简介概要

Strontium ferrite powders prepared from oily cold rolling mill sludge by solid-state reaction method

来源期刊：Rare Metals2013年第5期

论文作者：Bo Liu Shen-Gen Zang Jian-jun Tian De-an Pan Hang-Xin Zhu

文章页码：518 - 523

摘要：Oily cold rolling mill（CRM）sludge is one of the pollutants emitted by iron and steel plants.Recycling oily CRM sludge can not only reduce pollution but also bring social and environmental benefits.In this study,using oily CRM sludge as sources of iron oxide,the strontium ferrite powders were synthesized in multiple steps including vacuum distillation,magnetic separation,oxidizing roasting,and solidstate reaction.The optimal technological conditions of vacuum distillation and oxidizing roasting were studied carefully.To consider the effects of Fe2O3/SrCO3mol ratio,calcination temperature,milling time and calcination time on magnetic properties of prepared strontium ferrite powders,the orthogonal experimental method was adopted.The maximum saturation magnetization（62.6 mAám2ág-1）of the synthesized strontium ferrite powders was achieved at the Fe2O3/SrCO3mol ratio of 6,5 h milling time,1250°C calcination temperature,and 1 h calcination time.Strontium ferrite powders synthesis method not only provides a cheap,high quality raw material for the production of strontium ferrite powders,but also effectively prevents the environmental pollution.

稀有金属 (英文版) 2013,32(05),518-523

Strontium ferrite powders prepared from oily cold rolling mill sludge by solid-state reaction method

Advanced Material and Technology Institute, University ofScience and Technology Beijing

摘要：

Oily cold rolling mill (CRM) sludge is one of the pollutants emitted by iron and steel plants.Recycling oily CRM sludge can not only reduce pollution but also bring social and environmental benefits.In this study, using oily CRM sludge as sources of iron oxide, the strontium ferrite powders were synthesized in multiple steps including vacuum distillation, magnetic separation, oxidizing roasting, and solidstate reaction.The optimal technological conditions of vacuum distillation and oxidizing roasting were studied carefully.To consider the effects of Fe2O3/SrCO3mol ratio, calcination temperature, milling time and calcination time on magnetic properties of prepared strontium ferrite powders, the orthogonal experimental method was adopted.The maximum saturation magnetization (62.6 mAám2ág-1) of the synthesized strontium ferrite powders was achieved at the Fe2O3/SrCO3mol ratio of 6, 5 h milling time, 1250°C calcination temperature, and 1 h calcination time.Strontium ferrite powders synthesis method not only provides a cheap, high quality raw material for the production of strontium ferrite powders, but also effectively prevents the environmental pollution.

收稿日期：6 July 2013

基金：supported by the National Key Technology R&D Program (Nos. 2012BAC02B01, 2012BAC12B05, 2011BAE13B07, and 2011BAC10B02);the National High Technology Research and Development Program of China (No. 2012AA063202);the National Natural Science Foundation of China (Nos. 51174247 and 51004011);the Science and Technology Program of Guangdong Province, China (No. 2010A030200003);the Ph.D. Programs Foundation of the Ministry of Education of China (No. 2010000612003);

Strontium ferrite powders prepared from oily cold rolling mill sludge by solid-state reaction method

Bo Liu Shen-Gen Zang Jian-jun Tian De-an Pan Hang-Xin Zhu

Advanced Material and Technology Institute, University ofScience and Technology Beijing

Abstract：

Oily cold rolling mill (CRM) sludge is one of the pollutants emitted by iron and steel plants. Recycling oily CRM sludge can not only reduce pollution but also bring social and environmental benefits. In this study, using oily CRM sludge as sources of iron oxide, the strontium ferrite powders were synthesized in multiple steps including vacuum distillation, magnetic separation, oxidizing roasting, and solidstate reaction. The optimal technological conditions of vacuum distillation and oxidizing roasting were studied carefully. To consider the effects of Fe2 O3 / SrCO3 mol ratio, calcination temperature, milling time and calcination time on magnetic properties of prepared strontium ferrite powders, the orthogonal experimental method was adopted. The maximum saturation magnetization (62.6 mAám2ág-1) of the synthesized strontium ferrite powders was achieved at the Fe2 O3 /SrCO3 mol ratio of 6, 5 h milling time, 1250 °C calcination temperature, and 1 h calcination time. Strontium ferrite powders synthesis method not only provides a cheap, high quality raw material for the production of strontium ferrite powders, but also effectively prevents the environmental pollution.

