Innovative Methodology for Comprehensive Utilization of Spent MgO

Research Article pubs.acs.org/journal/ascecg

Innovative Methodology for Comprehensive Utilization of Spent MgO-Cr2O3 Bricks: Copper Flotation Junwei Han, Fen Jiao,* Wei Liu,* Wenqing Qin, Tan Xu, Kai Xue, and Tianfu Zhang School of Minerals Processing and Bioengineering, Central South University, 932, Lushan South Road, Changsha 410083, China ABSTRACT: Spent MgO-Cr2O3 bricks contain various heavy metals and are considered as a hazardous waste. In order to utilize the waste, a novel process to recover the copper from the spent bricks by flotation was developed. The mineralogical characteristics of the bricks, the mechanisms of metal penetration, and the effects of grinding time, emulsified kerosene dosage, collectors, and Na2S addition on the copper recovery were investigated. The results indicated that the spent bricks were mainly composed of periclase, magnesiochromite, copper, cuprite, and magnesium copper oxide. The copper penetrated into the MgO-Cr2O3 bricks from cracks and holes, and the copper oxides are easier than metallic copper, while the metallic copper is easier than its sulfides in infiltrating into the bricks. The optimum flotation parameters were 7 min of grinding time, 200 g/t of emulsified kerosene, 400 g/t of sodium isoamyl xanthate, and 400 g/t of Na2S. Based on these conditions, about 95 wt % Cu was recovered from the bricks by a closed-circuit test and the concentrate contained 21.4 wt % Cu, which can be returned to the copper smelting process as a feeding. The tailing only contained trace of impurities and can be reused as a raw material for new refractory bricks. KEYWORDS: Copper flotation, Magnesia-chrome bricks, Metal penetration, Spent refractory, Sulfidation

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INTRODUCTION Magnesia-chrome (MgO-Cr2O3) bricks are of wide use in many industries, such as steel refining, nonferrous metallurgy, and cement making, due to their attractive mechanical and refractory properties, including high hot temperature strength, resistance to slag attack, dimensional stability, etc.1−4 However, owing to chemical corrosion, thermal shock, and mechanical damage, the used bricks, in general, have to be replaced from every several months to two years, depending on the difference in their quality and application.5,6 Millions of tons of spent MgO-Cr2O3 bricks are therefore generated every year in the world. Most users preferred stockpiling or landfilling to recycling these spent refractories because of the lack of economic and legislative driving forces in the past decades. For example, at the end of the 20th century, more than 3 million tons of refractories were annually produced in the USA and 80,000 tons in Europe.7 Of these, more than 90% was landfilled with or without treatment. But only about 1% of the refractories from metal producers in Missouri was recycled.8 Compared with developed countries, the management of used refractories is more unreasonable in developing countries, such as China, India, and Iran. On the other hand, MgO-Cr2O3 refractories are produced using magnesia and chrome ores, resulting in massive chrome ores being depleted with the continuous development of related industries. More serious is that the heavy metal chromium (Cr) in the spent bricks can be transferred into © XXXX American Chemical Society

soil and water, which will seriously threaten the survival and development of human beings. This is attributed to the fact that Cr(III) can accumulate in body tissues and result in an abnormal cellular metabolic rate, while the toxicity of Cr(VI) is 100 times stronger than Cr(III).9,10 Cr(VI) can invade the human body through the digestive system, respiratory system, skin, and mucous membranes and has a potential carcinogenic risk.11,12 As a result, regulations treating spent MgO-Cr2O3 bricks as potentially hazardous wastes are being enforced,13 with the development of the society and the improvement of environmental awareness. Furthermore, the used bricks generally carry a considerable amount of valuable metals, such as Cu, Pb, Ag, Bi, Ni, and Sb, by infiltration and/or erosion during the metallurgical process. The stockpiling of these spent bricks not only occupies precious land but also causes a serious environmental threat and the waste of refractories and valuable metals. It is therefore urgent to develop an economical green technology to comprehensively utilize the secondary resources. In the recent decades, a number of studies have been conducted to solve the problems brought by spent MgO-Cr2O3 bricks. Improving refractory life and developing new chromefree refractories have nowadays received much attention as Received: May 27, 2016 Revised: August 27, 2016

