Extraction of Iron and Manganese from Pyrolusite Absorption Residue by Ammonium Sulphate Roasting–Leaching Process

metals

Article Extraction of Iron and Manganese from Pyrolusite Absorption Residue by Ammonium Sulphate Roasting–Leaching Process

Lin Deng, Bing Qu, Shi-Jun Su, Sang-Lan Ding and Wei-Yi Sun *

College of Architecture and Environment, Sichuan University, Chengdu 610065, China; [email protected] (L.D.); [email protected] (B.Q.); [email protected] (S.-J.S.); [email protected] (S.-L.D.) * Correspondence: [email protected]; Tel.: +86-028-8564-7252

Received: 21 December 2017; Accepted: 5 January 2018; Published: 8 January 2018

Abstract: The residue from desulfurization and denitriﬁcation of exhaust gas treatment process with pyrolusite ore as absorbent is regarded as a potential source of iron and manganese. In this study, an extraction process is proposed for recovery of iron and manganese with ammonium sulphate roasting followed by sulphuric acid leaching. Firstly, the conversion mechanism was analyzed through mineral phase analysis of roasting products at different roasting temperature by means of X-ray diffraction (XRD) technology. Then, the parameters of the roasting procedure such as roasting temperature and time, ammonium sulphate dosage, leaching temperature, leaching time, and sulphuric acid concentration are examined. The results implicate that the iron oxide and

manganese dioxide in the residue are ﬁrstly converted into the water-soluble (NH4)3Fe(SO4)3 and ◦ (NH4)2Mn2(SO4)3 at 200–350 C, and then the more stable NH4Fe(SO4)2 and MnSO4 are formed, at temperature higher than 350 ◦C. Under optimum conditions, 95.2% Fe and 97.0% Mn can be extracted. Reactant diffusion through inert layer of silicon dioxide was considered as the rate-limiting step for iron extraction with an activation energy of 20.56 kJ/mol, while, the recovery process of Mn was controlled by both reactant diffusion and chemical reaction with an activation energy of 29.52 kJ/mol.

Keywords: pyrolusite absorption residue; recovery; iron; manganese; ammonium sulphate; roasting

1. Introduction

SO2 and NOx emission from combustion of coal, fuel oils and waste have brought signiﬁcantly important effects on environment and human health. Various kinds of technologies [1,2] have been developed to control and reduce SO2 and NOx emissions worldwide. Among these methods, pyrolusite ore adsorption has been regarded as one of the most promising technologies [2,3]. This is because it can successfully combine the removal of pollutants from ﬂue gas, together with the manganese extraction from poor-grade pyrolusite ore [2,4]. Pyrolusite is a manganese oxide ore with manganese dioxide as its main component [5], which can oxidize the SO2 and NO, NO2 in the 2− − exhaust gas into SO4 and NO3 , respectively. These reactions can be written as the following:

2+ 2− MnO2(s) + SO2 = Mn + SO4 (1)

+ 2+ − 3MnO2 + 2NO + 4H = 3Mn + 2NO3 + 2H2O (2) 2+ − MnO2(s) + 2NO2 = Mn + 2NO3 (3)

Our previous studies [2,3] implicated that the removal efﬁciency of SO2 and NOx reached about 90% and 82% respectively, and the extraction efﬁciency of manganese can be maintained

Metals 2018, 8, 38; doi:10.3390/met8010038 www.mdpi.com/journal/metals Metals 2018, 8, 38 2 of 12 at 80~86%, with a pulp density of 500 g/L and a leaching temperature of 25 ◦C. The recovery products

(MnSO4 and Mn(NO3)2) meeting the national standard (HG/T 3817-2006) can be obtained through the further treatment of the absorption liquid, such as the removal of iron and heavy metal ions [6]. Unavoidably, the pyrolusite absorption residue (PAR) was produced in the process of desulfurization and denitrification using pyrolusite ore. The PAR is featured by high iron content, silicon dioxide and remaining manganese dioxide, and it also contains some heavy metal ions, such as cobalt, nickel, barium and calcium ions [7]. However, there are limited investigations on how to deal with these residues in order to reduce the risks of polluting environment. The PAR used to be an adsorbent for the treatment of Congo red and dyeing waste water, and satisfying removal efficiency were reported [8–10]. Lang et al. [11] pointed out that after adding limestone and bauxite, the residue can be used to generate sulphoaluminate cement. On the other hand, more than 45% iron oxide and 4% manganese dioxide were enriched in the PAR, which also can be regarded as a secondary source for iron and manganese. In the past few decades, many methods have been developed to recover iron and manganese from various mineral [12], wastes [13] and residues [14,15]. The reduction roasting-magnetic separation is widely used for iron recovery from red mud using coal [16], sodium carbonate [14] and pyrite [17] as reducing agent at 600–1300 ◦C and the iron recovery efficiency is generally over 85%. Oxalate also was adopted for iron leaching with the advantages of fast leaching rate, high extraction efficiency, attributing its higher chelate ability [18,19]. Manganese recovery can be realized from low-grade ◦ ◦ pyrolusite ore by roasting, using elemental sulphur at 350–500 C[20] or SO2 at 500 C[21] followed by sulphuric acid or water leaching; also by hydrometallurgical treatment processes including aqueous SO2 [15,22], FeSO4 [23], and biomass [24] However, these methods do not meet the requirements for the simultaneous recovery of iron and manganese. Rolf R.F [25] presented their work that 97% manganese and 47% iron can be extracted at the same time by firstly reducing with gaseous hydrogen, carbon monoxide or mixtures at 550~800 ◦C a then leached with ammonium sulphate solutions. Although the high recovery efficiency of both Mn and Fe were obtained, this process consumes more energy and used hazardous gases (H2, CO) as reducing gases. Recently, our group has found that ammonium sulphate, a widely available, low-cost, recyclable chemical agent can be used for extracting various metals containing Co, Ni, Mn and Al from low-grade nickel ore [26], zinc leaching residues [27], mine residues [28] and coal fly ash [29]. The extraction efficiency of manganese from the above minerals can achieve satisfying results i.e., higher than 80%. However, the iron leaching rate is lower, generally less than 50%. It is surprising that the iron recovery efficiency from ilmenite ore [30] and high-iron bauxite ore [31] could yield higher than 85%. The discrepancy of iron leaching efficiency obtained among these literatures [13,26,29–31] may be attributed to the different purpose of metal recovery; iron was generally regarded as impurities in these reports so that it was restricted to be extracted out. However, research aimed at simultaneous recovery of iron and manganese from minerals or other waste residue using ammonium sulfate as extracting agent has not been reported in literatures. In this paper, the feasibility of sulphating roasting of PAR with ammonium sulphate was comprehensively investigated. The role that ammonium sulphate plays in the sulphating roasting process and corresponding reaction mechanism are discussed. The roasting kinetics were analysed to elucidate the extraction characteristics of manganese and iron. The effect of ammonium sulphate-to-PAR ratio, roasting temperature, roasting time, leaching temperature, leaching time, and sulphuric acid concentration on the extraction behavior have been studied.

