“Zero-Waste”: a Sustainable Approach on Pyrometallurgical Processing of Manganese Nodule Slags

Article “Zero-Waste”: A Sustainable Approach on Pyrometallurgical Processing of Manganese Nodule Slags

Marcus Sommerfeld 1,* , David Friedmann 1, Thomas Kuhn 2 and Bernd Friedrich 1

1 Department of Process Metallurgy and Metal Recycling (IME), RWTH Aachen University, Intzestraße 3, D-52056 Aachen, Germany; [email protected] (D.F.); [email protected] (B.F.) 2 Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-241-80-95-200

Received: 19 September 2018; Accepted: 17 November 2018; Published: 23 November 2018

Abstract: A continuously growing demand for valuable non-ferrous metals and therefore an increase in their prices at the metal exchanges makes it necessary and profitable to investigate alternative metal resources. Polymetallic deep-sea nodules contain cobalt, copper, manganese, molybdenum and nickel, and are highly abundant on the sea floor. Developing a metallurgical process to recover the metal content from manganese nodules can close the predicted supply gap of critical metals like cobalt. This paper investigated a potential extraction process for valuable metals from manganese nodules supplied by the German Federal Institute for Geosciences and Natural Resources. The samples originated from the German license area of the Clarion-Clipperton Zone in the Pacific Ocean. Due to a low concentration of valuable metals in nodules, a pyrometallurgical enrichment step was carried out to separate cobalt, copper, molybdenum and nickel in a metallic phase. The manganese was discarded in the slag and recovered in a second smelting step as ferromanganese. To aid the experiments, FactSageTM was used for thermodynamic modeling of the smelting steps. To increase metal yields and to alter the composition of the metal alloys, different fluxes were investigated. The final slag after two reduction steps were heavy-metal free and a utilization as a mineral product was desired to ensure a zero-waste process.

Keywords: marine mineral resources; manganese nodules; pyrometallurgical treatment; zero-waste processing; direct slag reduction; thermodynamic modeling

1. Introduction The exploration of polymetallic deep-sea nodules as a multi-metallic resource is an ongoing research object since the first extensive exploration phase in the 1960s and 1970s. Due to an increase in metal prices and a continuously high demand for various metals, another extensive phase of exploration started at the beginning of this century. Several governmental and semi-governmental institutions signed contracts with the International Seabed Authority to investigate polymetallic deep-sea nodules and since 2010 private enterprises have also done so. The most promising area for exploitation of this marine resource is the Clarion-Clipperton Fracture Zone (CZZ) in the Pacific Ocean [1]. Around 15 kg of manganese nodules per square meter on average cover the seafloor in an area the size of five million square kilometers [2]. Metals of interest in deep-sea nodules are especially copper, cobalt and nickel. Due to the high abundance of nodules on the deep-sea floor, the reserves for those metals are significant [3,4]. Furthermore, the recovery of manganese, molybdenum and vanadium is of interest or at least the

Minerals 2018, 8, 544; doi:10.3390/min8120544 www.mdpi.com/journal/minerals Minerals 2018, 8, 544 2 of 13 enrichment of a marketable by-product can add value to an extraction process [3]. Table1 shows a comparison of estimated reserves for the mentioned valuable metals in manganese nodules and in land-based ore bodies, which highlights why manganese nodules are an interesting potential resource [4].

Table 1. Comparison of the estimated resources for metals in manganese nodules in the Clarion-Clipperton Fracture Zone (CCZ) and the land-based reserve base (million metric tons) [4].

Resource Co Cu Mn Mo Ni Nodules in CCZ 42 224 5929 12 278 Global land-based 13 1300 5200 19 150

The expected resources of cobalt, manganese and nickel in manganese nodules solely in the CCZ surpass the reserves of those metals on land [4]. Due to the abundance of high-grade manganese ore on land, utilizing the nodules as a source of copper and nickel only is of interest [5]. A process that additionally recovers cobalt is desirable due to the criticality of cobalt and its importance in the area of clean energy technology [6]. More than one half of global cobalt mine production currently comes from the Democratic Republic of the Congo and since the predicted consumption increases faster than the world cobalt refinery capacity, a limited supply is expected. Most cobalt is already mined as a by-product of nickel and copper production [7], therefore the recovery of cobalt while extracting nickel and copper from manganese nodules can be considered a viable option to close the forecasted supply gap. A rising demand is also predicted for manganese alloys, however, metallurgical grade ores on land are distributed more evenly than cobalt [8], which minimizes a supply risk. To optimize the profitability of a metallurgical process and to decrease the mass of produced by-products with low or no economic value, the production of marketable manganese alloys seems to be an apparent necessity to utilize manganese-rich streams. Several metallurgical processes underwent experimental trials. In general, different extraction processes for utilizing manganese nodules could be categorized into hydrometallurgical processes and pyrometallurgical enrichment processes with subsequent hydrometallurgical downstream processing to separate valuable metals. Already in the 1970s, the International Nickel Corporation (Inco) proposed a process, the so-called “Inco-process” [5], to utilize polymetallic deep-sea nodules as a resource for nickel, copper and cobalt. The process is based on well-known metallurgical unit operations like drying, selective reduction, smelting, oxidizing, sulfiding and converting. Valuable metals like nickel, copper and cobalt are concentrated in an early stage of the process by reduction into a metallic phase and manganese is mainly discarded in the manganese-silicate slag. Excess iron and manganese in the metal is converted to reduce the weight of the valuable metal stream. A matte is produced by sulfiding the reduced metal. Compared to the weight of the nodules, the matte weighs only 5 wt % and is processed further by hydrometallurgical unit operations to separate nickel, copper and cobalt. Due to the abundance of high-grade manganese deposits on land, the recovery of manganese from the manganese-silicate slag was not investigated in detail by Inco [5]. A simplified flowchart of the pyrometallurgical enrichment process by Inco is displayed in Figure1. In comparison to the pyrometallurgical treatment of polymetallic nodules, there are also several hydrometallurgical processes under investigation; however there are a few major disadvantages with these processes, especially, the high consumption of chemicals like acids or bases and considerable amounts of leach residues and precipitation by-products that seem to be a high burden for the environment [9]. A sole hydrometallurgical process can only be economically feasible if reagents are cheap or can be recycled easily. The major disadvantage of a pyrometallurgical process is the energy consumption of drying and smelting of nodules [10], however, less extensive downstream processing to recover valuable metals and the possibility of manganese recovery from slag can possibly outweigh these disadvantages [9,11]. Minerals 2018, 8, 544 3 of 13 Minerals 2018, 8, x FOR PEER REVIEW 3 of 13

Minerals 2018, 8, x FOR PEER REVIEW 3 of 13

Figure 1. SimplifiedSimpliﬁed flowchart ﬂowchart of the pyrometallurgical Inco-process [[5].5].

