Nickel leaching from low-grade nickel matte using aqueous ferric chloride solution
School of Metallurgy,Northeastern University
作者简介：*Zhi-Qiang Ning e-mail:ningzq@smm.neu.edu.cn;
收稿日期：19 July 2019
基金：financially supported by the National Basic Research Program of China (No.2014CB6434085);
Nickel leaching from low-grade nickel matte using aqueous ferric chloride solution
Zhi-Qiang Ning Hong-Wei Xie Qiu-Shi Song Hua-Yi Yin Yu-Chun Zhai
School of Metallurgy,Northeastern University
Abstract：
Nickel leaching from low-grade nickel matte(LGNM) using aqueous ferric chloride solution was studied.The influence of factors such as leaching temperature and concentration of ferric chloride on the nickel leaching ratio was investigated.The results show that increasing leaching temperature and concentration of ferric chloride increases the nickel leaching ratio.The overall nickel leaching process follows the unreacted shrinking core model,and the surface chemical reaction is the rate-controlling step.The activation energy and the reaction order of the nickel leaching process,controlled by the surface chemical reaction,were calculated to be 52.96 kJ mol-1and 0.5,respectively.Therefore,the kinetics equation for the nickel leaching was found to be 1-(1-α)1/3=7.18×104C0.5exp[-52,960/(RT)]t.
Keyword：
Nickel matte; Ferric chloride; Sulfur; Kinetics;
Received： 19 July 2019
1 Introduction
Nickel,which has good plasticity and corrosion resistance.is commonly used to produce alloys and stainless steel
Nickel and other valuable metals such as copper and cobalt were traditionally leached from nickel matte and separated into inpidual metal from the leaching solution by hydrometallurgical.This is because hydrometallurgy has the advantages of low energy consulnption,comprehensively recovery of valuable metals,environment friendliness and economical equipment investment.However,it was inevitable to produce the hazardous gases such as SO2,SO3 or H2S due to the use of sulfuric acid or hydrochloric acid to react with nickel matte containing sulfur
As shown in Fig.1.a new technology in which nickel and copper are simultaneously leached from low-grade nickel matte (LGNM) in an aqueous ferric chloride solution is established.This new technology can take full advantage of nickel,cooper and sulfur.Nickel and cooper are in the form of positive ions in aqueous solution,and sulfur is in the solid-state substance after leaching of LGNM using aqueous ferric chloride solution.Iron,nickel and cooper are still in the aqueous solution,and the solid sulfur can be recycled from the solid residuals using kerosene after filteration 1 21 1.The iron ion can be oxidized to ferric ions after adding hydrogen peroxide to the aqueous solution,and the ferric ions can be first removed and recycled from the aqueous solution by adjusting the pH value.Nickel and copper in the aqueous solution can then be separated by extraction
In this article,the influence of factors such as leaching temperature and the concentration of ferric chloride on nickel leaching ratio were investigated to optimize the leaching conditions and determine the nickel leaching kinetics of LGNM in the ferric chloride aqueous solution.
Fig.1 General process flow diagram
2 Experimental
2.1 Materials
Table 1 shows the major elements present in LGNM,examined by inductively coupled plasma optical emission spectrometer (ICP-OES)
X-ray diffraction (XRD) pattern of LGNM shown in Fig.2 indicates that the main phases consisted of sulfides of the major elements present in LGNM,such as(Fe,Ni)9S8,Cu5FeS4,CuS,NiS,CuS2 and FeS2.The free sulfur was not observed in the LGNM.The molecular formulas for these major phases may be written as FexNiySz,CuxFeySz,MS and MS2,because of isomorphism.
Scanning electron microscopy (SEM) image of ground LGNM in Fig.3 indicates that the sizes of ground LGNM particles are very tiny,and the effect of this factor on the nickel leaching ratio is ignored because most of them are less than 10μm.