2. Materials and Methods

2.1. Materials The Pyrolusite Absorption Residue (PAR) was generated during the ﬂue gas desulfurization process using iron-rich pyrolusite ore as desulfurizer. The obtained residue was washed with distilled water and milled to a size of 200 mesh after dried at 80 ◦C for 3 h prior to experiments. The chemical Metals 2018, 8, 38 3 of 12 Metals 2018, 8, 38 3 of 12 distilledcomposition water of and this milled residue to is a shown size of in 200 Table mesh1. The after average dried at iron 80 and°C for manganese 3 h prior contentsto experiments. were 30.1% The chemicaland 4.03%, composition respectively. of this The residue main minerals, is shown whichin Table are 1. evidentThe average from iron the X-rayand manganese diffraction contents pattern were 30.1% and 4.03%, respectively. The main minerals, which are evident from the X-ray diffraction (Figure1), are goethite (FeOOH) and silicon dioxide (SiO 2). The analysis grade ammonium sulphate pattern (Figure 1), are goethite (FeOOH) and silicon dioxide (SiO2). The analysis grade ammonium ((NH4)2SO4) was used in all experiments. sulphate (NH)SO) was used in all experiments. Table 1. Chemical composition (metal content) of the pyrolusite absorption residue (PAR) used Tablefor experiments. 1. Chemical composition (metal content) of the pyrolusite absorption residue (PAR) used for experiments.

ElementsElements Fe FeMn Mn Al Al Ca Ca Ba Ba Zn Zn Ni Ni Cu Cu Content Content(wt. %) (wt. %) 30.1 30.1 4.03 4.03 1.75 1.75 0.761 0.761 0.491 0.491 0.044 0.044 0.029 0.029 0.021 0.021

Figure 1. 1. TheThe X-ray X-ray diffraction diffraction of ofpattern pattern of of(a) (thea) thePAR; PAR; (b) (theb) theleaching leaching residue residue after after roasting-acid roasting- leaching.acid leaching.

2.2. Methods and Procedure 2.2. Methods and Procedure Roasting experiments were conducted in a fixed horizontal electric tube furnace (SK-G04123K Roasting experiments were conducted in a fixed horizontal electric tube furnace (SK-G04123K Tianjin Zhonghuan Lab Furnace Co., Ltd., Tianjin, China). The experimental apparatus is presented Tianjin Zhonghuan Lab Furnace Co., Ltd., Tianjin, China). The experimental apparatus is presented in in Figure S1. As the tube is heated the required temperature at a rate of 10 °C /min, then a Figure S1. As the tube is heated the required temperature at a rate of 10 ◦C /min, then a corundum corundum crucible loading mixture of PAR and ammonium sulphate was sent into the roasting crucible loading mixture of PAR and ammonium sulphate was sent into the roasting zone. At the end zone. At the end of the tube, the exhaust gas (NH3) was collected by NaOH solution. After each of the tube, the exhaust gas (NH3) was collected by NaOH solution. After each roasting experiment, roasting experiment, the roasted PAR was leached by water and diluted sulphuric acid to transfer the roasted PAR was leached by water and diluted sulphuric acid to transfer soluble sulphate salt soluble sulphate salt into solution. The leaching slurry was filtered with 0.45 μm membrane, and the into solution. The leaching slurry was filtered with 0.45 µm membrane, and the filtrate and residue filtrate and residue then were separated. The leaching liquid was collected to determine the recovery then were separated. The leaching liquid was collected to determine the recovery percentage of percentage of manganese and iron. The influence of different parameters including: ammonium manganese and iron. The influence of different parameters including: ammonium sulphate dosage, sulphate dosage, roasting temperature, and roasting time, as well as leaching temperature, time, and roasting temperature, and roasting time, as well as leaching temperature, time, and leaching acidity leaching acidity were studied in the way that one parameter was tested while keeping other were studied in the way that one parameter was tested while keeping other parameters constant. parameters constant. 2.3. Product Analysis 2.3. Product Analysis Spectrophotometry was employed for Fe analysis using MAPADA V-1200 VIS spectrophotometer. ManganeseSpectrophotometry was determined was by employed ICP-MS (NEXION for Fe 300X,analysis PERKINELMER, using MAPADA San Diego, V-1200 MA, USA)VIS spectrophotometer.equipped with an autoManganese sampler was (SC2 determined DX, ESI, PERKINELMER,by ICP-MS (NEXION San Diego,300X, PERKINELMER, MA, USA). A X-ray San Diego,diffraction MA, (XRD)USA) equipped equipment with (EMPYREAN, an auto sample PANalytical,r (SC2 DX, Almelo, ESI, PERKINELMER, The Netherlands) San wasDiego, used MA, to USA).identify A theX-ray solid diffraction components (XRD) of theequipment PAR and (EMPYREAN, roasting products. PANalytical, Almelo, The Netherlands) was used to identify the solid components of the PAR and roasting products.