Keyword：

Strontium ferrite powders; Oily cold rolling mill sludge; Solid-state reaction method; Recycling;

Author： Shen-Gen Zang e-mail: zhangshengen@mater.ustb.edu.cn;

Received： 6 July 2013

1 Introduction

Since their discovery in the 1950s, there has been a increasing interest in the hexagonal ferrites, also known a hexaferrites, which is still growing exponentially today[1, 2].Strontium hexaferrite (Sr Fe₁₂O₁₉) is widely used in permanent magnets, telecommunication, magnetic recording media, magneto-optics, microwave devices, and other fields, owing to its advantages, such as good combination of high magnetic properties, chemical stability, and low cost[3].Various preparation approaches and techniques, such as solid-state reactions[4–6], chemical co-precipitation[7, 8], sol–gel[9, 10], citrate precursor[11, 12], hydrothermal[13], and self-propagating high temperature synthesis methods[14], were developed.Among these, the most commonly used method in the industry is the solidstate reaction method with Sr CO₃and roll scale as the main raw material, for its economy and ease of operation.The quality of the strontium ferrite products is mainly determined by roll scale quality and processing parameters.With the rapid development of permanent magnet ferrite material, the supply situation of high-quality roll scales tends to become tenser.In addition, the high price of raw materials also brings a great burden to the production enterprises.Therefore, searching for a cheap replacement material of roll scale is an urgent task.

The oily CRM sludge, resulting from cold rolling processes, is characterized by a mixture of small metal particles, cooling fluids, lubricants, etc.According to incomplete statistics, more than 100, 000 tons of oily CRM sludge is generated each year in China.Although total iron content in the oily CRM sludge is relatively high, its recovery is not practiced due to high content of impurities such as complex organic components and soluble salts, etc.At present, oily CRM sludge is mainly reutilized as raw material in secondary smelt furnaces[15–17].The processing method has some advantages such as high treatment efficiency and low cost, but an unavoidable problem is that high organic components content of the oily CRM sludge results in increased emission of volatile organic compounds including dioxins and can lead to problems in waste gas purification systems, e.g., glow fires in electrostatic precipitators.Containing high content of iron alloy powders with fine particle size and high purity, the oily CRM sludge can be an excellent candidate in synthesizing strontium ferrite powders.However, there are few researches on the strontium ferrite powders preparation by oily CRM sludge.

In this study, using oily CRM sludge as raw materials, the strontium ferrite powders were synthesized in multiple steps including vacuum distillation, magnetic separation, oxidizing roasting, and solid-state reaction.The optimal technological conditions of vacuum distillation and oxidizing roasting were studied carefully.The organic components of oily CRM sludge can be distilled out and recovered, avoiding environmental pollution.In addition, the orthogonal experimental method was adopted to determine the process parameter for optimal properties of strontium ferrite powders.

2 Experimental

2.1 Materials and reagents

All reagents used in this study were of analytical grade and locally procured.The oily CRM sludge used in the experiment was obtained from the Baosteel Group Corporation in Shanghai Province, China.Table 1 shows its chemical composition.

2.2 Strontium ferrite powders preparation

Strontium ferrite powders were obtained through a multistep process, including vacuum distillation, magnetic separation, oxidizing roasting, and solid-state reaction.First, the oily CRM sludge was distillated under vacuum in a rotary electric furnace, whose heating part could rotate at a certain speed.Organic components of sludge were distilled out and condensed for further recovery, avoiding the pollution of the environment.Then, the magnetic separation and washing were carried out to separate the soluble salts and non-magnetic materials from the distillation residue.The powders after magnetic separation and washing were put into the rotary electric furnace.The furnace temperature was raised to the desired level at a rate of10°C?min^-1, while the rotating speed was fixed at50 r?min^-1and air was fed into the rotary electric furnace.When the furnace chamber reached the design temperature, it was held at this temperature for several hours to oxidize iron components into ferric oxide (Fe₂O₃) as much as possible.The resulted powders were dried, ball milled, and then mixed with strontium carbonate for several hours.Strontium ferrite powders were produced after the mixture went through the solid-state reaction in electric furnace.