The aim of this study was to to investigate investigate the the smel smeltingting reduction reduction operation operation from from the the Inco-process. Inco-process.

Inco proposed aa smeltingsmelting reductionreduction operationoperation withoutwithout thethe additionaddition ofof fluxesfluxes [5[5].]. However, to Figure 1. Simplified flowchart of the pyrometallurgical Inco-process [5]. decrease the the liquidus liquidus temperature temperature and and to to improve improve the the recovery recovery of ofmetals, metals, the theaddition addition of fluxes of fluxes can canbe beneficial be beneficialThe [3]. aim Previous [of3]. this Previous study research was research to exploredinvestigate explored the the smeloption theting of optionreduction adding of operation silica adding [3,12] from silica orthe silica [ Inco-process.3,12] and or silicalime [13]. and limeTo alter [13Inco ].the To smeltingproposed alter the abehavior smelting smelting and behaviorreduction to increase operation and to the increase withdegreeout theofthe reduction degreeaddition of of for reduction fluxes valuable [5]. forHowever, metals, valuable thisto metals, paper thisexplored paperdecrease the explored usage the liquidus of the different usage temperature ofadditives different and andto additives improve variations the and recovery in variations the added of metals, in amount the the addedaddition of fluxes amount of fluxes to the ofcan fluxesnodules. to TM TM theTo nodules.supportbe beneficial Tothe support design [3]. Previous the of designthe research experimental of explored the experimental the trialsoption of trialsFactSage adding FactSage silicaTM 7.0 [3,12] was7.0 or wassilicaused. used.and FactSage lime FactSage [13].TM is isa To alter the smelting behavior and to increase the degree of reduction for valuable metals, this paper athermochemical thermochemical modeling modeling program program [14]. [14]. In In this this article article liquidus liquidus temperatures temperatures of of smelting smelting operations, operations, explored the usage of different additives and variations in the added amount of fluxes to the nodules. activities of oxides, degree of reduction and the composition of phases like slag or alloys were predicted activitiesTo ofsupport oxides, the degree design ofof reductionthe experimental and the trials co mpositionFactSageTM 7.0of phaseswas used. like FactSage slag orTM alloysis a were TM bypredicted FactSagethermochemical by FactSage. modelingTM. program [14]. In this article liquidus temperatures of smelting operations, Furthermore,activities of theoxides,the possibilitypossibility degree of ofof reduction manganese manganese and recovery therecovery composition utilizing utilizing of thephases the resulting resulting like slag manganese-rich manganese-richor alloys were slag slag as aas resource a resourcepredicted to produce to by produce FactSage high highTM carbon. carbon ferromanganese ferromanganese (HC FeMn)(HC FeMn) in a second in a second smelting smelting reduction reduction process Furthermore, the possibility of manganese recovery utilizing the resulting manganese-rich slag wasprocess proposed. was proposed. The effects The of addingeffects limeof adding and magnesia lime and as magnesia a flux in theas seconda flux in smelting the second reduction smelting step on the compositionas a resource ofto theproduce final high alloy carbon and slag ferromanganese were examined (HC andFeMn) modeled in a second by FactSagesmelting TMreduction. Those fluxes reductionprocess step was on proposed.the composition The effects of of theadding fina limel alloy and magnesiaand slag as were a flux examined in the second and smelting modeled by are commonlyTM used in the production of ferromanganese in a submerged arc furnace from land-based FactSagereduction. Those step fluxes on the are composition commonly of usedthe fina in lthe alloy production and slag were of ferromanganese examined and modeled in a submerged by orearc bodies.furnaceFactSage ThefromTM main. Thoseland-based objective fluxes areore of commonly thebodies. addition Theused of mainin basic the obproduction fluxesjective was ofof totheferromanganese increase addition the of in manganese basic a submerged fluxes yield was and to toincrease decreasearc the furnace the manganese silicon from contentland-based yield and in ore the to bodies. produceddecrease The the alloymain si liconob [15jective]. content Figure of the2 in showsaddition the produced the of flowchartbasic fluxesalloy of was[15]. the to Figure process 2 investigatedshows increasethe flowchart by the this manganese paper. of the For yield process both and reduction to investigateddecrease operations, the si liconby thiscontent submerged paper. in the Forproduced arc furnacesboth alloy reduction were [15]. Figure recommended. operations, 2 shows the flowchart of the process investigated by this paper. For both reduction operations, Thesubmerged advantages arc furnaces of such awere furnace recommended. for processing The manganese advantages nodules of such were a furnace already for highlighted processing in previoussubmerged research arc [3 ,5furnaces]. For producingwere recommended. ferromanganese, The advantages the submerged of such a furnace arc furnace for processing is already the manganesemanganese nodules nodules were were already already highlightedhighlighted inin previousprevious research research [3,5]. [3,5].For producingFor producing dominantferromanganese,ferromanganese, furnace the in thesubmerged theindustry submerged arc [15 arc furnace]. furnace is is alrealready the the dominant dominant furnace furnace in the in industry the industry [15]. [15].

FigureFigure 2. Simpliﬁed 2. Simplified ﬂowchart flowchart of of th thee investigated investigated process. process.