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DOI: 10.1021/acssuschemeng.6b01163 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering potential methods to reduce the amount of spent MgO-Cr2O3 bricks and to avoid chromium pollution, respectively.14,15 Although the lifetime of the refractory can be improved by optimizing the lining concept, engineering the slag, and changing the process conditions,7,16 and thus substantially reducing the amount of the waste, it is unable to fundamentally solve the problems. A variety of chrome-free refractories, such as magnesia-alumina, magnesia-carbon, and alumino-silicate materials,17−24 have been successfully developed and applied to iron metallurgy and refractory linings for rotary kilns in the cement industry, but in many secondary metallurgy processes in ferrous metallurgy as well as in the metallurgy of copper and lead, MgO-Cr2O3 bricks are still used on a large scale because of their outstanding performances compared with these chrome-free alternatives.13,25 A more sustainable method to solve the problem is to recycle spent refractories. The typical recycling process is first to sort the refractory to avoid the mixture of different types, then to crush and grind them for liberating metals, slag, and other impurities, and to magnetically separate the Fe-rich chromite from the MgO-rich fraction.26 Finally the chromite can be recycled in the production of new MgO-Cr2O3 bricks, while the MgO-rich fraction can be used as a substitute for sand in concrete, after impurities removal. Additionally, the spent refractories are also recycled for some construction uses, magnesia-based castables, metal production, etc. Although many achievements in this field have been made, these developed technologies have still not been widely applied in industry because of lack of an economic driving force. Flotation is an economical and eco-friendly technology and is widely used in the mineral processing field.27 As mentioned previously, the spent bricks generally contain a considerable amount of valuable metals, such as Cu, Pb, Ag, Bi, Ni, and Sb, which are mainly in the form of metals, sulfides, and/or oxides and can be recovered by flotation or sulfidizing flotation. In the present study, a novel method was proposed to utilize the spent MgO-Cr2O3 bricks that come from nonferrous metal smelters. The impurity metals are separated from the bricks by flotation and will be returned to the corresponding smelting process, while the rest only contain minor or trace impurities and thus can be reused as raw material for new refractory bricks. Since the mineral processing wastewater is usually recycled in a flotation process, there is no waste discharge in this process, and therefore, this technology is economical and eco-friendly. In this paper, the recovery of metal impurities, mainly Cu, from the spent MgO-Cr2O3 bricks that were generated in copper smelting industries was studied, and correspondingly, the mineralogical characteristics of the bricks and the flotation concentrate and tailing were investigated by XRF, ICP, XRD, SEM-EDS, and optical microscopy.

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Figure 1. Scheme diagram of different analysis on the spent bricks and flotation experiments. testing sieves and inductively coupled plasma (ICP, IRIS Intrepid II XSP), respectively. The chemical composition of the spent bricks was analyzed by X-ray fluorescence spectroscopy (XRF, Rigaku, ZSX Primus II) after the sample was ground to −74 μm, and the total content of copper was further determined using ICP after the sample powder was digested with a mixture of concentrated HCl and HNO3 (3:1, v/v).29,30 Its crystalline composition was analyzed by X-ray powder diffraction (XRD, Germany Bruker-axs D8 Advance). The study of grinding time was performed using a series of standard testing sieves and a ball mill, and 500 g of the prepared sample was used for each test. All techniques used were carried out in accordance with ISO standard. Deionized water was used throughout these analysis processes. The samples for microstructure analysis were made into a lump with a polished surface in advance.31 In addition, sodium sulfide (Na2S) as the activator, emulsified kerosene (EK) as the flocculant, sodium hexametaphosphate (SH) as the depressor, ammonium dibutyl dithiophosphate (ADD), sodium isobutyl xanthate (SIBX), sodium isoamyl xanthate (SIAX), Z-200 and ethyl thio carbamate (ETC) as the collector candidates, and terpilenol as the frother were used in the flotation experiments. Sodium sulfide, sodium hexametaphosphate, and terpilenol are of analytical grade, and the others are of industrial grade. Flotation Process. All experiments were carried out using an XFG flotation machine with a mechanical stirrer. Before each test, 500 g of the samples were ground with a wet ball mill to obtain an 80% passing of 74 μm. The ore slurry was then transferred into a 1.5 L flotation cell and conditioned for 1 min at an agitation speed of 1992 rpm. After this process, a given amount of Na2S was added into the flotation pulp for conditioning for 8 min. Afterward, 200 g/t of emulsified kerosene was added into the flotation cell and the pulp was conditioned for 6 min. Then, 400 g/t of collector was added for conditioning for 4 min and 50 g/t of terpilenol was added before scraping froth. The flotation froth was scraped every 3 s and a concentrate was collected after 4 min. Finally, the floated and unfloated samples were collected, filtered, dried, weighed, and analyzed for copper content by ICP. The flotation recovery was calculated based on solid weight distributions between the concentrate and the tailing. In addition, the flotation closed-circuit tests were carried out according to the flowsheet shown in Figure 2. Finally, the concentrates and tailings of the closed-circuit tests were also analyzed by XRD and optical microscopy combined with SEMEDS, whose detailedly sample preparation processes are the same as the corresponding analysis of the raw material.