2.2 General leaching procedure
1 L aqueous ferric chloride solution with a predetermined concentration was mixed with 10 ml of a 1 mol·L-1hydrochloric acid solution,to inhibit the hydrolysis of ferric ion,and then,the mixture was poured into a glass reactor with a double vane agitator.A condenser with continuous water supply was fitted to the reactor to avoid evaporation loss of the aqueous solution.The weighting LGNM was put into the reactor when the reactor was heated to the setting temperature with a precision of±1°by an electric-heated thermostatic water bath.Then,the agitator was started at a set stirring speed,and the leaching began.The leaching solution was obtained after separating the solid and the liquid by vacuum filtration at the scheduled leaching time.The mass value of nickel ion in the leaching solution was calculated by chemical titration
Table 1 Major elements of LGNM (wt%)
Fig.2 XRD pattern of LGNM
Fig.3 SEM image of LGNM
whereα(Ni) is nickel leaching ratio;m'(Ni) is the mass value of nickel in leaching solution;and m(Ni) is the mass value of nickel in LGNM.
2.3 Chemicals and reagents
Ferric chloride and hydrochloric acid used in the leaching experiments and the other reagents used in the chemical titration experiments were all chemically pure.The water used in all experiments was deionized purified water.
2.4 Characterization
The phases composition of the experimental samples were determined by XRD (Rigaku UltimaⅣ) with Cu Kαradiation of 0.15405 nm at 40 kV and 40 mA,under continuous scanning at a range (2θ) of 10°-90°and a scanning speed of 10 (°) min-1.The morphology of the experimental samples was characterized by S EM (FEI Quanta FEG 250).The chemical compositions of the experimental samples were examined by ICP-OES (ICAP 7000).
3 Results and discussion
3.1 Chemical reaction
The main chemical reactions that occur in the aqueous solution between ferric chloride and LGNM are as follows:
3.2 Influence of stirring speed
The influence of stirring speed on nickel leaching ratio is shown in Fig.4a.The leaching conditions were as follows:liquid-solid ratio was 1000:30 (ml:g),leaching temperature was 50°,leaching time was 2880 min (48 h),and concentration of ferric chloride in aqueous solution was1.0 mol·L-1.The nickel leaching ratio is 89.02%,90.27%and 90.54%corresponding to stirring speed of 100,300 and500 r·min-1.The nickel leaching ratio increased by 1.25%when the stirring speed changed from 100 to 300 r·min-1and by 0.27%when stirring speed changed from 300 to 500r·min-1.The LGNM particles can be suspended sufficiently in the ferric chloride solution when stirring speed is300 r·min-1 Therefore,a stirring speed of 300 r·min-1 was chosen for the experiments to eliminate the influence of stirring speed as variable in the nickel leaching study.
3.3 Influence of liquid-solid ratio
The influence of liquid-solid ratio on nickel leaching ratio is shown in Fig.4b as the leaching conditions were as follow:stirring speed was 300 r r·min-1,leaching temperature was 50°,leaching time was 2880 min,and concentration of ferric chloride in aqueous solution was1.0 mol·L-1.As shown in Fig.4b,the nickel leaching ratio reached the maximum value,which is 90.27%,when the liquid-solid ratio was 1000:30 (ml:g).The nickel leaching ratio slightly reduced by 0.22%,which is 90.05%,when the liquid-solid ratio was 1000:35 (ml:g),meaning that it leads to a decrease in effective leaching with too much solid mass.Thereforc,liquid-solid ratio of 1000:35 (ml:g) was used for the experiments in the nickel leaching study.
3.4 Influence of leaching temperature
The influence of leaching temperature on nickel leaching ratio is shown in Fig.5 as the leaching conditions were as follow:stirring speed was 300 r·min-1,liquid-solid ratio was 1000:30 (ml:g),and concentration of ferric chloride in aqueous solution was 1.0 mol·L-1.As shown in Fig.5,leaching temperature has a noticeable influence on nickel leaching ratio and the nickel leaching ratio increases with the increase in leaching temperature.The nickel leaching ratio changes from 23.07%to 90.27%with leaching temperature changing from 20°to 50°when the leaching time was 2880 min.