Metals 2018, 8, 38 4 of 12

3. Results and Discussion

3.1. Reaction Mechanisms

3.1.1. Reaction between PAR and Ammonium Sulphate In order to understand the roasting process of thermochemical treatment of the absorption residue (PAR) with ammonium sulphate, it is important to gain insight in the thermal decomposition of pure

(NH4)2SO4 over the temperature range studied, since it will affect greatly the thermochemical process. The published literatures revealed disagreement as to the nature of the intermediate species formed during thermal decomposition of ammonium sulphate. Nevertheless, a general consensus [30,32–34] is that the decomposition mechanism may involve the following steps:

>100 ◦C (NH4)2SO4−−−−→ NH4HSO4(s) + NH3(g) (4)

100−170 ◦C (NH4)2SO4 + NH4HSO4−−−−−−→ (NH4)3H(SO4)2 (5) 170 ◦C NH4HSO4−−−→ NH2HSO3(s) + H2O(g) (6)

200 ◦C 3NH2HSO3(s)−−−→ NH3(g) + 3SO2(g) + N2(g) + 3H2O(g) (7)

170 ◦C NH4HSO4 + NH2HSO3−−−→ (NH4)2S2O7 (8) 400 ◦C (NH4)3H(SO4)2−−−→ (NH4)2S2O7 + NH3 + H2O (9) 400 ◦C 3(NH4)2S2O7−−−→ 2NH3 + 6SO2 + 9H2O + 2N2 (10) 400 ◦C 3(NH4)2SO4−−−→4NH3 + 3SO2 + 6H2O + N2 (11) KiyouraandUrano [33] conﬁrmed the thermal decomposition of ammonium sulphate to ammonium bisulphate (Equation (4)) at temperature higher than 100 ◦C through release of ammonia, however, that is not being detected by X-ray diffraction method. This is because of the rapid reaction between ammonium sulphate and ammonium bisulphate to form triammonium hydrogen sulphate ◦ ((NH4)3H(SO4)2) (Equation (5)). Meantime, sulphamic acid is formed by release of H2O at 170 C (Equation (6)), which would further decompose into various gases including NH3, SO2,N2, and H2O (Equation (7)) [33]. Dixo [32] pointed out that the ammonium pyrosulphate ((NH4)2S2O7) from the reactions (Equations (8) and (9)) accounts for 90% of the products of ammonium sulphate during the thermal decomposition at 400 ◦C. But this is not consistent with the results in this article, where only triammonium hydrogen sulphate was detected by XRD. This may due to the formation of NH4HSO4 from the dehydration of (NH4)2S2O7. Studies [26,27,33] on the sulfation of iron oxides and MnO2 by ammonium sulphate at ◦ 300~500 C, have shown the major products to be (NH4)3Fe(SO4)3, NH4Fe(SO4)2, Fe2(SO4)3, Fe2O3, and (NH4)2Mn2(SO4)3, and MnSO4. Based on these reaction products, the chemical reactions taking place during the thermochemical treatment PAR in air atmosphere may be considered as follows:

FeOOH + 3(NH4)2SO4 → (NH4)3Fe(SO4)3 + 3NH3 + 2H2O (12)

FeOOH + 2(NH4)3Fe(SO4)3 → 3NH4Fe(SO4)2 + 3NH3 + 2H2O (13)

2NH4Fe(SO4)2 → Fe2(SO4)3 + 2NH3 + SO3 + H2O (14)

Fe2(SO4)3 → Fe2O3 + 3SO3 (15)

(NH4)2SO4 + 2MnO2 + 2SO2 → (NH4)2Mn2(SO4)3(s) (16)

(NH4)2Mn2(SO4)3(s) → 2MnSO4 + 2NH3 + SO3 + H2O (17) MetalsMetals 20182018, 8,, 838, 38 5 5of of 12 12

MnO SO →MnSO (18) MnO2 + SO2 → MnSO4 (18)

3.1.2.3.1.2. Characterization Characterization of of Roasting Roasting Product Product TheThe roasting roasting products products of of pyrolusite pyrolusite absorpti absorptionon residue residue (PAR) (PAR) with with ammonium ammonium sulphate sulphate at at temperaturetemperature of of 300–600 300–600 °C◦ Cwere were analyzed analyzed by byXRD XRD to study to study the theroasting roasting mechanism. mechanism. The results The results are presentedare presented in Figure in Figure 2. 2.

FigureFigure 2. 2. X-rayX-ray patterns patterns of of the the roasting roasting pr productsoducts at at different different temperatures. temperatures.

ComparedCompared with with the XRDXRD patternpattern of of the the residue residue (Figure (Figure1), the 1), XRDthe XRD diagram diagram of the productof the product roasted roastedat 300 ◦atC, 300 shown °C, inshown Figure in2, Figure indicates 2, thatindicates some that new some diffraction new diffraction peaks appeared peaks after appeared calcination, after calcination,but the change but the of the change mineral of the phase mineral was not phas obviouse was atnot 300 obvious◦C. With at 300 respect °C. toWith the respect analysis to result, the analysis result, the new materials are mainly NH)FeSO) , and the peaks are weak and the new materials are mainly (NH4)3Fe(SO4)3, and the peaks are weak and unobvious (Equation (12)). unobvious (Equation (12)). In disagreement with line 1, the peaks of new products NH)FeSO) In disagreement with line 1, the peaks of new products (NH4)3Fe(SO4)3 were more obvious than before werewith more increasing obvious roasting than temperature,before with increasing indicating ro thatasting more temperature, iron oxide was indicating sulphated. that Atmore the sameiron oxidetime, was the hematitesulphated. peak At wasthe alsosame observed, time, the suggesting hematite peak that the was goethite also observed, in the PAR suggesting was dehydrated that the at goethite350 ◦C toin formthe PAR a more was stable dehydrated hematite at (Equation350 °C to form (19)). a more stable hematite (Equation (19)).

