The transformation behaviour of ZnFe2O4 spinel particles in the CaO-FexO-SiO2-ZnO slag system

Treating zinc hydrometallurgical residues and other Zn-bearing solid waste in direct lead smelting processes has been recognized as the preferred solution for the sustainable development of the lead and zinc industry. However, new technical problems occurred when changing the starting material of lead-making processes, the pyrometallurgical process for treating zinc hydrometallurgical residues and zinc bearing solid waste usually had the CaO-FexO-SiO2-ZnO slag. Along with the increasing of Zn content in the raw material, high ZnO slag formed, and the ZnFe2O4 spinel particles intended to precipitate from the molten slag. The understanding of the particle behaviour of ZnFe2O4 spinel was the core part of monitoring the slag properties. This study investigated the transformation behaviour of zinc ferrite spinel particles and the changes in glassy slag phase through the high-temperature phase equilibrium experimental technique, and the temperature was 1100 °C-1300 °C and the oxygen potential was 10−8 atm – 10−4 atm. It is found that the particle size of ZnFe2O4 spinel in the molten slag increased along with the increasing of the oxygen potential. The Zn/Fe mass ratio in spinel particles decreased when increasing of temperature and it increased when increasing of oxygen potential. The Fe/SiO2 mass ratio and the Zn content in glassy slag phase gradually increased along with increasing temperature and they decreased along with increasing oxygen potential. The growing and decomposition of ZnFe2O4 spinel was essentially a process of material exchange with the glassy slag phase.


Introduction
Global lead and zinc production in 2022 reached 12.30 million tons and 13.286 million tons, respectively.Lead and zinc consumption was about 12.40 million tons and 13.59 million tons in recent years (sources from International Lead and Zinc Study Group).[3] Advanced technologies such as direct smelting of lead sulfides, [4,5] direct reduction of high lead containing slag and direct leaching of complex zinc sulfide concentrates have been widely used all over the world.In addition, the Pb & Zn integrated co-smelting technology has been proved as a high resource utilization and excellent environmental performances model, which is a consensus in the field of Pb-Zn smelting industry and scientific research.Along with the depletion of the high-quality Pb-Zn mineral resources, there was more pressure in raw material supplying, and new technical and environmental problems occurred.Sustainable development has turned out to be the key idea in this field.Complex resources and secondary materials were having larger proportion in starting materials.Secondary resources, such as lead-silver residues, iron precipitations and lead past, have been co-treated with lead concentrates.The co-smelting technology can effectively utilize the heat releasing from the oxidation of lead sulfide concentrate, and it can also achieve the harmless treatment and resource utilization of solid wastes, especially the hazardous. [6]The development of modern lead smelting technologies such as Kivcet, oxygen-enriched side-blowing, oxygen-enriched bottom-blowing and QSL, have already started to co-smelt secondary materials with lead concentrates.All of them had industrial practices, for example, the Canadian Trail Smelter, [7] Jiangtong Lead and Zinc Company, [8] Zhongjin Lingnan Danxia Smelter. [9], etc.The pyrometallurgical processes owned the advantages of fast reaction rate, large capacity, low pollution emission and stable final products, which showed a good potential in co-treating complex Pb concentrate and Zn hydrometallurgy residues [10] .Researches and practices have shown that the co-processing of hydrometallurgical zinc smelting residues in the pyrometallurgical lead smelting process was the preferred method in line with the production reality and industrial development direction, there were significant environmental benefits.With its Kivcet furnace, Jiangxi Copper Lead and Zinc Company dealt with lead-containing materials such as zinc leaching residue, lead sulfate sludge, and neutralization residue.Moreover, it had realized good performance for more than six years of practice. [8]The leadsilver residue from zinc hydrometallurgy has also been successfully treated in the furnace, which made full use of resources and reduces the environmental risks.However, the current co-processing of zinc hydrometallurgical residues in direct lead smelting process caused prominent problems, such as the high zinc content in slag, deterioration of slag properties, difficult separation of slag and metal phase, etc.At the early stage of Trail plant operation, it produced many furnace knots in the period of treating high Zn charge with the Kivcet furnace, resulting in the reduced reduction capacity of the coke filter layer, which made the lead content of the slag increase and the capacity of the kiln to process zinc smelting slag decrease. [7]Slag-metal separation was difficult in the Kivcet furnace of the Danxia smelter when processing high proportion of goethite residues, and one sticky slag layer has been found in the molten bath. [9]The increase of zinc content in the raw material was the major reason for the above industrial difficulties, and zinc mainly entered the slag phase in the oxidation smelting stage.While under the currently direct lead smelting process conditions, high zinc slag was prone to have saturated zinc ferrite spinel, leading to the increase of slag viscosity, poor slag-metal separation and so on, which seriously affected the industrial practice.The analysis about the behavior of spinel in high Zn smelting slag and the formation of slag performance control mechanism was the way to solve above problems.The behavior of ZnFe2O4 spinel in CaO-FeXO-SiO2-ZnO slag system is the core knowledge of slag, which is also the bottleneck of technology development.[12][13][14][15][16][17][18][19][20][21] But there was a lack of systematic understanding about the transformation behavior of ZnFe2O4 spinel.In this paper, the transformation behavior of ZnFe2O4 spinel particles in CaO-FeXO-SiO2-ZnO slag system has been systematically studied with high-temperature phase equilibrium experimental technique.Obtained results can help in optimizing these processing parameters, and it was also useful for the treatment of zinc bearing solid waste and the synthesis of related spinel materials.