Figure 2. Simplified flowchart of the investigated process.

Minerals 2018, 8, x FOR PEER REVIEW 4 of 13 Minerals 2018, 8, 544 4 of 13 By combining two smelting reduction processes, the following advantages were expected: • ValuableBy combining metals two are smelting concentrated reduction in processes, an alloy the in followingthe first advantagesreduction werestep, expected:to minimize hydrometallurgical downstream processing. • Valuable metals are concentrated in an alloy in the ﬁrst reduction step, to minimize • By production of marketable ferromanganese, value can be added to the entire production hydrometallurgical downstream processing. process. • By production of marketable ferromanganese, value can be added to the entire production process. • After two reduction processes, it is envisaged to generate a heavy-metal free slag, which can be • After two reduction processes, it is envisaged to generate a heavy-metal free slag, which can be used as a construction material to avoid landfilling of slag and therefore reduce the burden on used as a construction material to avoid landﬁlling of slag and therefore reduce the burden on the environment. the environment.

2.2. Thermochemistry Thermochemistry

2.1.2.1. Thermochemistry Thermochemistry of of Manganese Manganese Nodule Nodule Smelting Smelting TheThe purpose purpose of of the the first ﬁrst reduction reduction step step in in Figure Figure 2 is to discard manganese in the slag and to enrichenrich the the valuable valuable metals metals in in an an alloy, alloy, to to simplify simplify further further downstream downstream processing processing [5]. [5]. For For a a successful successful slaggingslagging operation, operation, the the activity activity of of the the oxide oxide that that sh shouldould stay stay in in the the slag slag has has to to be be low. low. A A component component withwith a highhigh activityactivity has has the the tendency tendency to leave to leave the system the system [16], for [16], example for example by reduction by reduction and dissolving and dissolvingin a metallic in a phase metallic in this phase case. in Forthis manganese(II)case. For manganese(II) oxide, this oxide, can be this expressed can be expressed by Equation by Equation (1): (1):

MnO(l) + C(s) Mn(l) + CO(g) (1) MnO(l) + C(s) ⇌ Mn(l) + CO(g) (1) Wherewhere (s)(s) denotesdenotes components components in in a solida solid state; state; (l) denotes(l) denotes components components in aliquid in a liquid state; andstate; (g) and denotes (g) denotescomponents components in gaseous in gaseous state. state. Therefore,Therefore, to to design design a suitable slag composition aa lowlow activityactivity ofof manganese(II)manganese(II) oxide oxide in in the the slag slag is isdesirable desirable to to discard discard most most of theof the manganese manganese in the in slag.the slag. The inﬂuenceThe influence of different of different ﬂuxes fluxes on the activityon the activityof manganese(II) of manganese(II) oxide was oxide simulated was simulated with FactSage with FactSageTM and displayedTM and displayed in Figure in3 Figureat the liquidus3 at the liquidustemperature temperature of the slag of andthe slag a pressure and a pressure of 1 atm. of 1 atm.

0.7

0.6

0.5

0.4

0.3 CaO FeO 0.2 Al2O3 Na2B4O7 Activity of MnO in slag 0.1 TiO2 SiO2 0 -5 0 5 10 15 20 25 30 35 Addition of flux in wt % FigureFigure 3. 3. InfluenceInﬂuence of of fluxing ﬂuxing on on MnO-activity MnO-activity based based on on FactSage FactSageTMTM. .

FluxingFluxing by by Al Al22OO3,3 ,TiO TiO2,2 ,FeO, FeO, Na Na2B2B4O4O7 and7 and SiO SiO2 resulted2 resulted in in an an activity activity decrease decrease of of manganese(II) manganese(II) oxideoxide in in the the slag, slag, however, however, after after the theaddition addition of 25 of wt 25 % wtNa %2B4 NaO7 2theB4 Oactivity7 theactivity increased increased again. CaO again. is notCaO an is appropriate not an appropriate fluxing fluxingagent for agent the for smelting the smelting of manganese of manganese nodules, nodules, due dueto an to increase an increase in manganese(II)in manganese(II) oxide oxide activity activity in the in theslag. slag. The The most most promising promising results results were were reached reached by the by theaddition addition of SiOof SiO2 as 2aas flux. a flux. Therefore, Therefore, in the in experimental the experimental trials, trials, a special a special emphasis emphasis was given was given to the to effect the effectof SiO of2 onSiO the2 on smelting the smelting operations. operations. Different Different additions additions of SiO of SiO2, mixtures2, mixtures of offluxes fluxes containing containing SiO SiO2 2andand furthermorefurthermore three three other other pure pure oxides were investigated.investigated. TheThe fluxesfluxes andand thethe amountamount areare listed listed in in Table Table2, TM 2,together together with with the the liquidus liquidus temperature temperature of of the the slag slag predicted predicted by by FactSage FactSageTM. .

Minerals 2018, 8, x FOR PEER REVIEW 5 of 13 Minerals 2018, 8, 544 5 of 13 Table 2. Effect of flux on liquidus temperature and basicity, modeled with FactSageTM for smelting nodules. Table 2. Effect of ﬂux on liquidus temperature and basicity, modeled with FactSageTM for

smelting nodules.No 15% 25% 30% SiO2- 20% SiO2-TiO2, 25% 20% 20% Fluxing Addition SiO2 SiO2 CaO 10% Al2O3 TiO2 Na2B4O7 Fe2O3 30% 20% SiO2-TiO2, 20% TLiqFluxing in No Addition 15% SiO2 25% SiO2 25% TiO2 20% Fe2O3 1514 1263 1285 1365SiO 2-CaO 10% 1190 Al2 O3 1204 Na13022B4O 7 1403 °C ◦ TLiq in C 1514 1263 1285 1365 1190 1204 1302 1403 MixturesMixtures of components of components were were equimolar; additions additions were were in wtin %.wt %.