2.3 Analytical methods

X-ray diffraction (XRD) analysis was performed using Philips APD-10 X-ray diffractometer with Cu Ka radiation, 40 k V voltage, and 150 m A current at 10°C?min^-1scanning rate, from 10°to 90°.Thermogravimetry and differential scanning calorimetry (TG-DSC) (NETZSCH STA 409 C/CD, Germany) of the oily CRM sludge was carried out with a heating rate of 10°C?min^-1in Ar.Particle size distribution of obtained iron oxide red sample was determined using laser particle size analyzer (Seishin LMS-30) .Vibrating sample magnetometer (VSM) (Micro Sense EV-9, USA) was used to investigate the magnetic properties of synthesized Sr Fe₁₂O₁₉powders at room temperature.The morphology and size of particles were observed in field-emission scanning electron microscope (FE-SEM, ZEISS ULTRA 55, Germany) .

3 Results and discussion

3.1 Vacuum distillation

In order to investigate the distillation process of the oily CRM sludge, thermal analysis (TG-DSC) was carried out and the results are shown in Fig.1.From Fig.1, TG curves can be roughly pided into three stages, which are92.5–243.2°C, 243.2–400.0°C, and 400.0–454.1°C.In92.5–243.2°C, the weight loss rate of the sample is about4%due to water evaporation.When the temperature rises to 243.2–400°C, part of oil is distilled, which leads to larger slope of TG curve.In this period, the weight loss rate of the sample is about 5%.As the temperature continues to rise to above 400°C, the weight loss speed of sample in this stages is the fastest compared with prior to the two stages, indicating that high distillation temperature oil of oily CRM sludge begins to be distilled.When the temperature is over450°C, with the increase of annealing temperature, the TG curve becomes flat.Accordingly, the distillation temperature of oily CRM must be above 450°C.

Table 1 Chemical analysis of oily CRM sludge (wt%) 下载原图

Table 1 Chemical analysis of oily CRM sludge (wt%)

Fig.1 TG-DSC curves of oily CRM sludge in argon

To study the effect of distillation temperature and holding time, a series of experiments were performed under vacuum with different distillation temperatures, ranging from 450 to 600°C, different holding times ranging from 1to 3 h, when the rotation speed was set at 50 r?min^-1.The efficiency comparison of different treatment processes is shown in Fig.2.Organic residue of the oily CRM sludge drops to 0.16%when heated at 600°C for 3 h.Compared with sample distilled at 600°C, sample distilled at 700°C for 3 h has slightly lower residual organic matter content (0.12%) .Taking into accounts all these results discussed above and economic factors, the optimal parameters of organic residue removal are:600°C distillation temperature and 3 h holding time.

3.2 Oxidizing roasting

After vacuum distillation, the washing and magnetic separation were carried out to separate the soluble substances and non-magnetic substances of the distillation solid residues.Then, the powders after magnetic separation and washing were put into the rotary electric furnace and oxidized at different temperatures (700, 750, 800, and 850°C) for different time, while the rotating speed was fixed at 50 r?min^-1and air was fed into the furnace.The effects of roasting temperature and holding time on the oxidation degree of the CRM sludge are shown in Fig.3.From Fig.3, with the increase of roasting temperature, Fe₂O₃content of samples increases rapidly first, and then tends to be stable when temperature is higher than 800°C.Fe₂O₃contents of all samples obtained at 800°C are more than 98%.Considering the cost, 800°C for roasting temperature is better in this study.Figure 4 shows XRD patterns of obtained ferric oxide powders at 800°C for different time.From Fig.4, all samples have a pure a-Fe₂O₃structure (JCPDS No.87-1166) .

Fig.2 Efficiency comparison of different treatment processes

In order to further optimize the experimental parameters, ferric oxide powders obtained at 800°C for different oxidation time were mixed with strontium carbonate at Fe₂O₃/Sr O mole ratio of 6, respectively.Then the mixtures were ball milled for 6 h.After milling, the mixtures were calcined at a rate of 10°C?min^-1in flowing air atmosphere up to 1150°C, and maintained at the temperature for annealing time 2 h.Table 2 shows the saturation magnetization of resulted Sr Fe₁₂O₁₉powders.The results show that the saturation magnetization of obtained Sr Fe₁₂O₁₉powders slightly decreases by increasing the oxidizing roasting time.The explanation can be that prolonged oxidizing roasting time reduces the material reactive in solid-state reaction.In view of this, optimal roasting time is1 h.

