Iron Recovery from Discarded Copper Slag in a RHF Direct Reduction and Subsequent Grinding/Magnetic Separation Process

Transcription

1 minerals Article Iron Recovery from Discarded Copper Slag in a RHF Direct Reduction Subsequent Grinding/Magnetic Separation Process Zhicheng Cao 1,2, Tichang Sun 1, *, Xun Xue 1,2 Zhanhua Liu 2 1 Key Laboratory High-Efficient Mining Safety Metal Mines, Ministry Education, University Science Technology Beijing, Beijing , China; caozhicheng@shenwu.com.cn (Z.C.); xuexun@shenwu.com.cn (X.X.) 2 Beijing Shenwu Envment & Energy Technology Corp., Beijing , China; liuzhanhua@shenwu.com.cn * Correspondence: suntc@ces.ustb.edu.cn; Tel.: Academic Editor: Massimiliano Zanin Received: 4 August 2016; Accepted: 20 October 2016; Published: 3 November 2016 Abstract: Studies on direct carbon-bearing pellets made from discarded copper slag have been conducted in this paper. They include influences coal content, limestone content, industrial sodium carbonate content, temperature, time s carbon-bearing pellets on effect. Finally, optimum conditions have been obtained. The pilot scale experiment results show that optimum conditions are mass proportion discarded copper slag, coal, limestone industrial sodium carbonate 100:25:10:3, temperature 12 C for time 35 min, three s (approximately 42 mm) carbon-bearing pellets this was basis on which pilot tests in a rotary hearth furnace (RHF) were conducted. The products obtained from pilot tests under such conditions have an grade.35% with an recovery rate 89.70%. The mechanism research based on analysis results X-ray diffraction (XRD), scanning electron microscopy (SEM) energy dispersive spectroscopy (EDS) indicates that fayalite (2FeO SiO 2 ) magnetite (Fe 3 O 4 ) in copper slag are reduced into metallic Fe in direct (DR) process, mass heat transfer become stronger from bottom to top pellets, resulting in a rising recovery rate. Keywords: rotary hearth furnace; discarded copper slag; s carbon-bearing pellets; direct 1. Introduction At present, copper production in China mainly comes in a pyrometallurgical process. Generally, for one ton copper, 2.2 tons discarded copper slag is generated, in China, amount discharging copper slag exceeds 10 million tons per year [1], with accumulated amount copper slag over years exceeding 120 million tons [2]. Iron can reach about 48 million tons in this copper slag, based on a calculation 40% Fe content in slag. No such costs for operations such as mining or beneficiation (usually required for concentrates) have to incur for copper slag as it is discarded from a copper smelting plant. That is to say, copper slag is a kind costless raw material. Thanks to rich zinc content in such copper slag, same can also be recovered in form zinc oxide powder when element is recovered. Therefore, copper slag with 40% Fe content is a better resource economically than conventional deposits. Fe exists in copper slag mainly in form fayalite (2FeO SiO 2 ), which is hard to recover. Current disposal practice adopted by many enterprises is stockpiling, which not only occupies large pieces l, but also pollutes envment. Furrmore, a lot valuable metals (Fe, Cu, Zn, etc.,) Minerals 2016, 6, 119; doi:10.33/min

2 Minerals 2016, 6, settled in slag cannot be effectively utilized. Main methods in use for recovery from copper slag include direct beneficiation for recovery [3], oxidative modification [4,5] oxide for recovery, smelting for recovery [6,7], direct (DR) for recovery [8,9], etc. However, recovery rates in direct beneficiation oxidative modification are too low, whereas energy Minerals consumption 2016, 6, 119 smelting is too high. By using DR followed by2 a magnetic 11 separation process for recovery from copper slag, Yang [10] obtained powder 92.05% content with slag an cannot be recovery effectively rate utilized %, Main methods Wang in use for [11] obtained recovery from copper powder slag include 92.96% direct beneficiation for recovery [3], oxidative modification [4,5] oxide for recovery, content with an recovery rate 93.49%. Both ir tests have achieved good results. However, smelting for recovery [6,7], direct (DR) for recovery [8,9], etc. above However, researches, recovery whichrates arein technically direct beneficiation feasible, have oxidative beenmodification