•Conversion of Ca, Mg and Al elements are complete under mild roasting conditions.•(NH4)2SO4, which is used as extractant, is recyclable.•Capacity of CO2 mineral sequestration is huge.•Blast furnace slag is utilized comprehensively, and all products are valuable.Large quantities of CO2 and blast furnace slag are discharged in the iron and steel industry. Mineral carbonation of blast furnace slag can offer substantial CO2 emission reduction and comprehensive utilisation of the solid waste. In this study, a recyclable extractant, (NH4)2SO4, was used to extract calcium and magnesium from blast furnace slag (main phases of gehlenite and akermanite) by using low-temperature roasting to fix CO2 through aqueous carbonation. The process parameters and efficiency of the roasting extraction, mineralisation, and Al recovery were investigated in detail. The results showed that the extractions of Ca, Mg, and Al can reach almost 100% at an (NH4)2SO4-to-slag mass ratio of 3:1 and at 370 °C in 1 h. Adjusting the pH value of the leaching solution of the roasted slag to 5.5 with the NH3 released during the roasting resulted in 99% Al precipitation, while co-precipitation of Mg was lower than 2%. The Mg-rich leachate after the depletion of Al and the leaching residue (main phases of CaSO4 and SiO2) were carbonated using (NH4)2CO3 and NH4HCO3 solutions, respectively, under mild conditions. Approximately 99% of Ca and 89% of Mg in the blast furnace slag were converted into CaCO3 and (NH4)2Mg(CO3)2•4H2O, respectively. The latter can be selectively decomposed to magnesium carbonate at 100–200 °C to recover the NH3 for reuse. In the present route, the total CO2 sequestration capacity per tonne of blast furnace slag reached up to 316 kg, and 313 kg of Al-rich precipitate, 1000 kg of carbonated product containing CaCO3 and SiO2, and 304 kg of carbonated product containing calcium carbonate and magnesium carbonate were recovered simultaneously. These products can be used, respectively, as raw materials for the production of electrolytic aluminium, cement, and light magnesium carbonate to replace natural resources.Download high-res image (117KB)Download full-size imageRecent researches have been done to understand the reaction of (NH4)2SO4 with blast furnace slag and the mineral carbonation process with extracted calcium and magnesium.

With the depletion of high-grade manganese ores, Mn ore tailings are considered valuable secondary resources. In this study, a process combining high-gradient magnetic separation (HGMS) with hydrometallurgical methods is proposed to recycle fine-grained Mn tailings. The Mn tailings were treated by HGMS at 12,500 G to obtain a Mn concentrate of 30% Mn with the recovery efficiency of 64%. The Mn concentrate could be used in the ferromanganese industry. To recover Mn further, the nonmagnetic fraction was leached by SO2 in an H2SO4 solution. Hydrogen peroxide was added to the leachate to oxidize Fe2+ to Fe3+, and the solution pH was adjusted to 5.0–5.5 with ammonia to remove Al, Fe, and Si impurities. The purified solution was reacted with NH4HCO3, and a saleable product of MnCO3 with 97.9% purity was obtained. The combined process can be applied to Mn recovery from finely dispersed weakly magnetic Mn ores or tailings.

•A method of almost complete extraction of HNO3 and HAC from etching waste acid.•The MIBK + N235 mixtures extraction of HNO3 and HAC were very effective.•Extraction mechanism of the etching waste acid was established.The selective extraction of nitric and acetic acids from a simulated etching waste acid was investigated using a mixture of the extractants Alamine 336 (N235) and methyl isobutyl ketone (MIBK). The effects of dilution of the etching waste acid, N235 and MIBK concentrations, organic/aqueous (O/A) phase ratio, extraction temperature, and contact time were systematically studied. The results demonstrate that with an extractant mixture of the composition 12.5 vol% N235 and 87.5 vol% MIBK, 75% acetic acid and 85% nitric acid were extracted. However, only 3% phosphoric acid was co-extracted in a single-stage contact under the following conditions: phosphoric acid concentration of 385 g/L in the diluted acid, O/A ratio of 1.5:1, reaction temperature of 25–35 °C and reaction time of 10 min. The McCabe-Thiele analysis predicts that over 98% acetic acid and 99% nitric acid can be extracted, with co-extraction of only 5% phosphoric acid through a three-stage counter-current operation at an O/A ratio of 3:2. Further, ∼99% HNO3, 99% HAC, and 99% H3PO4 loaded in the organic phase were stripped using 0.25 mol/L NH3·H2O as the stripping agent at an O/A ratio of 1:2. Therefore, it is feasible to achieve almost complete separation of nitric acid and acetic acid from the waste acid to obtain purified phosphoric acid and a binary compound fertilizer containing the nutritional elements nitrogen and phosphorus. The compositions of the extraction complexes of different acids were determined to be H3PO4·2N235, HAC·1.5N235 and HNO3·N235 for the N235 extraction and HAC·2MIBK for the MIBK extraction of HAC. The results can explain the extraction tendencies of the three acids obtained above. Analyses of the FT-IR spectra of the loaded organic phases indicate that the N235 extraction of HNO3 occurred by the ion pair association mechanism, while the acetic acid extraction using N235 or MIBK was realized through hydrogen bonding.