2FeOOHs) → FeOs) HOg) (19) 2FeOOH(s) → Fe2O3(s) + H2O(g) (19) As the calcination temperature continues to increase to 400 °C, the NH)FeSO) diffraction ◦ peak formedAs the calcinationon lines 1–2 temperature disappears, continuesand a new to peak increase is formed to 400 in C,line the 3.( ThisNH4 peak)3Fe( SOis considered4)3 diffraction as NHpeakFe formedSO), which on lines is1–2 a more disappears, stable andferric a newsulphate peak ammonium is formed in salt line formed 3. This from peak isthe considered reaction ) ) betweenas NH4 FeNH(SO 4)Fe2, whichSO isand a moreFeOOH stable (Equation ferric sulphate(13)). When ammonium the roasting salt temperature formed from is the increased reaction ) upbetween to 450 (°C,NH more4)3Fe (NHSO4)Fe3 andSOFeOOH was produced,(Equation (13)).because When both the the roasting correspondent temperature diffraction is increased peak up ◦ NH ) HSO ) andto 450intensityC, more increase.NH4Fe And(SO 4the)2 was diffraction produced, peaks because of both the correspondent have disappeared, diffraction indicating peak and thatintensity it has increase. been decomposed And the diffraction completely peaks at 500 of ( NH°C, 4and)3H (theSO4 )peak2 have of disappeared,ferric sulphate indicating is observed, that it indicatinghas been decomposedthe occurrence completely of Equation at 500 (14).◦C, When and the the peak roasting of ferric temperature sulphateis continues observed, to indicating increase theto ◦ 600occurrence °C, the peak of Equation of FeO (14). can When be detected, the roasting which temperature indicates that continues some of to the increase ferric to sulfate 600 C, occurs the peak to decompose.of Fe2O3 can be detected, which indicates that some of the ferric sulfate occurs to decompose. OweOwe to to the the low low manganese manganese content content in inthe the PAR, PAR, it is it difficult is difﬁcult to examine to examine the thephase phase conversion conversion of manganeseof manganese dioxide dioxide during during the theroasting roasting process process by byX-ray X-ray diffraction diffraction (XRD) (XRD) technology. technology. However, However, thethe roasting roasting mechanism mechanism of of manganese manganese dioxide dioxide wi withth ammonium ammonium sulphate sulphate can can be be analyzed analyzed on on the the basisbasis of of the the extraction extraction data data from from Figure Figure 3b.3b. It can be seen from Figure 33bb thatthat 56.4%56.4% MnMn waswas extracted at roasting temperature of 300 °C and roasting time of 240 min, which implicated that the

Metals 2018, 8, 38 6 of 12 manganese dioxide has been reduced into the water-soluble Mn. The following paper [27,30] examine the decomposition gas of ammonium sulphate, and the results show that the NH3 is the main gas when the temperature is below 380 °C, while, SO2 and H2O were mainly generated at 520 °C. Therefore, the sulfation-roasting process of manganese dioxide at lower temperature can be described through Equation (13). And this small amount of SO2 gas in Equation (16) was produced from the decomposition of sulphamic acid (NHHSO) (Equation (7)). At a higher temperature, more SO2 was generated by Equation (11), which can directly react with manganese dioxide and form Metals 2018, 8, 38 6 of 12 manganese sulphate (Equation (18)) [21].

3.2.extracted Roasting at Kinetics roasting temperature of 300 ◦C and roasting time of 240 min, which implicated that the manganese dioxide has been reduced into the water-soluble Mn2+. The following paper [27,30] The roasting kinetics of PAR with ammonium sulphate was studied by water leaching in the examine the decomposition gas of ammonium sulphate, and the results show that the NH is the main temperature range of 275–400 °C. Other conditions were: 12 of ammonium sulphate-to-PAR3 ratio, gas when the temperature is below 380 ◦C, while, SO and H O were mainly generated at 520 ◦C. 60 °C of leaching temperature, and 120 min of leaching 2time. The2 effects of roasting temperature and Therefore, the sulfation-roasting process of manganese dioxide at lower temperature can be described time on manganese and iron extraction are shown in Figure 2a. With increasing roasting through Equation (13). And this small amount of SO gas in Equation (16) was produced from the temperature, the proportion of manganese and iron leached2 from the PAR increase. As for the iron decomposition of sulphamic acid (NH HSO ) (Equation (7)). At a higher temperature, more SO was roasting process, the extracted amount2 increased3 linearly at a roasting temperature of 2752 °C. generated by Equation (11), which can directly react with manganese dioxide and form manganese However, the maximum efficiency of Fe extracted was reached at roasting temperature of 300– sulphate (Equation (18)) [21]. 375 °C, within the roasting time studied.

Figure 3. The effect of roasting temperature on (a) iron and (b) manganese with sulphating Figure 3. The effect of roasting temperature on (a) iron and (b) manganese with sulphating roastingroasting-water leaching. water leaching.

Increasing the roasting temperature has little effect on further increasing the extraction 3.2. Roasting Kinetics efficiency of iron. Table S1 shows the iron extraction from various minerals, such as mine tailings [28], Thenickel roasting laterite kinetics [26,35] ofand PAR ilmenite with ammonium [30], using sulphateammonium was sulfate studied as bya roasting water leaching agent. These in the ◦ resultstemperature implicate range that of the 275–400 iron extrC.action Other efficiency conditions is were: either 12 lower of ammonium or higher. sulphate-to-PARThis situation may ratio, be ◦ related60 C of to leaching the content temperature, of silica andin the 120 ore, min whic of leachingh may retard time. The the effectsdiffusion of roasting of ammonium temperature sulphate and throughtime on manganesethe ash layer and from iron the extraction surface to are the shown center in of Figure the PAR.2a. With When increasing the silica roasting is high, temperature,the recovery ratethe proportionof iron is low of manganese [26,28,35], and and iron vice leached versa the from recovery the PAR rate increase. of iron As is for high the ironwhen roasting the content process, of ◦ siliconthe extracted is low amount[30,31]. increasedAbout 95.6% linearly Fe can at a be roasting extracted temperature from the ofroasting 275 C. product However, of thepure maximum FeOOH ◦ underefficiency the ofsame Fe extractedconditions, was which reached further at roasting verify temperaturethe above-mentioned of 300–375 mechanismC, within that the the roasting iron roastingtime studied. is greatly influenced by the content of silicon dioxide. Water leaching process after ammoniumIncreasing sulphate the roasting roasting temperature in these literatures has little [26,28,35] effect on further is an important increasing factor the extraction resulting efficiencylow iron recovery.of iron. Table The S1results shows demonstrate the iron extractionthat it is fromnecessary various to adopt minerals, acid suchleaching as mine after tailingsammonium [28], sulphatenickel laterite roasting [26 ,in35 ]order and ilmeniteto improve [30 the], using iron ammoniumrecovery efficiency. sulfate as a roasting agent. These results implicateIt is thatindicated the iron from extraction Figure efficiency 3b that isthe either extr loweraction orof higher. manganese This situation increases may with be relatedroasting to temperature.the content of The silica leaching in the ore, efficiency which may of mangan retard theese diffusionrises from of ammonium56.4% to 99.3% sulphate as the through roasting the temperatureash layer from increases the surface from to 300 the °C center to 400 of the°C PAR.at a ro Whenasting the time silica of 240 is high, min. the The recovery leaching rate behavior of iron of is manganeselow [26,28,35 differs], and vicegreatly versa from the recoverythat of iron rate ofduri ironng isthe high roasting when theprocess, content which of silicon seems is low not [ 30to, 31be]. Aboutaffected 95.6% by the Fe content can be extractedof SiO2 infrom the residue. the roasting The insensitivity product of pure of manganese FeOOH under roasting the same process conditions, to SiO2 which further verify the above-mentioned mechanism that the iron roasting is greatly influenced by the content of silicon dioxide. Water leaching process after ammonium sulphate roasting in these literatures [26,28,35] is an important factor resulting low iron recovery. The results demonstrate that it is necessary to adopt acid leaching after ammonium sulphate roasting in order to improve the iron recovery efficiency. It is indicated from Figure3b that the extraction of manganese increases with roasting temperature. The leaching efficiency of manganese rises from 56.4% to 99.3% as the roasting temperature increases Metals 2018, 8, 38 7 of 12