done onlyare intoo low, bench whereas test laboratory, energy butconsumption a lack industrial smelting equipment to process is too high. copper By using slag with DR followed coal-based by a DR magnetic in large scale. In practice, separation coal-based process DR for facilities recovery include from copper tunnel slag, kilns Yang [12], [10] rotary obtained kilns [13], powder rotary 92.05% hearth furnaces (RHF) [14,15], content with etc. an The tunnel recovery kilnrate features 81.01%, a low capacity, Wang [11] high obtained energy consumption, powder 92.96% a severe content with an recovery rate 93.49%. Both ir tests have achieved good results. However, production envment, while rotary kiln for DR process is usually applied in large-scale to above researches, which are technically feasible, have been done only in bench test laterite nickel ore, which has negative effects on operation due to ringlet formation resulting laboratory, but a lack industrial equipment to process copper slag with coal-based DR in large scale. from high In practice, temperature. coal-based Therefore, DR facilities include tunnel facilities kilns [12], with rotary lowkilns energy [13], rotary consumption hearth furnaces are in urgent dem(rhf) for [14,15], large-scaled etc. The tunnel treatment kiln features industrial a low capacity, production. high energy This paper consumption, takes copper a severe slag from a domestic production copper smelting envment, plant while as object rotary kiln study. for In DR studies, process is basic usually applied pilot plant in tests have been large-scale conducted to laterite following nickel ore RHF, DR which grinding/magnetic has negative effects separation operation process due to ringlet flow, formation resulting from high temperature. Therefore, facilities with low energy mechanism has been analyzed. consumption are in urgent dem for large-scaled treatment industrial production. This paper takes copper slag from a domestic copper smelting plant as object study. In studies, basic 2. Experimental pilot plant tests have been conducted following RHF DR grinding/magnetic separation process flow, mechanism has been analyzed Materials The 2. Experimental raw material in tests is discarded copper slag (hereinafter referred to as copper slag) a domestic copper smelting plant. Its particle size is mm accounts for 74.48% total Materials The major chemical composition copper slag is listed in Table 1 as below. The Fe content in The raw material in tests is discarded copper slag (hereinafter referred to as copper slag) copper slag is up to 40.78% Zn content in it is 1.35%. Both m have certain recovery values. a domestic copper smelting plant. Its particle size is mm accounts for 74.48% total. The major chemical composition copper slag is listed in Table 1 as below. The Fe content in Table 1. Chemical composition discarded copper slag. copper slag is up to 40.78% Zn content in it is 1.35%. Both m have certain recovery values. Elements TFe Table FeO1. Chemical Cu CaO composition MgO SiOdiscarded 2 Al 2 Ocopper 3 Na 2 slag. O K 2 O Pb Zn S Content (wt %) Elements TFe FeO 0.25 Cu 1.74 CaO 2.08 MgO SiO Al2O Na2O 0.61 K2O 0.51 Pb 1.35 Zn 0.19 S Content (wt %) Figure 1 is X-ray diffraction (XRD) patterns selected copper slag. With reference to chemical Figure composition 1 is X-ray diffraction Table 1, (XRD) it can patterns be seen that selected copper mineral slag. With in reference slag is to mainly chemical composition Table1, it can be seen that mineral in slag is mainly fayalite fayalite (2FeO SiO 2 ), which is generally hard to recover in those conventional beneficiation processes. (2FeO SiO2), which is generally hard to recover in those conventional beneficiation processes. The The anthracite, anthracite, with with aa fixed fixed carbon carbon content content 82.47%, 82.47%, a volatile a volatile matter matter 6.51% 6.51% an ash an content ash content 11.35%, 11.35%, is usedis as used a reducing as a reducing agent agent for for tests. tests. The The limestone with a CaCO3 CaCOcontent 3 content %, %, pure industrial pure industrial alkali with alkali a sodium with a sodium carbonate carbonate content content 98% 98% are are used as additives. Figure 1. XRD patterns received copper slag. Figure 1. XRD patterns received copper slag.