2.2.2.2. Thermochemistry Thermochemistry of of Ferromanganese Ferromanganese Production Production

WithWith FactSage FactSageTMTM thethe influence inﬂuence of of lime lime and and a a mixture mixture of of 50 50 wt wt % % lime lime and and 50 50 wt wt % % magnesia magnesia was was investigated.investigated. The The manganese manganese yield yield and and the the silicon silicon cont contentent in in the the alloy alloy were were of of interest interest and and are are shown shown inin Figure Figure 44.. The simulation was carried out under the assumptionassumption that graphite, slag and the alloy werewere in in a a thermal thermal equilibrium equilibrium at 1500 °C.◦C.

84 10

82 8 CaO 80 6

78 4 CaO+MgO Mn-recovery 76 2 CaO Manganese yield in % in yield Manganese 74 0 % wt in content Silicon 0 102030 CaO+MgO Si-content Addition of flux in wt % FigureFigure 4. 4. InfluenceInﬂuence of of fluxing ﬂuxing on on the the manganese manganese yi yieldeld and and silicon silicon content content in in the the alloy. alloy.

AccordingAccording to to the the simulation, simulation, lime lime and and the the mixture mixture of oflime lime and and magnesia magnesia had had a positive a positive effect effect on theon themanganese manganese yield yield and and the the silicon silicon content content inin the the ferromanganese. ferromanganese. The The difference difference between between the the additionaddition of of solely solely lime lime and and the the mixture mixture on on the the sili siliconcon content content was was rather rather small, small, however, however, there there was was a a differencedifference in in the the yield yield of of manganese. manganese. The The effect effect of of lime lime on on the the yield yield was was better better than than the the effect effect of of the the limelime and and magnesia magnesia mixture, thoughthough thisthis difference difference decreased decreased if if the the addition addition of of fluxes fluxes was was increased increased up upto 30to 30 wt wt %. %. The The highest highest yield yield and and lowest lowest silicon silicon content content were were achievedachieved whenwhen 3030 wt % of fluxes fluxes are are added.added. BasedBased onon this this simulation, simulation, five differentfive different slag compositions slag compositions were considered were considered for the experiments. for the experiments.Additionally, Additionally, magnesia was magnesia considered was as well.considered Table3 asshows well. the Table investigated 3 shows the fluxes investigated and the liquidus fluxes andtemperature the liquidus of the temperature slag before reduction,of the slag based before on reduction, FactSageTM basedand theon FactSage basicity (B)TM ofand the the slag basicity according (B) ◦ ofto the Equation slag according (2). The liquidusto Equation temperature (2). The liquidus of the slag temperature without fluxing of the wasslag 1279withoutC. fluxing was 1279 °C. wt %MgO + wt %CaO B = wt %MgO + wt %CaO (2) B = wt %SiO2 (2) wt %SiO2

Table 3. Effect of ﬂux on liquidus temperature and basicity, modeled with FactSageTM for Table 3. Effect of flux on liquidus temperature and basicity, modeled with FactSageTM for ferromanganese production. ferromanganese production. Fluxing 5% CaO 15% MgO 15% CaO-MgO * 15% CaO 30% CaO

◦ Fluxing 5% CaO 15% MgO 15% CaO-MgO * 15% CaO 30% CaO TLiq in C 1308 1409 1335 1351 1401 BTLiq in °C 0.48 1308 1409 0.91 1335 0.91 1351 0.91 1401 1.55 * MixtureB of CaO0.48 and MgO was0.91 50 wt % CaO and 0.91 50 wt % MgO; additions 0.91 were in wt1.55 %. * Mixture of CaO and MgO was 50 wt % CaO and 50 wt % MgO; additions were in wt %. Even if ﬂuxing by lime and magnesia increases the liquidus temperature, after reduction of manganese(II) oxide, ﬂuxes are needed to keep the liquidus temperature under the process temperature

Minerals 2018, 8, x FOR PEER REVIEW 6 of 13 Minerals 2018, 8, 544 6 of 13 Even if fluxing by lime and magnesia increases the liquidus temperature, after reduction of manganese(II) oxide, fluxes are needed to keep the liquidus temperature under the process ◦ temperatureof 1500 C. This of 1500 prerequisite °C. This wasprerequisite met by all was options met by presented all options inTable presented3. In a in simpliﬁed Table 3. In ternary a simplified phase ternarydiagram, phase containing diagram, the containing main oxides the MnO,main oxides SiO2 and MnO, CaO, SiO the2 and path CaO, of thethe slagpath systemof the slag during system the TM processduring the can process be visualized. can be visualized. Such a diagram Such a isdiag shownram is in shown Figure in5, Figure simulated 5, simulated by FactSage by FactSagewithTM a withconstant a constant oxygen oxygen pressure pressure of 0.01 of atm 0.01 as atm a boundary as a boundary condition. condition.

Figure 5. IsothermalIsothermal liquidus liquidus curves curves of ofthe the MnO-SiO MnO-SiO2-CaO2-CaO slag slag system system and andpath path of fluxing of ﬂuxing and reduction.and reduction.

Such a diagram could only roughly support the development of the process, because the main oxides were solely investigated. However, it was obvious that an addition of alkaline earth metals, in this case lime, was necessary to keep the liquidusliquidus temperature low, while manganese(II) oxide was extracted to a large extent. The liquidus temperatur temperaturee of the nodule slag before reduction and fluxing ﬂuxing ◦ was around 1280 °C,C, displayed by the black dot in th thee diagram diagram (Figure 5 5).). FluxingFluxing withwith limelime increasedincreased ◦ the liquidusliquidus temperaturetemperature by by 4 4 C°C per per percent percent of of added added lime lime and and is displayed is displayed by theby greythe grey line. line. The pathThe pathof manganese of manganese reduction reduction is displayed is displayed by the blue, by the red blue, and purplered and lines purple for three lines different for three additions different of additionslime. While of manganese(II)lime. While manganese(II) oxide reduced, oxide the reduced, liquidus the temperature liquidus temperature of the slag of increased the slag inincreased each of inthe each three of investigated the three investigated cases, however, cases, thehowever, biggest the increase biggest occurred increase whenoccurred only when 5 wt only % of 5 lime wt % was of limeadded. was Fluxing added. with Fluxing more with than more 15 wt than % lime 15 wt was % lime theoretically was theoretically necessary necessary to sustain to a liquidsustain slag a liquid with slaga maximum with a maximum of 10 wt %of manganese(II)10 wt % manganese(II) oxide. The oxide. liquidus The liquidus temperature temperature of such aof slag such was a slag around was ◦ 1425aroundC. 1425 The components°C. The components of the slag of with the theslag highest with the melting highest points melting were points predicted were by predicted FactSage TMby FactSageand are indicatedTM and are by indicated the black phaseby the boundaries. black phase The boun chemicaldaries. formulasThe chemical of those formulas components of those are componentsgiven in Figure are5 given, while in aFigure silicate 5, denotedwhile a silicate in parenthesis denoted in was parenthesis only a dilute was componentonly a dilute in component the main

