Mechanical Activation On Structure Of Nickel Slag And Its Carbothermal Reduction

Nickel slag is a waste slag produced in the nickel smelting process.The slag has a high iron content and contains different amounts of valuable metals such as nickel,copper,cobalt,gold,and silver.The comprehensive recovery of iron,cobalt and nickel has important social benefits and is expected to create certain economic benefits.Since the iron in the nickel slag exists in the form of Fe₂Si O₄,the conventional reduction effect is not good.Pretreatment of the nickel slag before reduction is an effective measure to promote subsequent reduction.In this paper,the effects of mechanical activation on nickel slag structure change and carbothermal reduction were studied.The particle size distribution,specific surface area,crystalline phase,surface morphology,functional groups and mechanical energy storage of nickel slag after activation were characterized.Effect of nickel slag structure changes.Using high-purity graphite as a reducing agent,the nickel slag activated at different times was directly reduced in a high-temperature furnace.The effects of mechanical activation on the reduction rate,metallization rate,and iron particle size of nickel slag were analyzed.The research results obtained are as follows:（1）Mechanical activation can effectively reduce the particle size of nickel slag particles and increase mechanical energy storage.When the activation time is increased to 12h,the average particle size of nickel slag is 0.531μm.The grain size of Fe₂Si O₄ phase in the slag is 16.3nm,the dislocation density is 168.3×10¹⁴m^-2,the amorphous fraction is35.5%,the corresponding surface Gibbs free energy is 0.257 k J·mol^-1,the grain boundary energy storage is 0.0667 k J·mol^-1,the dislocation energy storage is 2735.3 k J·mol^-1,and the amorphous energy storage is 26.16 k J·mol^-1,which is equivalent to the total Energy storage is 1790.4 k J·mol^-1.（2）Mechanical activation not only changes the relevant parameters of the Fe₂Si O₄ phase in the nickel slag powder,but also destroys the Si-O-Si bond and the Si-O bond in the Fe₂Si O₄,making the structure of the difficult-to-reduced Fe₂Si O₄ simple and turning it into free Fe and Si O₂,easier to reduce.（3）It can be seen from the thermogravimetric curve that the carbothermal reduction reaction rate of nickel slag activated at 4 hours and nickel slag at 12 hours is significantly different,and the start temperature and end temperature of the reaction are greatly reduced.The longer the mechanical activation time,the faster the reaction rate.（4）Temperature and time are the key factors influencing the carbon thermal reduction of nickel slag.Raising the reduction temperature and extending the holding time can effectively promote the reduction of nickel slag.The metallization rate of nickel slag activated for 12 hours after 30min reduction at 1273K can reach 72%.When the reduction time is extended to 70min,the metallization rate can be increased to 83.12%.（5）Nickel slag with a longer mechanical activation time,due to its finer particle size and increased mechanical energy storage,is more conducive to the reduction of nickel slag and the growth of iron particles in the reduction product.Mechanical activation of nickel slag can effectively promote the reduction reaction of nickel slag.By comparing the products of nickel slag activated for 4 hours and 12 hours after reduction at 1273K for 50min,it can be found that the nickel slag iron particles with a long activation time have obvious aggregation.Analysis of the reduction product shows that the activation effect is the best at 8 hours.Although the activation effect continues to increase after 8hours,the increase rate decreases.