3 Minerals 2016, 6, Theoretical Analysis Mixed with reducing agent additives, copper slag is made into carbon-bearing pellets (12 16 mm), which should be dried before being charged into a RHF. After passing preheating zone, middle temperature zone, high temperature zone cooling zone, % oxides in pellets are reduced into metallic Fe. Furrmore, ZnO in pellets are reduced into metallic Zn, which is collected by a bag-filter system. In order to speed up reactions, an appropriate amount limestone industrial sodium carbonate are added to pellets, major chemical reactions taking place are shown as below Reduction Mechanism Fayalite Reduction reactions fayalite can be obtained from combination Equations (1) (2) its result is shown in Equation (3). 2FeO(s) + 2C(s) 2Fe(s)+2CO(g) r G m θ = T (J mol 1 ) (1) 2FeO(s) + SiO 2 (s) Fe 2 SiO 4 (s) r G m θ = T (J mol 1 ) (2) Fe 2 SiO 4 (s) + 2C(s) 2Fe(s) + SiO 2 (s) + 2CO(g) r G m θ = T (J mol 1 ) (3) From calculations, it can be seen that Fe 2 SiO 4 is more difficult to reduce than FeO because dissociation temperature former is higher than that latter. Since both Equations (1) (3) are intensely endormic, to raise roasting temperature will help accelerate reducing reaction rate. Besides, Equation (1) can be formed from combination following two reactions: Recovery Mechanism Zn C(s) + CO 2 (g) 2CO(g) (4) FeO(s) + CO(g) Fe(s) + CO 2 (g) (5) ZnO in dust is reduced at high temperature into metallic Zn, which is vaporized separated in form metal vapor before being reoxidized while flowing along with flue gas in flue duct, being collected in form oxides. The mechanism includes several reactions under high temperature: ZnO(s) + C(s) Zn(g) + CO(g) (6) ZnO(s) + CO(g) Zn(g) + CO 2 (g) (7) ZnFe 2 O 4 (s) + 2Fe(s/l) Zn(g) + 4FeO(s) (8) The above reactions are endormic, so higher temperature, more conducive to reactions are. At higher temperature, rmodynamic conditions for Zn vaporization will be greatly improved Reaction Mechanism Additives Since copper slag is a kind acidic slag, addition calcium oxide in DR process results in following reactions: Fe 2 SiO 4 (s)+2cao(s) +2C(s) Ca 2 SiO 4 (s) + 2Fe(s) + 2CO(g) r G m θ = T (J mol 1 ) (9) Na 2 CO 3 (s) + SiO 2 (s) Na 2 SiO 3 (s) + CO 2 (g) (10) From above reactions, it can be seen that higher temperature is, more conducive to reactions are. Fe 2 SiO 4 can also be reduced into metallic at higher reducing

4 /% Minerals 2016, 6, temperature. By adding CaO in DR process, temperature Fe 2 SiO 4 can be effectively lowered, Minerals thus 2016, 6, 119 DR capability has been increased DR Fe 2 SiO 4 has been enhanced. 4 Silicates, 11 formed by addition sodium carbonate as basic oxides to be combined with SiO 2, can destroy temperature. By adding CaO in DR process, temperature Fe2SiO4 can be olivine structure copper slag increase FeO activity, thus accelerating effectively lowered, thus DR capability has been increased DR Fe2SiO4 has been oxides. enhanced. Silicates, formed by addition sodium carbonate as basic oxides to be combined with SiO2, can destroy olivine structure copper slag increase FeO activity, thus 2.3. Experimental Procedure accelerating oxides. The testing order is followed with basic tests, carried out first before results being verified at 2.3. pilot Experimental plant tests. Procedure The procedures are as follows: mixing raw materials pelleting drying DR roasting The testing order wateris cooling followed with grinding/magnetic basic tests, carried separation out first before final DR results product. being verified The focus at is on pilot quality plant tests. final The procedures DR product are as follows: mixing recovery raw rate, materials with pelleting calculation drying method DR used as follows: roasting water cooling grinding/magnetic separation final DR product. The focus is on quality final DR product ε = (W 1 recovery β)/(w rate, α) with 100% calculation method used as follows: (11) Where ε is recovery rate, ε W= (W1 1 is β)/(w weight α) 100% magnetically separated powder, (11) β is Fe content Where ε is powder, recovery W israte, W1 weight is weight discarded magnetically copper slag, separated α is powder, content β is copper Fe slag, content respectively. powder, W is weight discarded copper slag, α is content copper slag, respectively. 3. Results Discussions 3. Results Discussions The early tests indicated that when one pellets were under roasting, highest temperature The early couldtests only indicated be keptthat 1200 when C, one because pellets higher were temperature under roasting, could cause highest pellets melting, which temperature is not conducive could only to be kept RHF at 1200 production. C, because When higher twotemperature s pellets could cause werepellets laid for melting, roasting, which is temperature not conducive could to be increased RHF production. nowhen pelletstwo melting s took pellets place, were thus alaid good for pellet roasting, shape can be maintained. temperature Under could thisbe condition, increased many no pellets factorsmelting that affect took Fe place, content thus a good recovery pellet shape ratecan be powder maintained. were Under also examined, this condition, such many as factors amount that affect Fe content coal, quantity recovery rate additives, such as powder were also examined, such as amount coal, quantity additives, such as limestone industrial sodium carbonate, temperature time. limestone industrial sodium carbonate, temperature time Effect Reduction Coal Dosage on DR Process 3.1. Effect Reduction Coal Dosage on DR Process The exploratory experiments show that 5% bentonite should be added into copper slag The exploratory experiments show that 5% bentonite should be added into copper slag pellets, with a drop strength (number non-breakage a pellet drop at height 0.5 m onto a pellets, with a drop strength (number non-breakage a pellet drop at height 0.5 m onto a steel steel plate) plate) carbon-containing carbon-containing pellets pellets being being reached reached over over six six times, times, thus thus meeting meeting requirement requirement RHF process. RHF process. The examinations The examinations effect effect coal dosage coal dosage on on product s product s Fe content Fe content recovery rates recovery were carried rates were outcarried under out following under conditions: following conditions: limestone dosage limestone dosage 15%, sodium carbonate 15%, sodium dosage carbonate 0.5%, dosage 0.5%, temperature temperature 1250 C, 1250 C, time time 30 min, 30 grinding min, fineness grinding fineness mm accounting mm accounting for 72.1% for 72.1% copper slag copper mix slag mix magnetic magnetic field strength field strength ka/m The result ka/m. isthe shown result in is Figure shown 2 in below. Figure 2 below Reduction coal dosage/% Figure 2. Effect coal dosage on DR process.