EXPERIMENTAL SECTION

Materials. The spent MgO-Cr2O3 refractory bricks used in this study were obtained from the converter furnaces of a copper smelting plant in Guangdong Province, China. Prior to use, a representative brick was selected to prepare specimens for the microstructure analysis of different layers by optical microscopy (Leica DMRXP) and scanning electron microscopy (SEM, JEOL, JSM-6490LV) with energy dispersive spectroscopy (EDS, JEOLJSM-6490LV).28 The spent bricks were crushed to −3 mm and mixed thoroughly, and samples were taken by coning and quartering methods for further analysis and flotation experiments. A scheme of the different analysis on the spent bricks and flotation experiments is shown in Figure 1. The particle size distribution of the sample and the copper content of different size particles was first investigated using a series of standard

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RESULTS AND DISCUSSION Characteristics of the Spent Bricks. Table 1 shows the chemical composition of the spent MgO-Cr2O3 bricks, and the results indicate that the spent bricks contain 4.83% Cu and minor or trace Zn, Ni, Pb, Sn, and Sb etc., which mainly come B


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Figure 3. XRD pattern of the spent MgO-Cr2O3 bricks. Figure 2. Flotation flowsheet of the closed-circuit process of the spent bricks.

from the copper smelting process through metal penetration. It is therefore desired to recover these metals (mainly Cu) from the refractories by a flotation process. The XRD patterns of the spent bricks shown in Figure 3 indicate that the used bricks are mainly composed of periclase (MgO), magnesium copper oxide (Mg1−xCuxO), cuprite (Cu2O), metallic copper (Cu), and magnesiochromite [(Mg,Fe)(Cr,Al)2O4], which was formed as a result of reaction between slag (Al2O3, FeO) and refractory (MgO, Cr2O3) components according to [MgO]refr + [Cr2O3]refr + (Al 2O3)slag + (FeO)slag = [(Mg, Fe)(Cr, Al)2 O4 ]refr

(1) Figure 4. Copper distribution in different size particles of the spent MgO-Cr2O3 bricks.

where [ ] designates a solid refractory component and ( ) refers to a component dissolved in the slag. Copper is mainly in the forms of cuprite, metallic copper, and magnesium copper oxide. The copper distribution in different particle size ranges of the spent MgO-Cr2O3 bricks is presented in Figure 4. It is seen from Figure 4 that the copper distributed in the particle size ranges of +180 μm and −74 μm is higher than that in the intermediate particle size ranges, suggesting that those particles containing Cu are liable to overcrushing in the grinding process, which is bad for the copper recovery from the spent refractory. In order to investigate the microstructure of the spent bricks and gain insight into the mechanism of metal penetration, the microstructures and mineral composition of a representative brick at different sections (from the hot face to the other side, marked as 1, 2, 3, and 4 in sequence) were analyzed by optical microscopy and SEM-EDS, the results of which are shown in Figure 5. Combined with XRD results, EDS analysis indicates that the phases A to D in Figure 5 are metallic copper, cuprite, magnesiochromite, and periclase, respectively, some of which

contain minor or trace impurities. It can be seen from Figure 5(b) that there are a considerable amount of metallic copper particles embedded into the cracks and holes of the refractory brick. Most of these copper particles are relatively large and nonspherical, and have clear edges and corners, which suggests that the copper can be easily recovered by flotation with xanthate, because the floatabilities of large size, nonspherical, and prismatic particles are better than those of small size and equivalent round particles. It is found that the copper particles are surrounded by cuprite, which is generated during the copper matte converting process according to the following reaction: Cu 2S + 1.5O2 (g ) = Cu 2O + SO2 (g )