Metals 2018, 8, 38 7 of 12 from 300 ◦C to 400 ◦C at a roasting time of 240 min. The leaching behavior of manganese differs greatly fromlayer thatcan ofbe ironascribed during to thethe roastingfact that process,the mang whichanese seems dioxide not was to be sulphated affected byinto the soluble content manganese of SiO2 in thesulphate residue. by SO The2 gas insensitivity generated of from manganese the reactions roasting (Equations process (7) to SiOand2 (11)).layer Compared can be ascribed with tothe the liquid fact thatammonium the manganese sulphate dioxide in the roasting was sulphated process, into SO soluble2 gas more manganese easily diffused sulphate through by SO2 SiOgas2 generatedlayer into fromthe unreacted the reactions residue (Equations surface. (7) and (11)). Compared with the liquid ammonium sulphate in the roastingThe process,extraction SO process2 gas more of iron easily and diffused manganese through from SiO pyrolusite2 layer into adsorbent the unreacted residue residue (PAR) surface. with ammoniumThe extraction sulphate processas a roasting of iron agent and can manganese be simulated from by pyrolusite so-called adsorbent a shrinking residue model. (PAR) The withsimulation ammonium resultssulphate of iron are as ashown roasting in agentFigure can S2, be showing simulated that by the so-called iron extraction a shrinking process model. is Thecontrolled simulation by the results chemical of ironreaction are on shown the PAR in Figure surface S2, at showingthe roasting that temperature the iron extraction of 275 °C, process and is ◦ iscontrolled controlled by bythe the diffusion chemical of reactionthe reactants on the through PAR surface the inert at or the product roasting layer temperature at 300–375 of °C. 275 TheC, ◦ andapparent is controlled activation by theenergy diffusion was estimated of the reactants as 20.56 through kJ/mol the (Figure inert or 4), product supporting layer atthe 300–375 diffusionC. Themechanism apparent for activation iron extraction. energy Figure was estimated S3 shows as th 20.56e simulation kJ/mol (Figureresults 4of), the supporting manganese the extraction diffusion mechanismprocess. From for Figure iron extraction. S3, the manganese Figure S3 showsleaching the process simulation is controlled results of by the both manganese reactant extraction diffusion process.and surface From chemical Figure S3,reaction the manganese at a roasting leaching temp processerature is of controlled 300–375 by°C. both When reactant the temperature diffusion and is ◦ surfaceincreased chemical to 400 °C, reaction the manganese at a roasting extraction temperature rate is of limited 300–375 byC. SO When2 diffusion the temperature though the isinert increased layer. ◦ toFrom 400 theC, Figure the manganese 4, the apparent extraction activation rate is limitedenergy byof SOmanganese2 diffusion recovery though was the inertdetermined layer. From as 29.52 the FigurekJ/mol.4 , the apparent activation energy of manganese recovery was determined as 29.52 kJ/mol.

(a) (b)

Figure 4. The determination of apparent activation energy of (a) iron and (b) manganese. Figure 4. The determination of apparent activation energy of (a) iron and (b) manganese.

3.3. Effect of Conditions Applied for Roasting and Acid Leaching 3.3. Effect of Conditions Applied for Roasting and Acid Leaching

3.3.1. Effect of Ammonium Sulphate: PAR Mass Ratio The dosage of ammonium sulphate is the essentialessential factor for the roasting process to obtain higher manganese and iron recovery efficiency. The effect of NH)SO: PAR mass ratio on the higher manganese and iron recovery efficiency. The effect of (NH4)2SO4: PAR mass ratio on the manganese and iron extraction rate is shown in Figure5 5.. OtherOther conditionsconditions werewere as as follows: follows: 450450 ◦°CC of roasting temperature,temperature, 3030 min min of of roasting roasting time, time, 90 90◦C °C of leachingof leaching temperature, temperature, 60 min 60 ofmin leaching of leaching time, andtime, sulphuric and sulphuric acid concentration acid concentration of 0.5 of M. 0.5 M. It cancan bebe seen seen from from Figure Figure5, the 5, ironthe extractioniron extraction increased increased with increasing with increasing the ammonium the ammonium sulphate dosage.sulphate This dosage. is because This is increasing because increasing the amount the of amount ammonium of ammonium sulphate enhanced sulphate the enhanced drive force the ofdrive the force of the diffusion of NH)SO through the inert layer. As the ratio of ammonium sulphate to diffusion of (NH4)2SO4 through the inert layer. As the ratio of ammonium sulphate to PAR is over 6:1,PAR the is extractedover 6:1, iron the increaseextracted slowly, iron andincrease about slowly, 97.6% Fe and has about beenextracted. 97.6% Fe Manganese has been extractionextracted. seemsManganese to be extraction greatly influenced seems to be by greatly the ammonium influenced sulphate-to-PAR by the ammonium ratio. sulphate-to-PAR The leaching ratio. efficiency The increasedleaching efficiency from 51.5% increased to about from 97.0%, 51.5% when to theabout ratio 97.0%, is increased when the from ratio 2:1 is to increased 12:1. Continuing from 2:1 increase to 12:1. inContinuing ammonium increase sulphate: in PARammonium ratio over sulphate: 12:1 does notPAR result ratio in over obvious 12:1 improvement does not result upon in manganese obvious leaching.improvement The strongupon manganese dependence leaching. of manganese The strong extraction dependence upon the of manganese dosage of ammonium extraction upon sulphate the dosage of ammonium sulphate can be attributed to more SO2 gas generated by decomposition of