5 /% /% Fe recocery /% /% /% /% /% Minerals 2016, 6, Minerals 2016, 6, Figure 2 shows that, with increase dosage coal, Fe content Minerals powder Figure 2016, lies 26, shows 119 between that, % with 87%, increase dosage recovery rate firstly coal, slightly Fe rises content n 5 declines, 11 refore powder optimum lies between dosage% 87%, coal is set at 25%. recovery With rate lower firstly dosage slightly rises n Figure 2 shows that, with increase dosage coal, Fe content coal, declines, parts refore oxides optimum remain dosage unreduced, whereas coal is set at higher 25%. With dosage lower dosage coal powder lies between % 87%, recovery rate firstly slightly rises n will hinder coal, agglomeration parts oxides reduced remain unreduced, grain, thuswhereas causing higher negative dosage impacts on declines, refore optimum dosage coal is set at 25%. With lower dosage subsequent grinding/magnetic coal will hinder separation agglomeration for recovery. reduced grain, thus causing negative coal, parts oxides remain unreduced, whereas higher dosage impacts on subsequent grinding/magnetic separation for recovery. coal will hinder agglomeration reduced grain, thus causing negative 3.2. Effect Limestone Dosage on DR Process 3.2. impacts Effect on Limestone subsequent Dosage grinding/magnetic on DR Process separation for recovery. The examination effect limestone dosage on product s Fe content recovery The examination effect limestone dosage on product s Fe content recovery rates 3.2. were Effect conducted. Limestone The proportion Dosage on DR copper Process slag, coal industrial sodium carbonate rates were conducted. The proportion copper slag, coal industrial sodium was fixedthe atexamination 100:25:0.5, with effect or conditions limestone dosage product s grinding/magnetic Fe content recovery separation carbonate was fixed at 100:25:0.5, with or conditions grinding/magnetic remaining rates were separation unchanged. conducted. remaining The The unchanged. result proportion is The shown result in is Figure copper shown 3slag, in below. coal industrial sodium Figure 3 below. carbonate was fixed at 100:25:0.5, with or conditions grinding/magnetic separation remaining unchanged. 100 The result is shown in Figure 3 below Limestone dosage/% Figure 3. Effect Limestone limestone dosage/% dosageon on DR DR process. Figure 3 shows that, Figure with 3. Effect increase limestone in dosage dosage on DR limestone, process. grade has a Figure 3 shows that, with increase in dosage limestone, grade has a tendency tendency to rise, recovery rate rises first n declines. Furrmore, with limestone to rise, Figure 3 shows recovery that, with rate rises increase first n in declines. dosage Furrmore, limestone, with limestone grade has dosagea dosage 10% copper slag, grade can reach 88.79% with recovery rate 10% tendency 86.75%. copper to rise, However, slag, with limestone grade recovery canrate dosage reach rises 88.79% first n 15% withdeclines. 20%, partial Furrmore, recovery melting rate with upper 86.75%. limestone However, withdosage carbon-containing limestone 10% dosage copper pellets obstructs 15% slag, 20%, heat transfer partial grade can to melting reach lower 88.79% s upper with pellets, thus recovery carbon-containing rate reducing pellets 86.75%. effect obstructs However, is negatively heat with impacted. transfer limestone Therefore, to dosage lower optimal s 15% dosage pellets, 20%, partial thus limestone melting reducing is set at 10%. effect upper is negatively carbon-containing pellets obstructs heat transfer to lower s pellets, thus reducing impacted. Therefore, optimal dosage limestone is set at 10% effect Effect is negatively Industrial impacted. Sodium Therefore, Carbonate Dosage optimal on dosage DR Process limestone is set at 10% Effect Industrial Sodium Carbonate Dosage on DR Process 3.3. Effect Examinations Industrial Sodium effect Carbonate industrial Dosage sodium on carbonate DR Process dosage on product s Fe content Examinations recovery rate were effect conducted. industrial The proportion sodium carbonate copper dosage slag, product s coal Fe industrial content Examinations effect industrial sodium carbonate dosage on product s Fe content recovery sodium ratecarbonate were conducted. was fixed Theat proportion 100:25:10, with copper slag, or conditions coal industrial recovery rate were conducted. The proportion copper slag, coal industrial sodium carbonate grinding/magnetic was fixed separation at 100:25:10, remaining with unchanged. or conditions The result is shown in Figure 4. sodium carbonate was fixed at 100:25:10, with or conditions grinding/magnetic separation grinding/magnetic remaining separation unchanged. remaining The result unchanged. is shownthe in Figure result is 4. shown in Figure 4. Fe Fe recovery grade Industrial sodium carbonate dosage/% Figure 4. Effect industrial Industrial sodium sodium carbonate dosage dosage/% on DR process. Figure Effect industrial sodiumcarbonate carbonatedosage dosageon on DR DR process. process.