Fig.4 Influence of a stirring speed and b liquid-solid ratio on nickel leaching ratio (α)
Fig.5 Influence of leaching temperature on nickel leaching ratio
On the one hand,the nickel leaching ratio would be expected to increase with higher leaching temperature.On the other hand,the higher leaching temperature leads to the consumption of more energy due to the high specific heat of aqueous solution.In fact,the nickel leaching reaction tends to reach completion when the nickel leaching ratio is90.27%.Therefore,leaching temperature should not be higher than 50°.
3.5 Influence of concentration of ferric chloride in aqueous solution
The influence of concentration of ferric chloride in aqueous solution on the nickel leaching ratio is shown in Fig.6.The leaching conditions were as follows:stirring speed was 300r·min-1,liquid-solid ratio was 1000:30 (ml:g),and leaching temperature was 50°.As shown in Fig.6,the nickel leaching ratio improves slightly with the increase in concentration of ferric chloride in aqueous solution.Excess ferric chloride is necessary to ensure that ferric ions have full contact with LGNM particles and the nickel leaching reaction is sufficient.The nickel leaching ratio is 90.27%when the concentration of ferric chloride is 1.0 mol·L-1with leaching time of 2880 min,while it is less than 90%when the concentration of ferric chloride is less than1.0 mol·L-1.The increment of the nickel leaching ratio is only 2.54%when the concentration of ferric chloride is1.2 mol·L-1.Therefore,the suitable choice for the concentration of ferric chloride is 1.0 mol·L-1.
Fig.6 Influence of concentration of ferric chloride on nickel leaching ratio
3.6 Kinetics analysis
The nickel leaching process from LGNM in the ferric chloride aqueous solution is a typical liquid-solid reaction system.Nickel and other metallic elements form soluble chlorides in the ionic state in the aqueous solution,and sulfur of LGNM is oxidized by ferric ions to form elemental sulfur attached to the particles'surface.The whole leaching process can be analyzed using the unreacted shrinking core model consisting of the following steps:diffusion through the fluid (external diffusion),the chemical reaction at the surface of particles,and diffusion through the product layer (internal diffusion).According to this model,the nickel leaching rate between LGNM particle and ferric ions should be controlled by either step one alone or the last two (mixed control) steps
As we know,the kinetics process of nickel leaching should be fitted using Eq.(6) if the external diffusion is the rate-controlling step.Similarly,the leaching kinetics process should,respectively,comply with Eqs.(7)-(9) if the chemical reaction control,internal diffusion control or mixed control is the rate-controlling step.
whereαis nickel leaching ratio,and ke,kr,ki and km are the apparent rate constant of the external diffusion control,the chemical reaction control,the internal diffusion control and the mixed control,respectively.
The experimental data presented in Fig.5 were used to fit Eqs.(6)-(9),and the analysis results are shown in Fig.7.The apparent rate constants and correlation coefficient values (R2) are given in Table 2 according to the unreacted shrinking core model,which are determined from the straight-line in Fig.7 (dotted line).It can be seen that the linear relationship is the best for chemical reaction control,and it is the worst for mixed control from Fig.7and Table 2.Furthermore,the correlation coefficient values of mixed control are all less than 0.9.Therefore,the
Fig.7 Plots of unreacted shrinking core model versus time at different temperatures:a external diffusion control,b chemical reaction control,c internal diffusion control and d mixed control
Table 2 Apparent rate constant and correlation coefficient values
mixed control is not the rate-controlling step for the nickel leaching process.
According to Arrhenius equation,it is represented as
where k is the reaction rate constant,k0 is the pre-exponential factor,E is the reaction activation energy,R is the mole gas constant (8.314 kJ·mol-1),and T is the thermodynamic temperature.
The activation energies can be calculated by using the apparent rate constants to make a plot of In k versus T-1.The Arrhenius plots of each step are shown in Fig.8.The activation energies are listed in Table 3.The apparent activation energy was calculated to be 47.15,52.96 and94.55 kJ·mol-1 for external diffusion control,chemical reaction control and internal diffusions control,respectively.As we know,the apparent activation energy is usually between 8 and 10 kJ·mol-1 for external diffusion control,greater than 40 kJ·mol-1 for chemical reaction control,and between 8 and 20 kJ·mol-1 for internal diffusions control.Furthermore,leaching temperature has very weak influence on leaching ratio for external diffusion control and internal diffusion control,while it has a strong influence on leaching ratio for chemical reaction control.In accordance with these results,chemical reaction control is the rate-controlling step for the nickel leaching process from LGNM in an aqueous ferric chloride solution,and the nickel leaching kinetic equation can be expressed as Eq.(11).