•A novel method of almost complete separation of Fe(II) and Ti(IV) is reported.•The effects of various process factors and extraction mechanisms were investigated.•The composition of the extracted complex was HFeCl4·5(2-octonol).Separation of ferrous iron from titanium(IV) in a simulated ilmenite hydrochloric acid leachate by simultaneous oxidation and 2-octanol extraction was investigated. The effects of extraction time, organic/aqueous (O/A) phase ratio, temperature, and hydrochloric acid concentration were studied. Using pure oxygen as the oxidant and 100% (v/v) 2-octanol as extractant, the oxidation of ferrous iron and subsequent ferric iron extraction reached 99.3% and 86.1%, respectively. In addition, with an O/A ratio of 1:3, a hydrochloric acid concentration of 9.35 mol/L at 35 °C for 30 min, titanium co-extraction was not observed. Over two stages of cross-current operation, the total iron extraction reached 99.6%, again with no titanium co-extraction. Furthermore, ∼100% iron stripping was obtained through a single contact, using distilled water as the stripping liquor with an O/A ratio of 1:1. Thus, an almost complete separation of iron(II) from titanium(IV) in the simulated leachate was achieved. The composition of the extracted complex was determined to be HFeCl4·5(2-octanol). The kinetics of both the iron(III) extraction and stripping were rapid, while the iron(II) oxidation was slow despite a new reactor with significantly enhanced gas–liquid contact being employed. Further enhancement of the oxidation through catalysis is expected, and so an improved method of iron(II) extraction from titanium for use in a hydrochloric acid TiO2 pigment production process is proposed.

The relationship between hydrolysis conditions and hydrous titania polymorphs obtained in a titanyl sulfate and sulfuric acid solution was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The results revealed that the feeding rate of the titanyl sulfate stock solution, the concentration of sulfuric acid, and the seed dosage of rutile crystal could significantly affect the hydrolysis rate, thus influencing the titania crystal phase. Hydrous TiO2 in the form of rutile, anatase, or the mixture of both could be obtained in solutions of low titanium concentrations and 2.5wt% to 15wt% sulfuric acid at 100°C. When the hydrolysis rate of titanium expressed by TiO2 was more than or equal to 0.04 g/(L·min), the hydrolysate was almost phase-pure anatase, while the main phase state was rutile when the hydrolysis rate was less than or equal to 0.01 g/(L·min). With the hydrolysis rate between 0.02 and 0.03 g/(L·min), the hydrolysate contained almost equal magnitude of rutile and anatase. It seems that although rutile phase is thermodynamically stable in very acidic solutions, anatase is a kinetically stable phase.

•A novel method for preparation of synthetic rutile from sulfated ilmenite is proposed.•The new process can be applied in ilmenite which contains high MgO and CaO contents.•The new process is environmental friendly, energy-saving and facile.•The present process can be integrated with the sulfate TiO2 process.This paper describes a novel process for preparation of synthetic rutile using an intermediate, sulfated ilmenite from the sulfate TiO2 process as the feedstock. The synthetic rutile can be obtained by selective thermal decomposition of the sulfated ilmenite, followed by targeted leaching for removal of various impurities. The results of the decomposition unit showed that almost all the TiOSO4 in the sulfated ilmenite decomposed to TiO2, while the iron component mainly existed in the form of sulfates in the optimal thermal decomposition conditions, i.e., a roasting temperature of 540 °C and a roasting time of 120 min under air flow or in stagnant air or a roasting temperature of 510 °C and a roasting time of 120 min under nitrogen flow. The thermal decomposition can be divided into three stages. The sulfates of titanium and iron in the sulfated ilmenite were first decomposed to TiO2 and water-insoluble FeOHSO4, respectively, at a temperature less than 500 °C. The FeOHSO4 was further converted into water-soluble Fe2O(SO4)2 at 500–560 °C. Finally, the Fe2O(SO4)2 was decomposed to Fe2(SO4)3 and Fe2O3 at a temperature above 560 °C. The water-soluble metal sulfates, the water-insoluble FeOHSO4/Fe2O3 and the SiO2 in the TiO2-containing slag can be removed through leaching by using water, dilute sulfuric acid and sodium hydroxide, respectively. The results showed that 70–85% of the iron, as well as a majority of the magnesium, calcium, etc. impurities, could be leached by water, and up to 92% of the total iron could be removed after the subsequent acid leaching in a 15 wt% H2SO4 solution, while the silicon removal reached 65% in a 5 wt% NaOH solution. A synthetic rutile with a TiO2 content over 90 wt% and total MgO + CaO less than 1 wt% was obtained under the optimal conditions listed above. The present process can be integrated with the sulfate TiO2 process, producing titania pigment and synthetic rutile simultaneously. The advantages of the TiO2 beneficiation process include the moderate reaction conditions, the capability of recycling the H2SO4 completely and the recovery of a large part of the iron, as well as comprehensive utilization of the waste sulfuric acid discharged from the sulfate TiO2 production process.