride, aluminium fluoride,β-alumina and carbon with mass fraction lower than 2 percent, can be reused in aluminum smelter for primary aluminum producing. Sediment of first stage sedimentation is calcium fluoride and can be recycled as industrial raw material.