6 /% Minerals 2016, 6, Minerals 2016, 6, Figure 4 shows that, both grade recovery rate show a tendency to increase at first n decrease with increase in dosage industrial sodium carbonate. This is because more more silicates are formed from combination sodium carbonate with SiO 2 2,, which destroys silicate mineral structure copper slag, slag, enhances oxides. oxides. However, However, a higher a higher dosage dosage sodium sodium carbonate carbonate leads to leads top top carbon-containing pellets pellets melting, melting, thus obstructing thus obstructing furr furr lower s lower s pellets. pellets. As a result, As a result, optimal optimal dosage dosage industrial industrial sodium carbonate sodium carbonate is set at 3%. is set at 3% Effect Roasting Temperature on DR Process From all above tests, optimal proportion copper slag, coal, limestone industrial sodium carbonate was fixed at 100:25:10:3. Tests on temperature were conducted on such a basis, with details shown in Figure 5. Figure Figure Effect Effect temperature on DR DR process. process. Figure 5 shows that, with increase in temperature, grade Figure 5 shows that, with increase in temperature, grade recovery rate have a tendency to first rise before declining, at temperature recovery rate have a tendency to first rise before declining, at temperature 12 C, 12 C, an grade.18% can be obtained at highest recovery rate 89.91%. As an grade.18% can be obtained at highest recovery rate 89.91%. As temperature temperature continues rising, rmodynamic conditions for mass heat transfer in lower continues rising, rmodynamic conditions for mass heat transfer in lower pellet pellet deteriorate. Therefore, optimal temperature is 12 C. deteriorate. Therefore, optimal temperature is 12 C Effect Reduction Time on DR Process 3.5. Effect Reduction Time on DR Process Roasting temperture/ o C With With proportioning proportioning grinding/magnetic grinding/magnetic separation separation conditions conditions raw raw materials materials remaining unchanged, tests on time were conducted at temperature remaining unchanged, tests on time were conducted at a temperature 12 C, grade recovery rate showed a tendency to increase. With 12 C, grade recovery rate showed a tendency to increase. With time set at time 35 min, set at an 35 min, grade an.% grade.% an recovery an rate recovery.49% rate could.49% be obtained. could be obtained. Furr increase Furr in increase in time did not time result did in any not considerable result in any improvement, considerable improvement, optimal optimal time is set at 35 min, time from is set a at cost-saving 35 min, from perspective. a cost-saving Details perspective. are shown Details in Figure are 6. shown in Figure Effect Layers Carbon-Bearing Pellets on DR Process All above tests were conducted on two s carbon-bearing pellets, with such conditions as proportion, temperature, time grinding/magnetic separation remaining unchanged. Furr tests were carried out on impacts pellet s from 1 to 4 on. The details are shown in Figure 7.

