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A pyro-hydrometallurgical process is proposed in this study for the selective arsenic removal and the Pb and Sb recovery from lead softening slag, which incorporates NaOH roasting followed by water leaching, carbothermic reduction, and HNO3 leaching as process steps. Lead softening slag, containing 63.9 wt% Pb, 17.7 wt% Sb, and 4.4 wt% As, is first roasted with NaOH at 400°C, followed by water leaching at room temperature. The effect of the roasting temperature and NaOH dosage is investigated with regard to the removal yield of As, Pb, and Sb. The arsenic-free slag is reduced with carbon for Pb recovery as the antimonial lead. The reduction temperature and carbon dosage are optimized by applying FactSage modeling and performing experimental tests for selective Pb recovery. The residual slag is leached with HNO3 solution for the further removal of Pb and enrichment of Sb. The chemical and phase composition of the final leaching residue is determined to assess the possibility of using it as a feedstock for antimony trioxide production. The proposed process flowsheet enables the recovery of Pb and Sb as antimonial lead and antimony trioxide, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Slags are an indispensable tool for the pyrometallurgical industry to extract and purify metals at competitive prices. Large volumes are produced annually, leading to important economical and ecological issues regarding their afterlife. To maximise the recycling potential, slag processing has become an integral part of the valorisation chain. However, processing is often directed solely towards the cooled slag. In this article, the authors present an overview of the scientific studies dedicated to the hot stage of slag processing, i.e. from the moment of slag/metal separation to complete cooling at the slag yard. Using in-depth case studies on C2S driven slag disintegration and chromium leaching, it is shown that the functional properties of the cooled slag can be significantly enhanced by small or large scale additions to the high temperature slag and/or variations in the cooling path, even without interfering with the metallurgical process. The technology to implement such hot stage processing steps in an industrial environment is currently available. No innovative technological solutions are required. Rather, advances in hot stage slag processing seem to rely primarily on further unravelling the relationships between process, structure and properties. This knowledge is required to identify the critical process parameters for quality control. Moreover, it could even allow to consciously alter slag compositions and cooling paths to tailor the slag to a certain application. [Copyright &y& Elsevier]

Bauxite residue (red mud) that is generated from karst bauxite ores is rich in rare-earth elements (REEs). The REEs can be recovered from bauxite residue by direct acid leaching but the extraction yields are generally low. The extraction yields can be increased by increasing the acid concentration but this will increase the dissolution of iron as well. Large amounts of iron in the leach solution create problems in the further recovery processes. Therefore, a combined sulfation–roasting–leaching process was developed to selectively leach the REEs while leaving iron undissolved in the residue. In this process bauxite residue was mixed with water and concentrated H 2 SO 4 followed by drying, roasting and then leaching of the roasted product with water. Most of the oxides were converted to their respective sulfates during the sulfation process. During subsequent roasting, unstable sulfates (mainly iron(III) sulfate) decompose to their respective oxides. Rare-earth sulfates, on the other hand, are stable during roasting and dissolve during water leaching, leaving the iron oxides in the residue. The effect of the roasting temperature, roasting time and amount of acid on leaching of the different elements was studied. Decreasing the roasting temperature increased the dissolution of the REEs, but also that of iron and aluminum. Increasing the amount of acid led to higher REEs extraction. Acid to bauxite residue mass ratio beyond 0.75 at 650 °C increases the iron and aluminum dissolution due to increase in the iron(III) and aluminum sulfate amounts. The extraction of REEs slightly increased (<5%) with roasting time up to 2 h at 675 °C, but a further increase of the roasting time has a negative effect on the REEs extraction as the low amount of iron sulfate in the roasted mass increases the pH of the leach solution. About 60% of scandium and more than 90% of the other REEs can be dissolved at optimum conditions, while only a very small amount of iron (<1% of total iron) is solubilized. The residue after leaching was rich in Fe 2 O 3 , Al 2 O 3 , SiO 2 and CaSO 4· 0.5H 2 O. [ABSTRACT FROM AUTHOR]

• Arsenic is selectively removed from crude Sb 2 O 3 by leaching with HNO 3. • Conversion of Sb 2 O 3 to Sb(III)OHN releases As 2 O 3 from (As, Sb) 2 O 3 and contributes to the As removal. • The leaching residue can decompose to Sb 4 O 6 (g) and condense to Sb 2 O 3 (s). • A novel process for As removal and Sb recovery is proposed. The presence of arsenic (As) in crude antimony trioxide (Sb 2 O 3) impedes the Sb recovery and results in an environmental issue. Selective As removal from crude antimony trioxide would therefore enable using this residue in commercial Sb 2 O 3 production. This work uses a nitric acid leaching method to remove As from crude antimony trioxide. Arsenic oxide and antimony oxide are respectively converted to soluble H 3 AsO 4 and insoluble antimony(III) oxide hydroxide nitrate Sb(III)OHN during HNO 3 leaching. The As content in the crude antimony trioxide decreases from 3.0 wt% to 0.4 wt%. The As removal ratio reaches 86%, while the Sb leaching ratio is limited to 0.3%, confirming the high selectivity of HNO 3 leaching. The leaching residue is composed of cervantite (Sb 2 O 4) and Sb(III)OHN. By calcining the leaching residue in the air at 200 °C, it can be converted to a mixture of cervantite and valentinite (Sb 2 O 3). Finally, cervantite is decomposed above 1070 °C in N 2 into Sb 4 O 6 that can be condensed as Sb 2 O 3. A novel flowsheet is proposed for both As removal and Sb recovery from the crude antimony trioxide. [ABSTRACT FROM AUTHOR]