Experimental apparatus
The schematic picture of the high temperature phase equilibrium setup has been depicted in Figure 1.It is consisted of one vertical tube furnace (Hefei Kejing KSL-1700) with Si-Mo heating element, water cooling system, gas supplying system and a connected quenching device.Zirconia crucible (purity 99.9 wt %, inner ⌀, outer ⌀ and hight = 8, 10 and 20 mm, respectively), Cornell wire/platinum wire have been used as sample reaction vessel and suspension wire, respectively.The dissolution of zirconia or platinum into the slag can be neglected in these experiments.Samples were kept at the hot zone of the vertical furnace for a certain period.The temperature profile of the working tube has been prechecked with a S-type thermocouple, and the temperature accuracy was estimated to be ± 3 ℃.When the inquired equilibrium time reached, the suspension line was cut, and the sample dropped and quenched into an ice-water bath immediately.

Preparation of initial Material
Based on the industrial slag composition [7][8][9][10] , master slag was pre-synthesized from chemical pure oxides.Keeping the mass ratio of Fe/SiO2 and CaO/SiO2 as constants, and the value was 0.85 and 0.33, respectively.Zn, Fe, Si and Ca contents in the starting slag were 20.0 wt %, 23.91 wt %, 11.89 wt % and 5.83 wt %, respectively.
Accurately balanced oxides were well ground and mixed in an agate mortar.After that, the mixture was pressed into flakes in a pressing tool with a pressure of 1.5 MPa/cm 2 and followed by loading them into the zirconia crucible.The crucibles containing the samples were suspended in the high temperature zone of the furnace through platinum wires.Pre-mixed N2-O2 gas or CO-CO2-Ar (with the oxygen partial pressure of 10 −6 atm) has been used as carrying gas in master slag preparation process.The raw material sintering conditions were determined from the sample characterization, melting performance, and chemical reactions, and processing parameters were 1200 °C, 8h and 10 −6 atm.When the reaction time reached, the crucible was quenched and cooled in the low-temperature zone by adjusting the length of the platinum wire, and the experimental raw material was obtained by physically breaking it into several pieces, then it was fully ground in a mortar and pestle to obtain a powder material.Ultimately it was dried and put into a storing sample bag.