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Minerals 2018, 8, x FOR PEER REVIEW 7 of 13 silicate. A detailed description and the names of the phases in the FactSageTM database are listed inin the Table main S1 ofsilicate. the Supplementary A detailed description Information. and the Silicon names oxides of the was phases the component in the FactSage withTM the database highest aremelting listed point in Table for trialsS1 of withthe Supplementary low additions ofInform limeation. and calcium Silicon oxides silicates was having the component the highest with melting the highestpoint in melting the trials point with for more trials than with 15 wtlow % additions additional of lime. lime and calcium silicates having the highest melting point in the trials with more than 15 wt % additional lime. 3. Materials and Experimental Details 3. Materials and Experimental Details 3.1. Smelting of Manganese Nodules 3.1. SmeltingThe deep-sea of Manganese nodules Nodules studied in this research were a common sample from the German license areaThe in the deep-sea CCZ. The nodules average studied chemical in this content research of the were valuable a common metals sample is presented from the in German Table4[3 ].license area in the CCZ. The average chemical content of the valuable metals is presented in Table 4 [3]. Table 4. Average chemical content of valuable metals in dried deep-sea nodules from the German license Tablearea in 4. the Average Clarion-Clipperton chemical content Fracture of Zonevaluable (CCZ), metals analyzed in dried byICP-OES deep-sea and nodules ICP-MS from (wt the %) [German3]. license area in the Clarion-Clipperton Fracture Zone (CCZ), analyzed by ICP-OES and ICP-MS Co Cu Mn Mo Ni V (wt %) [3]. 0.16 1.17 31.2 0.06 1.36 0.06 Co Cu Mn Mo Ni V 0.16 1.17 31.2 0.06 1.36 0.06 The manganese nodules were dried before the analysis, because they contained signiﬁcant amountsThe manganese of water. In additionnodules were to physically dried before bound the moisture, analysis, water because of hydration they contained was present significant in the amountsnodules. of The water. loss onIn ignitionaddition ofto driedphysically nodules boun wasd moisture, still around water 15 wt of %.hydration The initial was ratio present of MnO in the to nodules. The loss on ignition of dried nodules was still around 15 wt %. The initial ratio of MnO to SiO2 in the measured samples was 3.2 [3]. SiO2 inThe the smelting measured experiments samples was were 3.2 [3]. carried out in a direct current (DC) electric arc furnace. The build-upThe smelting of the experiments furnace used were for carried the smelting out in operationa direct current is shown (DC) in electric Figure 6arc. The furnace. supplied The build-upelectrical of power the furnace of the used furnace for the is inﬁnitely smelting variable.operation is The shown top electrodein Figure 6. position The supplied is hydraulically electrical poweradjusted of bythe hand. furnace After is infinitely an experiment, variable. the The melt to isp pouredelectrode into position a coated is castinghydraulically mold. Theadjusted tapping by hand.operation After is an also experiment, shown in Figure the melt6. is poured into a coated casting mold. The tapping operation is also shown in Figure 6. Graphite electrode

Insulation

(Al2O3)

TI T01 TI T02

Transformer- rectifier Water cooled copper electrode

(a) (b)

Figure 6. Small-scaleSmall-scale electric electric arc arc furnace: furnace: ( (aa)) Schematic Schematic concept concept of of the the furnace; furnace; ( (bb)) tapping tapping of of the furnacefurnace after after an an experiment.

The smelting operation was carried out in a high purity graphite crucible. The The working volume of the furnace furnace was was 2 2 L. L. Approximately Approximately 3.5 3.5 kg kg of of no nodulesdules were were charged charged into into the the preheated preheated furnace furnace per per trial.trial. The The fluxes ﬂuxes shown shown in in Table Table 22 were smelted togethertogether withwith thethe nodules. For every slag composition, composition, fourfour experiments experiments were were conducted. conducted. The The graphite graphite from from the the top top electrode electrode and and crucible crucible were were used used as as a a reducingreducing agent. The The reduction reduction time, time, i. i.e.,e., the the time time after after all all material material is is fed fed into into the the crucible crucible was was 20 20 min. min. The reduction temperature was about 150 °C above the modeled liquidus temperature of the slag (Table 2).

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The reduction temperature was about 150 ◦C above the modeled liquidus temperature of the slag (Table2). To verify the most promising results and to generate enough homogeneous material for the investigation of ferromanganese production, a second furnace was used to increase the mass of smelted nodules per experiment. The scale-up furnace is similar to the furnace in Figure6, but 8 kg of nodules could be smelted per trial. The working volume of the furnace was 6 L. Six trials were carried out in this furnace and 48 kg of nodules were smelted in total. The temperature of the slag during the reduction time was 1400 ◦C and the reduction time was increased to 60 min, due to the increased mass of nodules. To provide sufficient reducing agent, 200 g of carbon in the form of coke with a particle size between 1–5 mm was added per trial further to the carbon supply from the crucible and top electrode. After the reduction time, all coke reacted with the slag and gas. Generated slag in this paper was analyzed by fused cast bead X-ray fluorescence spectroscopy (XRF) and the metal by a wavelength dispersive X-ray fluorescence spectroscopy (WDXRF) spectrometer. The carbon content in the metal was determined by an infrared absorption method after combustion in an induction furnace. The mass of the metallic phase and the slag was measured with a scale.