(2)

With going deeper into the brick, metallic copper particles are hardly found in the optical micrographs or SEM images. As shown in Figure 5c, there is a considerable amount of cuprite

Table 1. Chemical Composition of the Spent MgO-Cr2O3 Bricks (wt %)

a

Cua

Mg

O

Cr

Fe

Si

Al

Ca

S

Ti

P

Zn

4.83 Sn

36.1 Mn

29.5 Ni

4.90 Pb

4.28 V

3.11 W

1.90 K

1.87 Sb

0.44 Cl

0.14 As

0.13 Sr

0.12 Zr

0.10

0.10

0.09

0.09

0.05

0.03

0.03

0.02

0.02

0.01

0.01

0.01

Detected by ICP. C


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Figure 5. Image of the refractory brick (a) and its optical micrographs with corresponding SEM-EDS results at different cross sections: 1 (b), 2 (c), 3 (d), and 4 (e).

Effect of Grinding Time. As is well-known, the liberation of aimed minerals from gangues by grinding is a necessary condition for an effective flotation, but an extremely fine particle size causes difficulties during flotation separation.32 The effect of grinding time on the recovery of copper was therefore investigated, and the results are presented in Figure 6. It is seen from Figure 6 that with the grinding time increasing from 3 to 7 min the copper recovery and grade increase from 56.8% to 75.5% and from 18.3% to 20.6%, respectively. Further increasing the grinding time, both the copper grade and recovery, however, gradually decrease. The optimum grinding time was hence considered to be 7 min, for which about 80% of particles were smaller than 74 μm, and all further experiments were carried out for 7 min. Effect of Emulsified Kerosene Dosage. As discussed previously, the copper minerals in the bricks are liable to overcrushing, resulting in it being difficult to recover the

particles in the cracks and holes of the refractory brick but metallic copper has not been found. At section 3, only a small amount of cuprite was entered into the refractory brick through the cracks and holes (in Figure 5d). With further going deeper into the brick, copper minerals are not found in the brick at section 4 (in Figure 5e). This indicates that the amount of metal penetration gradually decreases as the depth into the brick increases. Additionally, it is also found from Figure 5 that copper sulfides hardly penetrated into the refractory bricks. It is thereby concluded that copper oxides are easier than metallic copper, while the metallic copper is easier than copper sulfides in infiltrating into MgO-Cr2O3 refractory bricks. This may be accounted by the fact that MgO-Cr2O3 refractory is made of oxidized materials and is hydrophilic, resulting in that it is difficult for metallic copper or copper sulfides to penetrate into the brick microstructure because of its large wetting angle with the refractory. D


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Figure 6. Effect of grinding time on the recovery of copper (200 g/t of EK, 400 g/t of SIBX, 50 g/t of terpilenol).

Figure 8. Effect of collectors on the recovery of copper (7 min of grinding, 200 g/t of EK, 50 g/t of terpilenol).

superfine particles of copper minerals. In order to improve copper recovery, emulsified kerosene as a flocculant was introduced into the flotation process,33 and the effect of emulsified kerosene dosage on the recovery of copper was studied, the results of which are shown in Figure 7. It is found

grade of copper concentrate obtained by flotation with SIAX is higher than 20%, which meets the quality requirement of copper smelting feeding, the copper recovery should be preferentially considered to remove the copper from the spent refractories as much as possible. As a result, the optimum collector was determined as 400 g/t of SIAX, under which the copper recovery reached 81.2%, while the copper grade of the concentrate was 20.8%. Effect of Na2S Dosage. Although a good flotation index has been obtained in the above experiments, it is possible to further increase the copper recovery from the spent bricks by sulfidizing with Na2S in advance, because some of the copper is in the form of oxides, such as cuprite and magnesium copper oxide, as mentioned in the section Characteristics of the Spent Bricks. The effect of Na2S dosage on the recovery of copper was therefore investigated, and the results are shown in Figure 9. It is seen from Figure 9 that the copper recovery and grade