NH)SO(Equation (11)). It can be seen from Figure 2 that ammonium sulphate has decomposed completely at 450 °C producing various gases including SO2 gas according to Equation (11), as the diffraction peak of NH)HSO) has not been observed. It has been demonstrated that

Metals 2018, 8, 38 8 of 12

can be attributed to more SO2 gas generated by decomposition of (NH4)2SO4 (Equation (11)). It can be seen from Figure2 that ammonium sulphate has decomposed completely at 450 ◦C producing various Metalsgases 2018 including, 8, 38 SO2 gas according to Equation (11), as the diffraction peak of (NH4) H(SO4) has8 of not 12 Metals 2018, 8, 38 3 2 8 of 12 been observed. It has been demonstrated that manganese dioxide (MnO2) can be sulfated to produce ◦ manganese dioxide sulphate (MnO with2)SO can2 gasbe sulfated at roasting to produce temperature manganese of 200–550 sulphateC (Equationwith SO2 gas (18)) at roasting [21]. So, manganese dioxide (MnO2) can be sulfated to produce manganese sulphate with SO2 gas at roasting temperaturethe SO2 gas resulted of 200–550 from °C decomposition (Equation (18)) of ammonium [21]. So, the sulphate SO2 gas would resulted react withfrom manganesedecomposition dioxide of temperature of 200–550 °C (Equation (18)) [21]. So, the SO2 gas resulted from decomposition of ammoniuminammonium the PAR tosulphatesulphate form water-soluble wouldwould reactreact manganese withwith manganesemanganese sulfate. dioxidedioxide Increasing inin amountsthethe PARPAR oftoto ammonium formform water-solublewater-soluble sulphate manganeseincreases the sulfate. yield of Increasing SO2 gas. Toamounts sum up, of an a ammoniummmonium sulphate sulfate-to-PAR increases ratio the of 12:1yield is of recommended SO2 gas. To manganese sulfate. Increasing amounts of ammonium sulphate increases the yield of SO2 gas. To sumforsum sulfating-roasting up,up, anan ammoniumammonium process. sulfate-to-PARsulfate-to-PAR ratioratio ofof 12:112:1 isis recommendedrecommended forfor sulfating-roastingsulfating-roasting process.process.

Figure 5. The effect of ammonium-to-sulphate ratio on the extraction of iron and manganese from Figure 5.5. TheTheeffect effect of of ammonium-to-sulphate ammonium-to-sulphate ratio ratio on on the the extraction extraction of iron of iron and manganeseand manganese from PAR.from PAR.PAR. 3.3.2. Effect of Roasting Temperature 3.3.2.3.3.2. EffectEffect ofof RoastingRoasting TemperatureTemperature The effects of roasting temperature in the range chosen 250−500 ◦C on the iron and manganese TheThe effectseffects ofof roastingroasting temperaturetemperature inin thethe rangerange chosenchosen 250250−−500500 °C°C onon thethe ironiron andand manganesemanganese extraction efﬁciency have been investigated at ammonium sulphate-to-PAR ratio of 12:1, roasting time extractionextraction efficiencyefficiency havehave beenbeen investigatedinvestigated atat ammoniumammonium sulphate-to-PARsulphate-to-PAR ratioratio ofof 12:1,12:1, roastingroasting of 30 min, leaching temperature of 90 ◦C, and leaching time of 60 min. The results are shown in timetime ofof 3030 min,min, leachingleaching temperaturetemperature ofof 9090 °C,°C, andand leachingleaching timetime ofof 6060 min.min. TheThe resultsresults areare shownshown inin Figure6. FigureFigure 6.6.

Figure 6. The effect of roasting temperature on the extraction of iron and manganese from PAR. Figure 6. The effect of roasting temperature on the extraction of iron andand manganese fromfrom PAR.PAR.

FigureFigure 66 showsshows thatthat roastingroasting temperaturetemperature hashas aa signsignificantlyificantly effecteffect onon roastingroasting reaction.reaction. WithWith anan increaseFigure of 6roasting shows temperature that roasting from temperature 250 to 325 has °C, athe significantly proportion effectof Fe leached on roasting from reaction.the PAR increase of roasting temperature from 250 to 325 °C, the proportion◦ of Fe leached from the PAR increaseWith an significantly. increase of roasting When the temperature roasting temperature from 250 to was 325 increasedC, the proportion from 325 °C of to Fe 450 leached °C, it fromwas increase significantly. When the roasting temperature was increased from 325 °C to 450◦ °C, it was◦ foundthe PAR that increase the iron significantly. leaching efficiency When the have roasting reached temperature maximum, was because increased the whole from iron 325 oxideC to 450 in theC, itfound was foundthat the that iron the leaching iron leaching efficiency efficiency have reac havehed reached maximum, maximum, because because the whole the wholeiron oxide iron oxidein the oreore areare essentiallyessentially extractedextracted out.out. ThisThis isis inin agagreementreement withwith thethe resultsresults presentedpresented inin FigureFigure 3a3a showingshowing thatthat ironiron extractionextraction isis nono longerlonger increaseincreasedd whenwhen roastingroasting temperaturetemperature isis higherhigher thanthan 325325 °C.°C. TheThe ironiron dissolutiondissolution ofof thethe unreactedunreacted ironiron oxideoxide duringduring thethe roastingroasting processprocess waswas causedcaused byby thethe subsequentsubsequent sulphuricsulphuric acidacid leachingleaching step.step. WhenWhen roasroastingting temperaturetemperature isis higherhigher thanthan 450450 °C,°C, itit waswas foundfound thatthat thethe ironiron leachingleaching decreaseddecreased slightly,slightly, whichwhich maymay bebe owingowing toto thethe occurrenceoccurrence ofof reactionreaction (Equation (15)) which produced the insoluble iron oxide (FeO). (Equation (15)) which produced the insoluble iron oxide (FeO).