Fig.3 Effect of annealing temperature and annealing time on Fe2O3content of cold rolling mill sludge

Fig.4 XRD patterns of samples prepared at different annealing time

Table 2 Effect of oxidizing roasting time on saturation magnetiza-tion of obtained strontium ferrite powders 下载原图

Table 2 Effect of oxidizing roasting time on saturation magnetiza-tion of obtained strontium ferrite powders

Table 3 Particle size distribution of iron oxide and strontium car-bonate ball milled for different times 下载原图

X₁₀, X₅₀, and X₉₀being corresponding particle size of 10%, 50%, and 90%cumulative distribution

Table 3 Particle size distribution of iron oxide and strontium car-bonate ball milled for different times

Table 4 Various parameters selected and their respective levels in the present experiment design 下载原图

Table 4 Various parameters selected and their respective levels in the present experiment design

To sum up the above discussion, optimal parameters of oxidizing roasting were:800°C roasting temperature and1 h roasting time.In the following experiments, iron oxide powders obtained under the optimal parameters were used as raw material.

3.3 Solid-state reaction

Among the processing parameters which affect the quality of strontium ferrite powders synthesized by solid-state reaction method, Fe₂O₃/Sr O mole ratio, milling time, calcination temperature, and calcination time are of primary importance.In order to study the effect of processing parameters, most of the researchers follow the traditional way, i.e., by varying one parameter and keeping others constant.However, this way does not account for the interaction between the processing parameters.Considering the above facts, the orthogonal experimental method is adopted to determine the process parameter for optimal properties of Sr Fe₁₂O₁₉powders.Table 3 shows particle size distribution of iron oxide and strontium carbonate ball milled for different times.The factor level of the orthogonal experiment is shown in Table 4.

Table 6 Range analysis results of orthogonal experiments 下载原图

Table 6 Range analysis results of orthogonal experiments

Table 5 Results of orthogonal experiments 下载原图

Table 5 Results of orthogonal experiments

Tables 5 and 6 show the results of orthogonal experiments and range analysis, respectively.As shown in Table 6, the effect significance sequences of factors on the saturation magnetization (M_s) and coercivity (H_c) are CBAD and ACDB, respectively.The better level combination for M_sis A₂B₁C₃D₁, while the better level combination for H_cis A₃B₂C₁D₂.Experiment was carried out according to the results of orthogonal experimental analysis and the results obtained were compared with the result of the eighth experiment of orthogonal experiments.

Table 7 Results of better experimental combinations 下载原图

Table 7 Results of better experimental combinations

Fig.5 XRD patterns of samples obtained under optimal parameters

The results of better experimental combination and the eighth experiment of orthogonal experiments are shown in Table 7.The combination of A₂B₁C₃D₁and A₃B₂C₁D₂achieves higher saturation magnetization (62.6 m A?m²?g^-1) and better coercivity (342.33 m T) , respectively, from which it can be seen that magnetic properties are improved.Figure 5 shows the XRD patterns of strontium ferrite powders prepared under different optimal processing conditions.From Fig.5, two samples have a pure Sr Fe₁₂O₁₉structure (JCPDS No.79-1412) .Figure 6 shows the morphology of strontium ferrite powders prepared with the optimal experimental parameters.It can be seen from Fig.6 that the synthesized powders have a particle size of*2 lm.

4 Conclusion

In this study, using oily CRM sludge as sources of iron oxide, the strontium ferrite powders were synthesized in multiple steps including vacuum distillation, magnetic separation, oxidizing roasting, and solid-state reaction.The results of vacuum distillation indicated that organic residue of the oily CRM sludge decreases to 0.16%when treated at 600°C, and the drum of the rotating furnace was rotated at 50 r?min^-1for 3 h.Organic components of sludge were distilled out and condensed for further recovery, avoiding the pollution of the environment.The optimal factors of oxidizing roasting are roasting temperature of 800°C and roasting time of 1 h.Too high roasting temperature and too long roasting time can reduce the reaction activity of obtained iron oxide powders.The maximum saturation magnetization (62.6 m A?m²?g^-1) of the synthesized strontium ferrite powders from oily CRM sludge is achieved at the Fe₂O₃/Sr CO₃mol ratio of 6, 5 h milling time, 1250°C calcination temperature, and 1 h calcination time.

Fig.6 SEM images of samples obtained under different optimal parameters:a A2B1C3D1and b A3B2C1D2

Strontium ferrite powders synthesis method not only provides a cheap, high quality raw material for the production of strontium ferrite powders, but also effectively prevents the environmental pollution.