Figure 7. Effect of EK dosage on the recovery of copper (7 min of grinding, 400 g/t of SIBX, 50 g/t of terpilenol).

that emulsified kerosene addition favors the copper recovery from the spent bricks by aggregating superfine copper particles. With the increase in emulsified kerosene dosage, the copper recovery gradually increases until reaching a maximum at 200 g/t, and thereafter, it has no significant variation, while the grade of copper continuously decreases in the investigated range. The reason for the decrease in copper grade is that the entrainment of gangue minerals was promoted by increasing emulsified kerosene dosage. Based on the above analyses, it is apparent that the optimum emulsified kerosene dosage was 200 g/t and all further experiments were performed with this dosage. Effect of Collector Kind. Figure 8 presents the effects of different collectors, including ammonium dibutyl dithiophosphate, sodium isobutyl xanthate, sodium isoamyl xanthate, Z200, and ethyl thio carbamate, on the flotation of copper. It can be seen from Figure 8 that the copper recovery with SIAX is obviously higher than those with other collectors although the copper grade is lower than that with ETC or Z-200. Since the

Figure 9. Effect of Na2S dosage on the recovery of copper (7 min of grinding, 200 g/t of EK, 400 g/t of SIBX, 50 g/t of terpilenol).

increase as the Na2S dosage increases from 0 to 400 g/t, indicating that the addition of Na2S can improve the floatability of the oxide copper particles in the refractories. When Na2S dosage further increases, however, both the copper recovery and grade significantly decrease. This is attributed to the copper flotation being depressed with more than 400 g/t Na2S, which can be deduced according to the rule that excess Na2S will E


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ACS Sustainable Chemistry & Engineering depress the flotation of metal minerals, especially sulfides. Therefore, the optimum Na2S dosage was considered as 400 g/ t. Closed-Circuit Test and Its Product Analysis. As mentioned above, the optimized flotation parameters are established as follows: 7 min of grinding time, 200 g/t of EK, 400 g/t of SIAX, and 400 g/t of Na2S. Based on the flotation condition experiments, the flotation closed-circuit process for the recovery of copper from the spent MgO-Cr2O3 bricks was investigated and the developed process has been given in Figure 2. The results of the closed-circuit test are listed in Table 2. It is Table 2. Results of the closed-circuit test (wt.%) Products

Yield

Grade (Cu)

Recovery (Cu)

Concentrate Tailing Feed

21.3 78.7 100

21.4 0.33 4.83

94.6 5.41 100

Figure 10. XRD patterns of the concentrate (a) and the tailing (b).

tailing mainly contains periclase, magnesiochromite, and magnesium copper oxide. This not only confirms the conclusion obtained from Table 3 but also testifies that the copper in the tailing is mainly dispersed in magnesium copper oxide, which is the major reason for limiting the further recovery of the copper. The optical micrographs and corresponding SEM images of the concentrate and tailing are presented in Figure 11. According to EDS analysis combined with Figure 5, it is revealed that the minerals A to D in Figure 11 are copper, cuprite, magnesiochromite, and periclase, respectively. The phase compositions of the concentrate and tailing obtained from Figure 11 are consistent with those from the XRD patterns shown in Figure 10. Additionally, it is found from Figure 11c and Figure 11d that there are some quartz and minor iron oxides present in the tailing according to their EDS patterns, which are also shown in Figure 11. The copper reported in the tailing was dispersed in the refractories as magnesium copper oxide, causing that it can hardly be further recycled by flotation. This supports the conclusion from XRD analysis. Therefore, this flotation process is very effective to separate the copper from the spent MgO-Cr2O3 bricks.