Sample characterization
The synthesized slag was examined by electron microscopy scanning and X-ray diffraction, the results were shown in Figure 2. The calculation of the CaO-FeXO-SiO2-ZnO slag system phase diagram was carried out through FactSage 7.1 and the results were obtained and shown in Figure 3.  From the microscopic morphology, the synthesized slag had different contrasts, which referred to two phases.The bright and regular shaped area was the spinel phase, the dark gray area was the glass slag phase, and the black area was the SiO2 precipitates, which has also been confirmed by the energy spectrum scan.The size of the spinel particles was statistically analyzed from obtained electron microscope images, and the result was 6.77 μm.The mean statistical analysis of the composition of glass slag and spinel in the synthesis slag was carried out from EDS results.The Zn/Fe mass ratio in spinel was 0.486, the Zn content in glass slag was 20.338 wt %, and the Fe/SiO2 mass ratio in glass slag was 0.135.The XRD results showed that the precipitated phase was ZnFe2O4.Combining XRD detection, SEM results and the calculated phase diagram of CaO-FeXO-SiO2-ZnO slag system, it can be determined that the slag is consisted of precipitated solid phase and glass slag liquid phase, and the precipitated solid phase was ZnFe2O4.

Experimental program
The successfully synthesized raw material, zirconia crucible, and 1700 °C vertical tube furnace were selected to experimentally determined the spinel conversion behavior by high-temperature phase equilibrium experiments.High ZnO fayalite slag was used to adjust two influencing factors: temperature and oxygen partial pressure.The temperature interval was 1100 °C -1300 °C, with five temperature points of 1100 °C, 1150 °C, 1200 °C, 1250 °C, 1300 °C, and oxygen partial pressure of 10 −8 atm, 10 −7 atm, 10 −6 atm, 10 −5 atm, 10 −4 atm standard gas (O2+N2), a total of 5*5 = 25 sets of experiments need to be completed, and the samples were quenched and cooled, dried, and stored in samples.The above 25 sets of experimental samples were characterized by using XRD detection method and SEM-EDS.

Effect of temperature
The experimental results were obtained by controlling a certain oxygen potential condition and investigating the phase transformation behavior of spinel particles in high ZnO fayalite slag with the change of temperature.The crystallinity of spinel was evaluated by X-ray diffraction, and the effect of temperature on crystallinity can be reflected in the variation of X-ray diffraction peak intensity under different oxygen potential conditions.The result was shown in Figure 4-6.As shown in Figure 4, it suggested that the crystallinity of ZnFe2O4 spinel increased with temperature increasing at an oxygen potential of 10 −8 atm.However, when slag samples were treated at 10 −4 atm, the conclusions were different, it decreased with temperature increasing.In terms of microscopic morphology, the average particle size of ZnFe2O4 spinel particles in equilibrium slag varied with temperature at different oxygen potentials shown in Figure 5.According to the data shown in Figure 5, the average size of ZnFe2O4 spinel particles tended to decrease with the increasing of temperature at different oxygen potentials.Solid spinel particles nucleated or precipitated in the molten slag phase, and the particle size had the correlation with temperature, hold time and oxygen partial pressure.Small particles might grow big with the mechanism of Ostwald Ripening or Nucleation-Growing.Samples prepared at low temperatures, such as 1100 ℃, more Fe(III) in the system at high pO2, and that was a privilege for big particles formation.Along with increasing the temperature or decreasing the pO2, the Fe (III) / Fe total ratio in the system decreased, which was a favor for spinel decomposition.Taking the oxygen potential of 10 −8 atm as an example, the backscattered electron microscopy results of the samples obtained at different temperature conditions were shown in Figure 6.The information of the picture in Figure 6 showed a clear support for the variation of the size of ZnFe2O4 spinel particles tends.The effect of temperature also reflected in the chemical composition.The Zn/Fe mass ratio of ZnFe2O4 spinel in slag samples with different oxygen potential was shown in Figure 7, and the variation of Fe/SiO2 mass ratio and Zn content in glass slag of samples with temperature was shown in Figure 8.  From Figure 7 and Figure 8, it has been demonstrated that the Zn/Fe mass ratio in spinel gradually decreased and the Fe/SiO2 mass ratio in glass slag phase gradually increased with temperature increasing under different oxygen potential conditions; the Zn content in glass slag phase showed an increasing trend with temperature increasing.

