Study On Recovery Of Tin From Fluorinated Tin Sludge In Electroplating Industry By Alkaline Leaching-zinc Cementation

In recent years,with the increasing demand for tin as well as the depletion of mineral resources,increasing attention has been paid to the development of recovery methods of tin from tin-containing wastes.In this paper,the alkaline leaching-zinc cementation process of fluorinated tin sludge generated from electroplating plant was further systematically studied on the basis of the previous study according to the principle of selective leaching of tin in alkaline solution.The parameters of alkaline leaching and cementation reaction were optimized and thermodynamic and kinetic analysis was carried out.During the cementation reaction process,activators and dispersants were screened and the cementation reaction mechanism was discussed.Zinc dust was recovered by alkaline electrodeposition technology(₉)4+<100mg/L),and sodium fluoride in alkaline leaching residue was recovered by the same ions effect.The main conclusions are as follows:?1?Alkaline leaching parameters of fluorinated tin sludge were further optimized.The optimum leaching conditions of fluorinated tin sludge were temperature of 90?,NaOH concentration of 120 g/L,stirring speed of 500 rpm?even mixing?,liquid-solid ratio of 12,and reaction time of 60 min.The maximum leaching proportion of tin could reach to 98.64%.The core shrinking model was used to study the kinetics of fluorinated tin sludge alkaline leaching.The results showed that the alkaline leaching process was controlled by external diffusion within 0-10 minutes,while controlled by interfacial chemical reaction within 10-90 minutes.The apparent activation energy of the leaching reaction was 42.92 kJ/mol.?2?The thermodynamic results of tin cementation by zinc dust in NaOH solution showed that the cementation reaction was a spontaneous redox reaction and the reaction was thorough.The addition of potassium antimony tartrate(Sb³⁺)can increase the reaction rate,shorten reaction time and disperse tin particles.The purity of tin can reach to 98.53%and Sn⁴⁺concentration in solution is less than 100 mg/L under the optimum reaction conditions with the temperature of 90?,NaOH concentration of 120 g/L,tin concentration of 20 g/L,stirring speed of 500 rpm,100mg/L Sb³⁺,molar ratio of Zn:Sn=2.6:1,reaction time of 45 min.The results of the study on the action mechanism of Sb³⁺showed that Sb³⁺reacts with zinc dust in NaOH solution and forms SnSb alloy with Sn produced at the same time,which provides a new reaction substrate for tin cementation by zinc dust.The ratio of cementation electromotive force?EMF?and the reduction current of tin cementation by zinc dust increased greatly in the presence of Sb³⁺,thus increasing the ratio of nucleation and grain growth of tin powders and resulting in more fine grains.On the other hand,the overpotential of hydrogen evolution shifts 0.1 V in the positive direction,and the reduction current of hydrogen evolution potential increases greatly compared with the blank.It showed that the side reaction of hydrogen evolution becomes easier and more intense when Sb³⁺is added to NaOH solution,and the tin product is further dispersed by bubbling.Dispersion of tin results in tin not adhering to the surface of zinc dust and increases the contact area between zinc dust and solution,thus increasing the cementation reaction rate and the purity of cemented tin.?3?The kinetics of tin cementation by zinc dust in the presence and absence of Sb³⁺was studied by shrinking core model.The results showed that the tin cementation reaction by zinc dust was affected by ash diffusion control in the absence of Sb³⁺,while the cementation of tin by zinc dust was controlled by chemical reaction in the presence of activator Sb³⁺.The activation energy of cementation reaction?Ea=64.83kJ/mol?in the presence of Sb³⁺was basically the same as that in the blank?Ea=60.44kJ/mol?,while the pre-exponential factor?A=1.95?in the presence of Sb³⁺was significantly higher than that in the blank?A=0.23?.Therefore,it can be concluded that the cementation reaction mechanism has not been fundamentally changed by adding Sb³⁺,and the reaction rate has increased only because of the dispersion,which results in the increase of the number of active sites in zinc dust surface.?4?Overall optimization of alkaline treatment process for fluorinated tin sludge was studied.Concentrations of NaOH and Sn⁴⁺in alkaline leaching solution were52.66 g/L and 25.95 g/L,respectively.The mass concentration of NaOH and Sn⁴⁺in leaching solution can be adjusted to 142.33 g/L and 20.42 g/L respectively by adding450 g/L NaOH solution to the leaching solution at the volume ratio of 10:3.The purity of tin powder can reach to 95.81%and Sn⁴⁺concentration in solution is less than100 mg/L under the optimum reaction conditions of temperature of 90?,stirring speed of 500 rpm,zinc dust dosage of Zn/Sn molar ratio Zn:Sn=3:1,100 mg/L Sb³⁺,reaction time of 90 min.The purity of zinc powder obtained by electrodepositing cemented solution?current density 1000 A/m²,electrolysis time 1h?can reach to96.55%.The main component of NaOH leaching residue is sodium fluoride and it can be recovered by leaching in deionized water and adding NaOH to the leaching solution.The maximum leaching proportion of sodium fluoride can reach to 85%under the optimum reaction conditions of 80?and liquid-solid ratio 20:1.With the optimum dosage of 40%NaOH,the purity of sodium fluoride product reaches to95.66%.Metallic tin can be recovered from fluorinated tin sludge by alkaline treatment process,and zinc dust can be recycled through electrolysis.Sodium fluoride in alkaline leaching residue can be recovered by deionized water leaching and adding NaOH.The whole process is technically economical.