7 /% /% /% /% Minerals 2016, 6, Reduction time/min Figure 6. Effect time on DR process Effect Layers Carbon-Bearing Pellets on DR Process All above tests were 15 conducted 25on two s 35 carbon-bearing 45 pellets, with such conditions as proportion, Reduction time/min temperature, time grinding/magnetic separation remaining unchanged. Furr tests were carried out on impacts Figure6. 6. Effect time on DR pellet s from 1 to 4 on. The details are shown process. in Figure Effect Layers Carbon-Bearing Pellets on DR Process All above tests were conducted on two s carbon-bearing pellets, with such conditions as proportion, temperature, time grinding/magnetic separation remaining unchanged. Furr tests were carried out on impacts pellet s from 1 to 4 on. The details are shown in Figure Carbon-containing pellets s Figure 7. Effect carbon-containing pellets s on DR process. Figure 7. Effect carbon-containing pellets s on DR process. It can be seen from Figure 7 that distribution thickness three s (about 42 mm) carbon-containing It can be seen from pellets Figure can 7have that distribution best thickness effect with three s highest (about 42 grade mm) recovery carbon-containing rate obtainable. pelletswhen can have carbon-containing best effect pellets with are in highest four s, heat grade transfer recovery to lower rate obtainable. s deteriorates When with carbon-containing Carbon-containing results pellets being arepellets in negatively four s, affected, heat transfer to lower grade s deteriorates recovery with rate Figure greatly 7. declining. results being negatively affected, grade recovery rate Effect carbon-containing pellets s on DR process. greatly declining. 4. The It can Pilot-Scale be seen Tests from Figure via RHF 7 that distribution thickness three s (about 42 mm) 4. The Pilot-Scale Tests via RHF carbon-containing pellets can have best effect with highest grade The optimal conditions obtained from basics tests are proportion copper slag, recovery The rate optimal obtainable. conditions When obtained carbon-containing from basicspellets tests are in four proportion s, heat transfer copper to slag, coal, limestone, industrial sodium carbonate fixed as 100:25:10:3, at a distribution lower s deteriorates coal, limestone, with industrial results sodium being carbonate negatively fixed affected, as 100:25:10:3, at a distribution grade thickness three pellet s, a temperature 12 C for a time 35 min. thickness recovery three rate greatly pellet s, declining. a temperature 12 These conditions were applied to pilot-scale tests on a RHF having C for a time 35 min. an outer diameter 10 m, These conditions were applied to pilot-scale tests on a RHF having an outer diameter 10 m, a processing capability 2 t/h. 4. The a processing Pilot-Scale capability Tests via RHF 2 t/h. The metallization pellets discharged from RHF went through a grinding/magnetic The The optimal metallization conditions pellets obtained discharged from from basics RHF tests went are through proportion a grinding/magnetic copper separation separation process, with a grinding fineness mm accounting for 72.10% total slag, a process, with coal, a grinding limestone, fineness magnetic field strength industrial mm ka/m sodium accounting first carbonate for 72.10% stage, fixed a grinding as 100:25:10:3, total fineness at a magnetic a distribution field mm thickness strength three pellet ka/m s, in first a stage, a temperature grinding fineness 12 C for a mm accounting time for % min. These conditions rest a magnetic were applied field to strength pilot-scale.54 tests ka/m on in a RHF second having stage. an outer The diameter content 10 m, obtained a processing products capability is.3% 2 t/h. with an recovery rate 89.70%. The metallization pellets discharged from RHF went through a grinding/magnetic separation process, with a grinding fineness mm accounting for 72.10% total a magnetic field strength ka/m in first stage, a grinding fineness mm

8 Minerals 2016, 6, accounting for 50.89% rest a magnetic field strength.54 ka/m in second stage. Minerals 2016, 6, The content obtained products is.3% with an recovery rate 89.70%. 5. Studies Mechanisms Product Analysis 5.1. Analysis Mechanism Distribution Thickness (Pellets Layers) In order to to explore mechanism effect effect s s carbon-bearing carbon-bearing pellets (thickness pellets (thickness s) on s) DR process, on DR XRDprocess, analysis XRD wasanalysis conducted was onconducted copperon slag copper metallization slag pellets metallization at different pellets locations different a locations 4- distribution a 4- thickness. distribution The thickness. details are The shown details inare Figure shown 8. The in Figure first, 8. The second, first, third second, fourth third, respectively, fourth, indicate respectively, pellets indicate from upper pellets (top) from to upper lower (top) (bottom). to lower (bottom). Figure Figure XRD XRD patterns patterns copper copper slag slag metallization metallization pellets. pellets. In Figure 8, four changes can be observed: (1) The phases fayalite (Fe 2SiO4) In Figure 8, four changes can be observed: (1) The phases fayalite (Fe