3.2. Smelting of Manganese-Rich Slag The homogenized, crushed and screened manganese-bearing slag from the scale-up experiments was used for the production of ferromanganese following the process shown in Figure2. For the smelting of manganese-rich slag, the small-scale electric arc furnace was used. 3 kg of slag was smelted together with fluxes in each experiment. The five combinations of fluxes shown in Table3 were investigated. Every experiment was carried out twice. The smelting temperature was 1500 ◦C. The reducing agent for the production of ferromanganese was the graphite from the crucible and top electrode as well. Furthermore, 6.2 wt % carbon was additionally added in the form of coke with a particle size between 1–5 mm. The reduction time was 60 min, after this time all coke had reacted with the slag and gas.

4. Results and Discussion

4.1. Smelting of Manganese Nodules To determine the efficiency of the smelting operation, the yield was calculated by using Equation (3): mxMetal ηx= 100% · (3) mxMetal + mxSlag η where x was the yield of element x; mxMetal was the mass of the element x in the metal; and mxSlag was the mass of the element x in the slag. The mass of an element in a single phase was calculated with information about the total mass of the phase and the concentration of the element in that phase. The yield for the valuable metals and iron are shown in Figure7 for the experiments in the furnace with a volume of 2 L. To determine the efficiency of the slagging operation for manganese, the reciprocal value of the fraction from Equation (3) was displayed. Due to the high manganese content in the slag and high yields for valuable metals, fluxing with Na2B4O7 or SiO2 seemed to be beneficial. However, Na2B4O7 is rather costly compared to SiO2, therefore only SiO2 was considered a viable option for a process on an industrial scale. In the trials, the addition of 15 wt % SiO2 showed an advantage over the addition of 25 wt % SiO2 and the mixtures containing SiO2 and other fluxes. Based on the results in Figure7, six scale-up trials were conducted. Compared to the small-scale trials, the SiO2-addition decreased to 9.4 wt %. The average chemical composition of the main oxides in the slag is shown in Table5. Minerals 2018, 8, x FOR PEER REVIEW 9 of 13

Co Cu Fe Mo Ni MnO 100100 100100

8080 8080

% 6060 6060 n i

Mineralsld 2018, 8, 544 9 of 13 e 4040 4040 Yield in % MineralsYi 2018, 8, x FOR PEER REVIEW 9 of 13 2020 2020 Co Cu Fe Mo Ni MnO 100100 100100 00 0 0 8080 No fluxingNo 15 %15% SiO2 25 %25% SiO230 %20 SiO2-CaO30% % SiO2-TiO2,20% 10 %25 Al2O3 %20% TiO220 % Na2B4O720% 20 %20% Fe2O38080

fluxing SiO2 SiO2 SiO2-CaO SiO2-TiO2, TiO2 Na2B4O7 Fe2O3 Efficiency of slagging operation in % % 6060 10 % Al2O3 6060 n i

ld Figure 7. Yield of nodule smelting and the efficiency of the slagging operation. e 4040 4040 Yield in % Yi Due to the high manganese content in the slag and high yields for valuable metals, fluxing with 2020 2020 Na2B4O7 or SiO2 seemed to be beneficial. However, Na2B4O7 is rather costly compared to SiO2, therefore only SiO2 was considered a viable option for a process on an industrial scale. In the trials, 00 0 0 the addition of 15 wt % SiO2 showed an advantage over the addition of 25 wt % SiO2 and the mixtures No fluxingNo 15 %15% SiO2 25 %25% SiO230 %20 SiO2-CaO30% % SiO2-TiO2,20% 10 %25 Al2O3 %20% TiO220 % Na2B4O720% 20 %20% Fe2O3 containing SiO2 and other fluxes. fluxing SiO SiO SiO -CaO SiO -TiO , TiO Na B O Fe O Efficiency of slagging operation in % Based on the results2 in Figure 7,2 six scale-up2 trials2 were2 conducted.2 Comp2 4ared7 to the2 small-scale3 10 % Al2O3 trials, the SiO2-addition decreased to 9.4 wt %. The average chemical composition of the main oxides in the slag is shownFigureFigure in 7. TableYieldYield of 5. nodule smelting and the effi efﬁciencyciency of the slagging operation.

Table 5. Average chemical composition of homogenized manganese-rich slag analyzed by fused cast DueTable to 5. the Average high manganesechemical composition content inof thehomogenized slag and highmanganese-rich yields for valuableslag analyzed metals, by fused fluxing cast with bead XRF (wt %). Na2B4beadO7 or XRF SiO (wt2 seemed%). to be beneficial. However, Na2B4O7 is rather costly compared to SiO2, 2 thereforeAl2O3 onlyCaO SiO was CuO considered FeO a viable MgO option for MnO a proces Nas 2onO an industrial NiO scale. P2O5 In theSiO trials,2 Al2O3 CaO CuO FeO MgO MnO Na2O NiO P2O5 SiO2 the addition of 15 wt % SiO2 showed an advantage over the addition of 25 wt % SiO2 and the mixtures 5.64 2.925.64 2.92 0.15 0.15 1.57 1.57 3.343.34 56.2 56.2 4.46 4.46 0.07 0.070.20 23.4 0.20 23.4 containing SiO2 and other fluxes. BasedThe average on the resultsphase indistribution Figure 7, six for scale-up the investigat trials wereed metallic conducted. elements Comp aredis shown to the in small-scale Figure 8. LossesThe are average neglected phase in distributionthis figure. The for thedisplayed investigated marg metallicin of error elements is the standard is shown deviation in Figure8 .of Losses all six trials, the SiO2-addition decreased to 9.4 wt %. The average chemical composition of the main oxides inaretrials. the neglected slag is shown in this in ﬁgure. Table The 5. displayed margin of error is the standard deviation of all six trials.