seen from Table 2 that the grade of copper concentrate was 21.4%, while the copper recovery was as high as 94.6%. On the other hand, the tailing only contained 0.33% Cu, which hardly affects its reutilization as a raw material for new refractory bricks. The results obtained from the closed-circuit test demonstrate that the objective to effectively separate copper from the refractories was achieved. In order to further investigate the concentrate and tailing, their chemical compositions, crystalline compositions, and microstructural features were analyzed using XRF and ICP, XRD, and optical microscopy combined with SEM-EDS, respectively. The chemical compositions of the concentrate and tailing are presented in Table 3. Combined with Table 1, it is found from Table 3 that, besides Cu, other heavy metals, including Sn, Zn, Ni, Pb, Sb, Ag, and Bi, were also enriched into the concentrate and the tailing only contained trace or no heavy metal impurities, indicating that the removal of these metal impurities from the refractories was well achieved. In addition, sulfur as another impurity element was also concentrated into the concentrate. As a result, the concentrate will be returned to the corresponding smelting process for the recycle of these valuable metals and sulfur, and the tailing only containing trace impurities can be reused as a raw material for new refractory bricks. On the other hand, the chemical composition of the new refractories is also given in Table 3. The results confirm the conclusion that the tailing can be used to produce new refractory bricks. Figure 10 shows the XRD patterns of the flotation concentrate and tailing. The XRD results indicate that the concentrate is mainly composed of periclase, magnesiochromite, magnesium copper oxide, cuprite, and copper, and the

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CONCLUSIONS

(1) The spent MgO-Cr2O3 bricks were mainly composed of periclase, magnesiochromite, magnesium copper oxide, metallic copper, and cuprite, and they contained 4.83 wt % Cu, which was mainly in the form of Cu, Cu2O, and Mg1−xCuxO (x = 0.05−0.22). Most of the copper particles were large sizes and nonspherical and had clear edges and corners and thus could be easily

Table 3. Chemical Compositions of the Concentrate and Tailing and the New Refractories (wt %)a

a

Elements

Cub

Mg

O

Fe

Cr

Si

Al

Ca

S

Sn

Zn

Ni

Concentrate Tailing Refractory Elements

21.4 0.33 0.13 Pb

35.7 36.4 37.2 P

28.5 32.2 30.6 Ti

4.55 4.06 4.36 Mn

4.00 4.94 5.19 Cl

2.70 2.75 2.55 V

1.78 1.80 2.00 Sb

1.73 1.52 1.50 Ag

0.53 0.15 0.11 K

0.30 0.04 0.03 Bi

0.26 0.05 0.03 Zr

0.23 0.04 0.04 Sr

Concentrate Tailing Refractory

0.15 0.06 0.02

0.14 0.10 0.11

0.13 0.14 0.12

0.08 0.10 0.11

0.06 − −

0.05 0.04 0.04

0.05 − −

0.02 − −

0.01 − −

0.01 − −

0.01 − −

− 0.01 −

“−” means not detected. bDetected by ICP. F


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Figure 11. Optical micrograph (a) and SEM image (b) of the concentrate, and optical micrograph (c) and SEM image (d) with some EDS patterns of the tailing.

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recovered by flotation. Cuprite is a copper oxide and could be recovered by sulfidation with Na2S, but it was difficult to recover the copper contained in magnesium copper oxide, which was the main factor limiting the copper recovery. (2) The copper entered into the refractory bricks from their cracks and holes through metal penetration, and the amount of copper gradually decreased when going deeper into the bricks. MgO-Cr2O3 refractory is made of oxidized materials and, hence, is hydrophilic, resulting in copper oxides being easier than metallic copper, while the metallic copper is easier than its sulfides in infiltrating into the bricks, depending on the difference in their wetting angles with the refractory. (3) Grinding time, EK dosage, collectors, and Na2S dosage had significant effects on the recovery of copper. The optimum process parameters were 7 min of grinding time, 200 g/t of EK, 400 g/t of SIAX, and 400 g/t of Na2S. Based on these conditions, about 95 wt % Cu was recovered from the bricks by a closed-circuit test and the concentrate contained 21.4 wt % Cu, which can be returned to the copper smelting process as a feeding. The tailing only contained trace impurities and thus could be reused as a raw material for new refractory bricks. Additionally, other valuable elements, including Sn, Zn, Ni, Pb, Ag, Bi, and S, were also enriched in the concentrate. This favors their reutilization.

AUTHOR INFORMATION

Corresponding Authors

*Fen Jiao. E-mail: [email protected]. Tel: +86 13549683403. Fax: +86 731 88830346. *Wei Liu. E-mail: [email protected]. Tel: +86 13787007421. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Innovation Project for Postgraduates of Central South University (2015zzts090) for funding this research.

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Innovative Methodology for Comprehensive Utilization of Spent MgO

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