Metals 2018, 8, 38 9 of 12 in the ore are essentially extracted out. This is in agreement with the results presented in Figure3a showing that iron extraction is no longer increased when roasting temperature is higher than 325 ◦C. The iron dissolution of the unreacted iron oxide during the roasting process was caused by the subsequent sulphuric acid leaching step. When roasting temperature is higher than 450 ◦C, it was found that the iron leaching decreased slightly, which may be owing to the occurrence of reaction Metals 2018, 8, 38 9 of 12 (Equation (15)) which produced the insoluble iron oxide (Fe2O3). It can be observed that roasting temperature had a more profound inﬂuenceinfluence on manganese extraction compared to that of iron iron.. For example, the extraction of manganese rises from 20.1% to ◦ ◦ about 97.33% when roasting temperature increases from 250 °CC to 450 °C.C. The manganese extraction ◦ occurs to decrease as roasting temperature is be beyondyond 450 °C.C. This may be related to the following reasons: (1) (1) the the more rapid decomposition of ammonium sulphate resulting in loss of SOSO2 before reacting with MnO22; (2) the decompositiondecomposition ofof MnOMnO22 producing MnMn33O4,, whose whose reactivity reactivity with SOSO2 is ◦ lower thanthan MnOMnO22.. InIn conclusion,conclusion, the the roasting roasting temperature temperature of of 450 450C °C was was selected selected to obtain to obtain higher higher iron ironand manganeseand manganese recovery. recovery.

3.3.3. Effect of Roasting Time 3.3.3. Effect of Roasting Time To investigate the effect of roasting time on the recovery of Fe and Mn from PAR, To investigate the effect of roasting time on the recovery of Fe and Mn from PAR, a series of a series of experiments were conducted at constant ammonium sulfate-to-PAR mass ratio of 12:1, experiments were conducted at constant ammonium sulfate-to-PAR mass ratio of 12:1, roasting roasting temperature of 450 ◦C, leaching temperature of 90 ◦C, and leaching time of 60 min. The results temperature of 450 °C, leaching temperature of 90 °C, and leaching time of 60 min. The results are are presented in Figure7. presented in Figure 7.

Figure 7. The effect of roasting time on the extraction of iron and manganese from PAR. Figure 7. The effect of roasting time on the extraction of iron and manganese from PAR. It was found that roasting time has a slight influence on Fe leaching. Only within 10 min, iron extractionIt was efficiency found that reaches roasting the maximum time has a values. slight inﬂuenceThe insensitivity on Fe leaching.of iron leaching Only within to calcination 10 min, timeiron extractionis mainly efﬁciencybe ascribed reaches to the the fact maximum that rapid values. conversion The insensitivity of iron oxide of iron on the leaching residue to calcinationsurface to solubletime is mainlyammonium be ascribed iron sulphate to the takes fact that place rapid at the conversion roasting temperature of iron oxide of on450 the °C, residue then the surface left over to soluble ammonium iron sulphate takes place at the roasting temperature of 450 ◦C, then the left over iron oxide in the slag is subjected to acid dissolution under leaching conditions of 90 °C, 0.5 M H2SO4 ◦ andiron oxide180 min. in the Within slag is subjectedthe roasting to acid time dissolution of 30 mi undern, the leachingmanganese conditions extraction of 90 rateC, 0.5reached M H2SO its4 maximum.and 180 min. The Within increased the roasting manganese time of leaching 30 min, thewith manganese increasing extraction roasting rate time reached is because its maximum. of the The increased manganese leaching with increasing roasting time is because of the reaction between reaction between ammonium sulphate and MnO2 in the residue. Thus, the roasting time of 30 min wasammonium chosen as sulphate the optimal and MnOvalue2 toin minimize the residue. energy Thus, consumption. the roasting time of 30 min was chosen as the optimal value to minimize energy consumption. 3.3.4. Effect of Leaching Temperature and Acidity 3.3.4. Effect of Leaching Temperature and Acidity The effect of leaching temperature and time on iron extraction from the residue (PAR) was The effect of leaching temperature and time on iron extraction from the residue (PAR) was investigated and is shown in Figure 8, at a constant ammonium sulphate-to-PAR mass ratio of 12:1, investigated and is shown in Figure8, at a constant ammonium sulphate-to-PAR mass ratio of 12:1, roasting temperature of 450 °C, and roasting time of 30 min. It can be seen that leaching temperature roasting temperature of 450 ◦C, and roasting time of 30 min. It can be seen that leaching temperature and sulphuric acid concentration have an important influence on iron extraction. Iron leaching efficiency increase with increasing leaching temperature and sulphuric acid concentration. Only 73.8% iron was leached out as the leaching conditions were set at 30 °C, 0.5 M sulphuric acid, and leaching time of 180 min, which further illustrated that there are parts of iron oxide still present the form of goethite or hematite in the roasting process. So, it is necessary to optimize reaction temperature and acid concentration to gain satisfying iron recovery efficiency. From the results in Figure 8, it becomes evident that the iron leaching amount could be increased to 90.9%, as leaching temperature was elevated 70 °C, at 0.5 M sulphuric acid and 180 min. However, increasing