100Table 5. Average chemical composition of homogenized manganese-rich slag analyzed by fused cast bead XRF (wt %). 80 Al2O3 CaO CuO FeO MgO MnO Na2O NiO P2O5 SiO2 5.64 2.92 0.15 1.57 3.34 56.2 4.46 0.07 0.20 23.4 The60 average phase distribution for the investigated metallic elements is shown in Figure 8. Losses are neglected in this figure. The displayed margin of error is the standard deviation ofSlag all six trials.40 Metal

100 20 Phase distribution in wt % wt in distributionPhase 80 0 Co Cu Fe Mn Mo Ni 60 Figure 8.8. Average phase distribution for metallic elementselements between slag and metalmetal forfor smeltingsmeltingSlag experimentsexperiments withwith 9.49.4 wtwt %% SiOSiO22-addition. 40 Metal The scale-up trials trials showed showed a asimilar similar high high yield yield for for cobalt, cobalt, iron, iron, molybdenum molybdenum and andnickel. nickel. The Thereduction reduction20 of manganese of manganese was was avoi avoidedded successfully. successfully. The The yield yield for for copper copper with with more more than than 90% was even higher than in the pre-trials. The high yield for iron of nearly 80% yielded an iron content in % wt in distributionPhase the alloy of over 50 wt %, therefore, the iron content had to be reduced by the subsequent oxidizing 0 and converting operation of the Inco-process to reduce the weight of the metal-bearing stream for downstream processing.Co Cu Fe Mn Mo Ni Figure 8. Average phase distribution for metallic elements between slag and metal for smelting

experiments with 9.4 wt % SiO2-addition.

The scale-up trials showed a similar high yield for cobalt, iron, molybdenum and nickel. The reduction of manganese was avoided successfully. The yield for copper with more than 90% was

Minerals 2018, 8, x FOR PEER REVIEW 10 of 13

even higher than in the pre-trials. The high yield for iron of nearly 80% yielded an iron content in the alloy of over 50 wt %, therefore, the iron content had to be reduced by the subsequent oxidizing and converting operation of the Inco-process to reduce the weight of the metal-bearing stream for downstream processing. Minerals 2018, 8, 544 10 of 13

4.2. Smelting of Manganese-rich Slag 4.2. Smelting of Manganese-Rich Slag 4.2.1. Analysis of Ferromanganese 4.2.1. Analysis of Ferromanganese Pre-trials showed that the content of manganese and silicon in the produced ferromanganese werePre-trials the critical showed parameters, that the content which ofhad manganese to be in and line silicon with in the the producedASTM standard ferromanganese for grade were A theferromanganese. critical parameters, The manganese which had tocontent be in linefor withthat grade the ASTM had standardto be above for grade78 wt A% ferromanganese. and the silicon Thecontent manganese could not content surpass for 1.2 that wt %. grade The average had to be va abovelues for 78 those wt % elements, and the based silicon on content two experiments could not surpassper slag 1.2system wt %. are The displayed average valuesin Figure for 9 those and elements,compared basedto the on values two experimentspredicted by per the slag FactSage systemTM aremodel. displayed The margins in Figure of9 errorand compared display the to theresult values of th predictede first and by second the FactSage trial carriedTM model. out for The each margins slag ofsystem. error display the result of the ﬁrst and second trial carried out for each slag system.

100 5

Trial 80 4

60 3 Model Mn-content

40 2 Trial

20 1 Silicon content in wt % incontent Silicon Model Si-content Manganese content in wt % Manganese 0 0 5% CaO 15% MgO 15% 15% CaO 30% CaO CaO-MgO

FigureFigure 9.9. ComparisonComparison ofof thethe manganesemanganese andand siliconsilicon contentscontents inin ferromanganeseferromanganese fromfrom experimentalexperimental trialstrials analyzedanalyzed byby WDXRFWDXRF andand thethe FactSageFactSageTMTM model under variation of fluxes. ﬂuxes.

BesidesBesides thethe experiments experiments with with an additionan addition of 15 of wt 15 % magnesia,wt % magnesia, all alloys all surpassed alloys surpassed the minimum the manganeseminimum manganese content of 78content wt %. of Those 78 wt results %. Those fairly resu matchedlts fairly the matched predicted the values predicted of the values FactSage of theTM model,FactSage exceptTM model, for the except experiments for the withexperiments 15 wt % with magnesia, 15 wt which% magnesia, were again which out were of line. again The out maximum of line. siliconThe maximum content silicon in all alloys content was in underall alloys 1.2 was wt %,under except 1.2 wt for %, the except trials for with the 15 trials wt %with lime. 15 wt However, % lime. theHowever, measured the temperaturemeasured temperature of the slag beforeof the tappingslag befo forre thetapping trials for with the 15 trials wt % with lime 15 was wt 75 %◦ limeC higher was than75 °C the higher planned than 1500 the planned◦C, even 1500 though °C, aeven process thou temperaturegh a process of temperature 1500 ◦C would of 1500 have °C been would enough, have tobeen sustain enough, a liquid to sustain slag during a liquid the slag reduction during shown the reduction according shown to Figure according5. Therefore, to Figure the higher 5. Therefore, silicon contentthe higher compared silicon content to the trialscompared with to a lowerthe trials addition with a oflower lime addition could be of explained lime could by be the explained increased by processthe increased temperature. process temperature. TheThe marginmargin ofof errorserrors dependsdepends mostlymostly onon thethe processprocess conditionsconditions sincesince aa steadysteady operationoperation inin thethe small-scalesmall-scale electricelectric arcarc furnacefurnace cannotcannot bebe guaranteedguaranteed duringduring thethe wholewhole experiment.experiment. AnotherAnother majormajor inﬂuenceinfluence is is the the composition composition of of the the used used raw raw material, material, which which probably probably underlies underlies inhomogeneity, inhomogeneity, even evenif homogenization if homogenization of the of supplied the supplied crushed crushed slag is slag carried is carried out by out mixi byng mixing before before the experiments. the experiments. With Withchemical chemical analyses, analyses, inaccuracies inaccuracies due to due sampling to sampling of the of metal the metal can be can possible. be possible. AA comparisoncomparison betweenbetween the trials and thethe modelsmodels inin thethe casecase ofof siliconsilicon werewere notnot useful.useful. AllAll fourfour trialstrials withwith a tappinga tapping temperature temperature of 1500 of 1500◦C had °C a ha signiﬁcantlyd a significantly lower silicon lower content silicon than content the prediction than the byprediction FactSage TMby .FactSage This couldTM. be This explained could bybe theexplained fact that by FactSage the factTM thatonly FactSage predicts theTM only thermodynamical predicts the equilibrium,thermodynamical and after equilibrium, 60 min of and reduction after 60 time min it isofnot reduction guaranteed time that it is the not equilibrium guaranteed is that already the reached.equilibrium In comparison is already reached. to the ASTM In comparison standard for to Gradethe ASTM A ferromanganese, standard for Grade the phosphorous, A ferromanganese, carbon and iron content of the alloys were no issue, because the tolerated maximum content of these elements was undercut easily. However, of concern is the high copper content around 0.4 wt % in the produced alloys. This content surpassed the amount of copper that was present in the manganese-rich slag as copper(II) oxide according to Table5. The reason for this Cu-enrichment in ferromanganese may be Minerals 2018, 8, 544 11 of 13 due to copper-rich metal droplets, which have still been in the charged nodule slag. Even if the slag is crushed and screened before the experiments, it can be possible, that small droplets remained in the slag and contaminated the ferromanganese. By improving the metal and slag phase separation a lower copper content could be possible.