NdFeB magnets currently dominate the magnet market. Supply risks of certain rare earth elements (REE), e.g. Nd and Dy, demand efficient recycling options that are applicable to different types and compositions with minimum use of chemicals and energy and with minimum waste generation. In this study, a combined pyro- and hydrometallurgical method is presented that is adjustable to all NdFeB magnets regardless of their composition. After completely transforming powdered samples into a metal nitrate mixture at room temperature for 1 h, a low-temperature (e.g. 200 °C) calcination and water leaching treatment resulted in 95–100% extraction efficiencies for Nd, Dy, Pr and Gd. The major impurity Fe (63.9 wt.%) almost completely remained in the resultant residue (e.g. < 1% extraction) forming a mixture of hematite and goethite as a useful by-product for related iron industries. Al and Co, representing minor impurities with individually < 2 wt.%, also mostly remained in the residue (i.e. < 40% extraction), thereby enabling the production of a liquid with very high REE purity. Such a solution then can be directly treated with subsequent shortened downstream processes without pre-treatments required for impurity removal. Due to decomposition reactions of impurities (including Fe) during the low-temperature calcination stage (e.g. 200 °C), the majority of consumed acid (i.e. the only chemical used) is recyclable resulting in a potentially environmentally-friendly and cost-effective flowsheet. [ABSTRACT FROM AUTHOR]

With an aim of producing high value cementitious binder, stainless steel refining slag containing a high amount of CaO in γ-dicalcium silicate form was activated with NaOH and Na-silicate as well as KOH and K-silicate solutions, followed by steam curing at 80 °C. Higher levels of alkali–silicate in the activating solution resulted in higher cumulative heat suggesting accelerated reaction kinetics. With respect to compressive strength, higher levels of alkali silicate resulted in higher strength and the mortars with Na activator were found to have higher early strength than the ones with K activator. The long term strength was found to be similar, regardless of the alkali metal. Thermogravimetric, QXRD and FTIR analyses showed an increase in the amount of reaction products (C S H type) over time, further confirming the reactivity of the crystalline slag. Batch leaching results showed lower leaching of heavy metals and metalloids with K activator compared to the Na activator. These results demonstrate that the alkali type and the ratio of hydroxide to silicates have a significant impact on the hydration and mechanical strength development of the stainless steel slag. The above findings can aid in the recycling and valorization of these type of slags which otherwise end up landfilled. [ABSTRACT FROM AUTHOR]

The binding properties of continuous casting stainless steel slag were studied when activated by a combination of Na silicate and 5 M NaOH as well as K silicate and 5 M KOH at steam curing temperatures of 60, 70, 80, 90, 100 and 110 °C. The slag mortars developed moderate compressive strength (∼5 MPa) at 60 °C which initially increased moderately and then steeply (>30 MPa) at higher curing temperatures. The porosity of K activated mortars was lower than for Na activated mortars. Thermal and ATR-FTIR analysis showed the formation of C–S–H type of reaction products for both activators at all temperatures, while the formation of brucite was promoted at higher curing temperatures. A much denser hydrated matrix with a better bonding with aggregates was formed at higher curing temperatures. Heavy metal leaching from the slag was dependent on its type and curing temperature of samples. [ABSTRACT FROM AUTHOR]

Highlights: [•] AOD slag can be valorized as building material by mineral carbonation. [•] Depending on the process strengths of more than 30MPa can be achieved. [•] XRD analysis show formation of amorphous carbonation products. [•] SEM analysis shows different morphology of calcite at different pressures. [•] Leaching of several heavy metals is reduced on carbonation. [ABSTRACT FROM AUTHOR]

Abstract: Treatment and disposal of Basic Oxygen Furnace (BOF) slag, a residue of the steel production process characterized by high basicity and propensity for heavy metal leaching, is a costly burden on metallurgical plants; a sustainable valorization route is desired. The stabilization of BOF slag utilizing hot-stage carbonation treatment was investigated; this approach envisions carbonation during the hot-to-cold pathway followed by the material after the molten slag is poured and solidified. Three experimental methodologies were employed: (i) in situ thermogravimetric analyzer (TGA) carbonation was used to assess carbonation reaction kinetics and thermodynamic equilibrium at high temperatures; (ii) pressurized basket reactor carbonation was used to assess the effects of pressurization, steam addition and slag particle size; and (iii) atmospheric furnace carbonation was used to assess the effect of carbonation on the mineralogy, basicity and heavy metal leaching properties of the slag. Free lime was found to be the primary mineral participating in direct carbonation of BOF slag. Initial carbonation kinetics were comparable at temperatures ranging from 500 to 800°C, but higher temperatures aided in solid state diffusion of CO2 into the unreacted particle core, thus increasing overall CO2 uptake. The optimum carbonation temperature of both BOF slag and pure lime lies just below the transition temperature between carbonation stability and carbonate decomposition: 830–850°C and 750–770°C at 1atm and 0.2atm CO2 partial pressures, respectively. Pressurization and steam addition contribute marginally to CO2 uptake. CO2 uptake progressively decreases with increasing particle size, but basicity reduction is similar independent of particle size. The solubility of some heavy metals reduced after carbonation (barium, cobalt and nickel), but vanadium and chromium leaching increased. [Copyright &y& Elsevier]