Metals 2018, 8, 38 10 of 12 and sulphuric acid concentration have an important influence on iron extraction. Iron leaching efficiency increase with increasing leaching temperature and sulphuric acid concentration. Only 73.8% iron was leached out as the leaching conditions were set at 30 ◦C, 0.5 M sulphuric acid, and leaching time of 180 min, which further illustrated that there are parts of iron oxide still present the form of goethite or hematite in the roasting process. So, it is necessary to optimize reaction temperature and acid concentration to gain satisfying iron recovery efficiency. From the results in Figure8, it becomes evident Metals 2018, 8, 38 10 of 12 that the iron leaching amount could be increased to 90.9%, as leaching temperature was elevated 70 ◦C, ◦ attemperature 0.5 M sulphuric up to acid 90 and°C greatly 180 min. shorten However, the increasingleaching time temperature to obtain up the to 90higherC greatly iron leaching shorten theefficiency leaching over time 90%. to obtain For example, the higher 95.2% iron leachingiron was efficiency extracted over at 90%.60 min For leaching example, time. 95.2% Acid iron wasconcentration extracted atis 60also min vital leaching factor time.to the Acid dissolution concentration of the is remaining also vital factoriron oxide. to the dissolutionFor example, of the remaining iron oxide. For example, the iron recovery efficiency increase from 63.1% to 76.9%, 73.8% to iron recovery efficiency increase from 63.1% to 76.9%, 73.8% to 90.9%◦ while◦ sulfuric acid 90.9%concentration while sulfuric increase acid from concentration 0.3 M to 0.5 increase M at 30 from °C 0.3and M 50 to 0.5°C Mrespectively. at 30 C and Although 50 C respectively. high iron Althoughextraction highefficiency iron extraction can be obtained efficiency by can increasi be obtainedng either by increasing leaching eithertemperature leaching or temperature sulfuric acid or sulfuricconcentration, acid concentration, it is better itto isimprove better to reaction improve reactiontemperature temperature rather than rather to than increase to increase sulfuric sulfuric acid acidconcentration. concentration. This is This because is because the leaching the leaching solution solution with high with acidity high acidity would would cause causeproblems problems in the in the subsequent recovery of Mn and Fe. Therefore, the selected optimum leaching conditions are: subsequent recovery of Mn◦ and Fe. Therefore, the selected optimum leaching conditions are: leachingleaching temperaturetemperature ofof 9090 °C,C, sulphuric acid concentration ofof 0.50.5 MM andand leachingleaching timetime ofof 6060 min.min.

Figure 8. The effect of leaching temperature, sulphuric acid concentration and leaching time on the Figure 8. The effect of leaching temperature, sulphuric acid concentration and leaching time on the extraction of iron and manganese. extraction of iron and manganese. 4. Conclusions 4. Conclusions In this study, the process of ammonium sulphate roasting followed by sulphuric acid leaching In this study, the process of ammonium sulphate roasting followed by sulphuric acid leaching was was developed to recover iron and manganese from pyrolusite absorption residue (PAR). The developed to recover iron and manganese from pyrolusite absorption residue (PAR). The experimental experimental results reveal that high recovery efficiency of manganese and iron were results reveal that high recovery efﬁciency of manganese and iron were simultaneously obtained, simultaneously obtained, compared to the methods reported in literatures posing importance only compared to the methods reported in literatures posing importance only on recovery of manganese, on recovery of manganese, igoring iron extraction. In this roasting reaction, ammonium sulphate igoring iron extraction. In this roasting reaction, ammonium sulphate mainly reacts with goethite and mainly reacts with goethite and manganese dioxide in the PAR and produce NH)FeSO) and◦ manganese dioxide in the PAR and produce (NH4)3Fe(SO4)3 and (NH4)2Mn2(SO4)3 at 200–350 C, NH)MnSO) at 200–350 °C, which can be converted to the thermochemically stable which can be converted to the thermochemically stable NH4Fe(SO4)2 and MnSO4, at higher roasting NHFeSO) and MnSO4, at higher roasting temperature. The sulfation of iron oxide process is temperature. The sulfation of iron oxide process is controlled by reactant diffusion through the surface controlled by reactant diffusion through the surface ash layer with an activation energy of 20.56 ash layer with an activation energy of 20.56 kJ/mol. Manganese extraction was limited by both of the kJ/mol. Manganese extraction was limited by both of the diffusion rate and the chemical reaction diffusion rate and the chemical reaction with an activation energy of 29.52 kJ/mol. The maximum with an activation energy of 29.52 kJ/mol. The maximum recovery rates of Mn and Fe were 97.0% recovery rates of Mn and Fe were 97.0% and 95.2%, respectively, at an ammonium sulphate-PAR mass and 95.2%, respectively, at an ammonium sulphate-PAR mass ratio of 12:1, roasting temperature of ratio of 12:1, roasting temperature of 450 ◦C, roasting time of 30 min, leaching temperature of 90 ◦C, 450 °C, roasting time of 30 min, leaching temperature of 90 °C, leaching time of 60 min, and leaching leaching time of 60 min, and leaching acidity of 0.5 M H2SO4. acidity of 0.5 M H2SO4.

Acknowledgments: This work was supported by the National Natural Science Foundation of China (NSFC-51304140 and NSFC-51374150) and Science and Technology Plan Projects of Sichuan Province, China (Grant No. 2015HH0067).

Author Contributions: Wei-Yi Sun and Shi-Jun Su provided research ideas; Lin Deng conducted a literature survey; Sang-Lan Ding conducted experimental design; Lin Deng and Bing Qu conducted experimental operations and data analysis; Lin Deng wrote the paper.

Metals 2018, 8, 38 11 of 12

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-4701/8/1/38/s1, Figure S1: Schematic diagram of the sulfation roasting equipment, Figure S2: The simulation results for iron roasting process, Figure S3: The simulation results for manganese roasting process, Table S1: Iron extraction from ores using ammonium sulphate roasting data reported in literatures. Acknowledgments: This work was supported by the National Natural Science Foundation of China (NSFC-51304140 and NSFC-51374150) and Science and Technology Plan Projects of Sichuan Province, China (Grant No. 2015HH0067). Author Contributions: Wei-Yi Sun and Shi-Jun Su provided research ideas; Lin Deng conducted a literature survey; Sang-Lan Ding conducted experimental design; Lin Deng and Bing Qu conducted experimental operations and data analysis; Lin Deng wrote the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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