4.2.2. Analysis of the Final Slag The ﬁnal slag was analyzed and FactSageTM was used to model the liquidus temperature after the experiments. The basicity was again calculated by Equation (2). Table6 shows the basicity and liquidus temperature for the investigated slags.

Table 6. Basicity and liquidus temperature of the ﬁnal slag modeled with FactSageTM.

Fluxing 5% CaO 15% MgO 15% CaO-MgO 15% CaO 30% CaO ◦ TLiq in C 1312 1470 1357 1203 1284 B 0.44 0.83 0.83 0.83 1.19

Magnesia seemed to not be useful in the process since the liquidus temperature was only 30 ◦C lower than the proposed process temperature and as discussed before, the manganese content in the produced alloy was insufficient. Therefore, lime only seems to be an option for fluxing, an addition of 5–15 wt % is recommended. The liquidus temperature was low enough to ensure a liquid slag during the process and the produced alloys were in line with the ASTM standard. Fluxing with 30 wt % lime was not recommended because of two reasons, (i) the usage of so much flux is uneconomical and (ii) the slag of the experiments with 30 wt % lime tends to disintegrate, which prohibits the use of the slag as a mineral product in the construction industry. For the development of a zero-waste process the final slag has to be a usable product. The content of heavy metals in the final slag should be as low as possible (Table7).

Table 7. Amount of heavy metal traces in the ﬁnal slag analyzed by fused cast bead XRF (wt %).

Fluxing 5% CaO 15% MgO 15% CaO-MgO 15% CaO 30% CaO

Cr2O3 <0.01 <0.01 <0.01 <0.01 <0.01 CuO 0.11 0.04 0.05 0.03 0.03 NiO 0.09 0.01 0.02 0.01 <0.01 V2O5 0.05 <0.01 0.01 <0.01 <0.01

The results showed that the concentration of most of the heavy metals in the slag was already low. The highest concentration was 0.11 wt % for copper(II) oxide in one sample. Nevertheless, only a leaching test can prove if the slag fulﬁlls the requirements for a heavy-metal free slag, according to regulations.

5. Conclusions This paper investigated the pyrometallurgical enrichment process of valuable metals from manganese nodules and explored the utilization of the manganese-bearing slag. Pre-trials and further scale-up experiments highlighted the beneficial effects of fluxing by SiO2 in the smelting reduction process of manganese nodules. Optimal parameters were a smelting ◦ temperature of 1400 C and the addition of 9.4 wt % SiO2 to recover over 90% and up to 100% of cobalt, copper, molybdenum, nickel and manganese was successfully discarded into the slag to a significant extent (>97%). The enrichment of valuable metals in an alloy while discarding gangue and manganese with the slag is an advantage of the process, since it significantly reduces the amount of material, which needs to be processed further by hydrometallurgical methods. Minerals 2018, 8, 544 12 of 13

The trials to produce ferromanganese showed that the manganese-bearing slag could be used as a resource to produce an alloy that conformed with the ASTM standard. Recovering manganese as a marketable alloy has two significant advantages, (i) the generation of revenue and (ii) by further reduction the total amount of slag is decreased and heavy-metals are reduced and collected in the metal, which is relevant for utilizing the slag as a mineral product, for example in the construction industry. Different fluxes and the amount of added fluxes were investigated. The results showed that the addition of 5–15 wt % of lime provided the best results. However, further studies and a scale-up are necessary especially for the second reduction step. Copper impurities are another problem in the alloy, which make further investigation of the copper behavior during the reduction and phase separation necessary. Furthermore, the utilization of the resulting slag as a mineral product has to be proven in supplementary studies, however, the chemical analysis does not show elevated contents for heavy-metals and is therefore promising.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/8/12/544/ s1, Table S1: Description of occurring phases from the phase diagram (Figure5). Author Contributions: M.S. designed, carried out and analyzed the ferromanganese experiments. D.F. designed, carried out and analyzed the manganese nodules smelting experiments. M.S. wrote the paper with significant contributions from T.K. and B.F. All authors read and approved the final manuscript. Funding: This research was funded by internal BGR funding under the grant number A-0203002.A. Acknowledgments: The German Federal Institute for Geosciences and Natural Resources (BGR) is acknowledged for the supply of raw nodules, financial and additional support. Conflicts of Interest: The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; and in the decision to publish the results. As part of the funding sponsor, T.K. contributed to writing the manuscript.

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