Fig.8 Arrhenius plot of unreacted shrinking core model:a external diffusion control,b chemical reaction control and c internal diffusion control
Table 3 Activation energies and nickel leaching kinetic equations
As shown in Fig.9,the experimental data of the nickel leaching ratio with different concentrations of ferric chloride were well fitted with chemical reaction control.Therefore,the nickel leaching kinetic equation should be expressed as Eq.(12) considering the change in concentration of ferric chloride in aqueous solution.
Fig.9 Arrhenius plots of chemical reaction control with different concentrations of ferric chloride
where K is a.constant,C is the concentration of ferric chloride in aqueous solution,and n is reaction order.
Equation (13) can be obtained from Eqs.(7) and (12)and can be expressed as Eq.(14) when T=50°.
The reaction order can be calculated by using the apparent rate constants and the concentration of ferric chloride in aqueous solution to make a plot of Inkr vs InC,as shown in Fig.10.The K value also can be calculated either by using C value of 1 mol·L-1 or the line intercept of Fig.10 according to Eqs.(12) and (14).As listed in Table 4,the reaction order is 0.5 and the K values calculated by the two methods are very close.The K value applied in the kinetic equation is the average of two methods.
Fig.10 Relationship between 1nC and Inkr
Table 4 Reaction order and nickel leaching kinetic equations
Fig.11 XRD pattern of solid residuals after leaching
3.7 Characterization of the solid residuals after leaching
The solid residuals after leaching,obtained from leaching at 50°for 2880 min with a stirring speed of 300 r·min-1,concentration of ferric chloride in aqueous solution of1.0 mol·L-1 and liquid-solid ratio of 1000:30 (ml:g) were characterized by XRD pattern,SEM observation and chemical composition analysis.
XRD pattern of the solid residuals after leaching is shown in Fig.11.It can be seen that there are free elemental sulfur(S),crystalline nickel chloride hexahydrate (NiCl2·6H2O)and copper chloride dihydrate (CuCl2.2H2O),and a small amount of metal oxides of nickel,iron and copper ((Ni,Cu,Fe)Fe2O4) in the solid residuals.Compared with Fig.2,it is obvious that metal sulfide in the LGNM was oxidized and broken by ferric chloride,the elemental sulfur was oxidized to free elemental sulfur,the metal ions formed soluble chloride salts and dissolved into the aqueous solution,and a small amount of metal oxides which could not react with ferric chloride remained in the solid residuals.
SEM image of the solid residuals after leaching is presented in Fig.12.It can be seen that the solid residuals after leaching are loose and porous,compared with Fig.3,as the particles of LGNM were leached and elemental sulfur was produced.
Fig.12 SEM image of solid residuals after leaching
4 Conclusion
The influence of leaching parameters on the nickel leaching ratio of LGNM with aqueous ferric chloride solution was examined.The results show that the nickel leaching ratio increases with the increase in leaching temperature,leaching time and concentration of ferric chloride.
The nickel leaching ratio is up to 90.27%when the leaching temperature is 50°,leaching time is 2880 min,concentration of ferric chloride in aqueous solution is1.0 mol-L-1 and liquid-solid ratio is 1000:30 (ml:g).
The nickel leaching process of LGNM in the ferric chloride aqueous solution follows the unreacted shrinking core model,and it was determined that the surface chemical reaction is the rate-controlling step for the nickel leaching process.The activation energy was calculated to be 52.96 kJ-mol-1.The reaction order is 0.5.The kinetics equation for nickel leaching can be expressed as1-(1-α)1/3-7.18×104C0.5.exp[-52960/(RT)]t.
参考文献