magnetite (Fe3O4) do not exist anymore after DR in a RHF; (2) Differences 2 SiO in 4 ) magnetite major peak (Fe heights 3 O 4 ) do not exist anymore after DR in a RHF; (2) Differences in major peak heights metallized metallized pellets from 1 to 4 mean differences in ir Fe contents, namely, pellets from 1 to 4 mean differences in ir Fe contents, namely, 1 > 2 > 1 > 2 > 3 > 4; (3) Occurrence diffraction peak values dissociate 3 > 4; (3) Occurrence diffraction peak values dissociate SiO SiO2 in pellets 1, 2 3 indicates a complete effective, with 2 in pellets 1, 2 all SiO 2 released 3 indicates a complete effective, with all SiO from fayalite phase that is conducive to an increase in 2 released from fayalite phase that recovery rate through is conducive to an increase in recovery rate through grinding magnetic separation; grinding magnetic separation; (4) Apparent appearance diffraction peak impurities (4) Apparent appearance diffraction peak impurities (augite Ca(Fe,Mg)Si (augite Ca(Fe,Mg)Si2O6) in metallization pellets 4 also indirectly indicates 2 O 6 ) in that metallization pellets 4 also indirectly indicates that effect pellets in effect pellets in distributed 4 is not as good as those in 1, 2 3. distributed 4 is not as good as those in 1, 2 3. Therefore, it can be concluded that pellets in a distribution thickness Therefore, it can be concluded that pellets in a distribution thickness three s is comparatively more complete, which is beneficial for recovery in three s is comparatively more complete, which is beneficial for recovery in subsequent subsequent grinding/magnetic separation process. grinding/magnetic separation process. Figure 9 shows results analysis by using SEM EDS conducted at different Figure 9 shows results analysis by using SEM EDS conducted at different locations locations metallization pellets, with a, b, c, d respectively indicating 1, 2, 3 4. It can metallization pellets, with a, b, c, d respectively indicating 1, 2, 3 4. It can be clearly be clearly seen that crystal chains are formed from agglomeration in 1, 2 3, seen that crystal chains are formed from agglomeration in 1, 2 3, which is which is conducive to recovery in subsequent grinding/magnetic separation operations. The conducive to recovery in subsequent grinding/magnetic separation operations. The pellets pellets 4, however, show scattered particles in pellets, indicating a less effective 4, however, show scattered particles in pellets, indicating a less effective, with, with a lower recovery than those in first three s. a lower recovery than those in first three s.

9 Minerals 2016, 6, 119 Minerals 2016, 6, Figure9.9.SEM SEMimages images EDS EDS analysis analysis results (a)(a) 1; (b) 2; 2; Figure results metallization metallizationpellets. pellets. 1; (b) (c) 3; (d) 4. (c) 3; (d) Product Analysis 5.2. Product Analysis The chemical analysis powder obtained from pilot plant tests is shown in Table 2, The chemical analysis powder obtained from pilot plant tests is shown in Table 2, chemical analysis ZnO-rich dust obtained from bag filter system is shown in Table 3. be chemical ZnO-rich dust obtained from bag system is shown in Table It can seen thatanalysis metallic (MFe) accounts for majority filter magnetically separated 3. It powder, can be seen that metallic (MFe) accounts for majority magnetically separated with a metallization rate 99.09%, less contents impurities such as sulfur powder, a metallization rate reduced 99.09%, less contents impurities sulfur phosphorous,with which is a good quality powder. The zincoxide contentsuch aszno-rich phosphorous, which is a good quality reduced powder. The zinc oxide content ZnO-rich dust, obtained from bag filter system, is 68.53%, which can be a good raw material for zinc smelters. dust, obtained from bag filter system, is 68.53%, which can be a good raw material for zinc smelters. Table 2. Chemical composition obtained powder from grinding/magnetic separation. Table 2. Chemical composition obtained powder from grinding/magnetic separation. Elements Tfe Mfe CaO Content (wt %) Elements Tfe Mfe CaO Content (wt %) Elements Content (wt %) Elements MgO SiO2 Al2O3 Cu C Mn P S MgO SiO2 Al2 O3 Cu C Mn P S Table 3. Chemical composition obtained ZnO rich dust TFe CaO MgO SiO2 Al2 O3obtained K2O Na 2O PbO ZnO C S P Table 3. Chemical composition ZnO rich dust TFe CaO MgO SiO2 Al2 O3 K2 O Na2 O PbO ZnO C S P Figure 10 is XRD patterns SEM image powder from grinding/magnetic Content (wt %) separation. It can be seen that in powder obtained from grinding/magnetic separation exhibits a high diffraction peak, with certain impurities in pyroxene phase. The SEM Figure 10 islarger XRD SEM image powder powderfrom from grinding/magnetic grinding/magnetic image shows patterns particles in metallization separation,itwith certain phase wrapped separation. can be seen impurity that in augite powder obtained within. from grinding/magnetic separation exhibits a high diffraction peak, with certain impurities in pyroxene phase. The SEM image shows larger particles in metallization powder from grinding/magnetic separation, with certain impurity in augite phase wrapped within.