Effect of oxygen potential
The experimental results were obtained by controlling a certain temperature condition and investigating the phase transformation behavior of spinel particles in high ZnO fayalite slag with the change of oxygen potential.The effect of oxygen potential on crystallinity can be reflected in the variation of X-ray diffraction intensity under different oxygen potential conditions.The results were shown in Figure 9. From Figure 9, It has been suggested that the crystallinity of ZnFe2O4 spinel increased with increasing oxygen potential at all temperature conditions.Combined with Figure 4, we found that the fraction of high ZnO fayalite slag does not change too much in the range of all conditions of this study.In terms of microscopic morphology, the average particle size of ZnFe2O4 spinel particles in equilibrium slag varied with oxygen potentials at different temperature shown in Figure 10.According to the information shown in Figure 10, the average size of ZnFe2O4 spinel particles tended to increase with the increase of oxygen potential at different temperatures.Taking an example at 1300 ℃, the backscattered electron microscopy results of the samples obtained at different temperature conditions were shown in Figure 11.As shown in Figure 11, ZnFe2O4 spinel particles grown larger along with increasing the oxygen partial pressure.The Zn/Fe mass ratio of ZnFe2O4 spinel in these samples was shown in Figure 12, and the Fe/SiO2 mass ratio and Zn content in glassy slag have also been shown in Figure 13.From Figure 12 and Figure 13, it has been demonstrated that the Zn/Fe mass ratio in spinel gradually increased with increasing oxygen potential under different temperature conditions; the Fe/SiO2 mass ratio in glass slag phase gradually decreased with increasing oxygen potential; the Zn content in glass slag phase had a decreasing trend with increasing oxygen potential.

Summary and conclusions
The phase equilibrium and the spinel particle transformation have been experimentally studied in the high ZnO contain fayalite slag by controlling different oxygen potential conditions and temperatures.The individual experimental data has been summarized and the variation pattern has been organized, and the following conclusions has been obtained: (1) the average size of spinel particles Variation pattern.The average size of ZnFe2O4 spinel particles tended to decrease with the increase of temperature, and gradually increased with the increase of oxygen potential.(2) Spinel phase components variation pattern.The Zn/Fe mass ratio of ZnFe2O4 spinel phase gradually decreased with temperature increasing and gradually increased with increasing oxygen potential.
(3) The change of components in the glass slag phase.In ZnFe2O4 fayalite slag, the variation pattern of Fe/SiO2 mass ratio in the glass slag phase remained consistent, gradually increased with temperature and gradually decreased with oxygen potential; the Zn content increased with temperature and decreased with oxygen potential.(4) In the high ZnO fayalite slag, the variation of Zn/Fe mass ratio in ZnFe2O4 spinel particles was exactly opposite to the variation of Fe/SiO2 mass ratio and Zn content in the glass slag.Therefore, the growth and decomposition process of ZnFe2O4 spinel is essentially a process of material exchange with the glass slag phase.

Figure 1 .
Figure 1.Schematic of the experimental equipment.

Figure 2 .
Figure 2. XRD result and microscopic morphology of synthesized slag.

Figure 4 .
Figure 4. Recorded XRD patterns of slag samples at different conditions.Left: at an oxygen potential of 10 −8 atm; Right: at an oxygen potential of 10 −4 atm.

Figure 8 .
Figure 8.The Fe/SiO2 mass ratio and Zn content of glass slag with the variation of oxygen potential and temperature.

Figure 10 .
Figure 10.Variation of the average size of ZnFe2O4 spinel particles in equilibrium slag with the variation of oxygen potential and temperature.

Figure 12 .
Figure 12.The Zn/Fe mass ratio variation of spinel with the variation of oxygen potential and temperature.
The Zn/Fe mass ratio variation of spinel with temperature in samples, at different oxygen potentials.