10 Minerals 2016, 6, Minerals 2016, 6, (a) (b) Figure 10. XRD patterns (a) SEM image (b) powder from grinding/magnetic separation. Figure 10. XRD patterns (a) SEM image (b) powder from grinding/magnetic separation. 6. Conclusions 6. Conclusions (1) Basic tests pilot scale plant tests have been conducted on discarded copper slag from a (1) Basic Chinese testscopper pilot smelting scale plant, tests by adopting have been a direct conducted in discarded a RHF copper a subsequent slag from a grinding/magnetic Chinese copper smelting separation plant, process, by adopting from which a direct optimal conditions in are a RHF acquired aas subsequent follows: grinding/magnetic proportion discarded separationcopper process, slag, from which optimal coal, limestone conditions areindustrial acquired as sodium follows: carbonates proportion 100:25:10:3, discarded copper slag, temperature 12 coal, C limestone for industrial time 35 sodium min, carbonates pellets 100:25:10:3, in three s (about 42 temperature mm thick). The 12 pilot-scale C for tests in a RHF time have been 35 min, carried pellets out under in three such conditions s (about 42 mm obtained thick). The powder pilot-scale products tests have in an a RHF grade have been carried.35% out with under an such recovery conditions rate 89.70%. obtained powder products have an grade (2).35% Analysis with an mechanism recoveryshows rate that 89.70%. fayalite magnetite in copper slag are reduced (2) Analysis into metallic Fe mechanism in direct shows that fayalite process, magnetite mass in heat copper transfer slag become are reduced stronger into metallic from Fe bottom in direct to top, process, exhibiting a higher mass diffraction heat transfer peak. In become addition, stronger from in metallization pellets turns into crystal chains from a dispersion state, resulting in a bottom to top, exhibiting a higher diffraction peak. In addition, in rising recovery rate. metallization pellets turns into crystal chains from a dispersion state, resulting in a rising recovery rate. Acknowledgments: This work was financially supported by Beijing Science Technology Special Project (Grant No. Z ). Acknowledgments: This work was financially supported by Beijing Science Technology Special Project (Grant Author No. Contributions: Z ). Zhicheng Cao, Tichang Sun XunXue conceived designed experiments; Zhicheng Cao Zhanhua Liu performed experiments; Zhicheng Cao Tichang Sun analyzed data; Author Zhicheng Contributions: Cao wrote Zhicheng paper. Cao, Tichang Sun XunXue conceived designed experiments; Zhicheng Cao Zhanhua Liu performed experiments; Zhicheng Cao Tichang Sun analyzed data; Zhicheng Conflicts Cao Interest: wrote The paper. authors declare no conflict interest. Conflicts Interest: The authors declare no conflict interest. References References 1. Jiang, P.G.; Wu, P.F.; Hu, X.J.; Zhou, G.Z. Current state research comprehensive utilization copper slag, proposal new technology. China Min. Mag. 2015, 25, Jiang, P.G.; Wu, P.F.; Hu, X.J.; Zhou, G.Z. Current state research comprehensive utilization copper 2. Zhao, K.; Chen, X.L.; Qi, Y.H.; Zhen, C.L.; Shuai, X.F. Study on impact factors in recovering slag, proposal new technology. China Min. Mag. 2015, 25, copper from copper slag by carbon. Env. Eng. 2012, 30, Zhao, K.; Chen, X.L.; Qi, Y.H.; Zhen, C.L.; Shuai, X.F. Study on impact factors in recovering copper 3. Zeng, J.L.; Xiao, K.M. Research on using dispersant agent to recovery in furnace slag. Nonferr. Met. Sci. Eng. from copper slag by carbon. Env. Eng. 2012, 30, , 2, Zeng, J.L.; Xiao, K.M. Research on using dispersant agent to recovery in furnace slag. Nonferr. Met. 4. Hu, G.; Xu, S.P.; Li, S.G. Steam gasification apricot stones with olivine dolomite as downstream Sci. Eng. 2011, 2, catalysts. Fuel Process. Technol. 2006, 87, Hu, G.; Xu, S.P.; Li, S.G. Steam gasification apricot stones with olivine dolomite as downstream 5. Yang, T.; Hu, J.H.; Wang, H.; Li, L. Enrichment Fe3O4 by roasting dilution copper slag from catalysts. electric furnace Fuel Process. smelting. Technol. Chin. 2006, J. Process. 87, Eng. 2011, [CrossRef] 11, Yang, Yuan, T.; S.Q.; Hu, J.H.; Dong, Wang, J.; Wang, H.; Li, C.; L. Wang, Enrichment Z.J. Study Feon 3 Oprocesses 4 by roasting for comprehensive dilution copper treatment slag from extractive electric furnace copper smelting. tailings Chin. nickel J. Process. smelting Eng. slag. 2011, Chin. 11, J Rare Met. 2014, 38, Yuan, Mansoor, S.Q.; Dong, B.; Keneths, J.; Wang, C. Electrical C.; Wang, Z.J. electronic Study on conductivity processes forcao-sio2-feox comprehensive slags treatment various extractive oxygen copper potentials: tailings Part 1. Experimental nickel smelting results. slag. Met. Chin. Mater. J. Rare Trans. Met. 2006, 2014, 37, 38,

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