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Sr-bearing sludge is a hazardous waste that is commonly generated by nuclear power plants and mineral refining operations. In this work, Sr-bearing sludge was simulated and then cleanly recycled into high-purity strontianite with hematite nanoparticles as a byproduct via a novel hematite precipitation route. The sludge contained 26.1% Fe, 3.5% Sr, and Si impurities. After dissolution in 1.2 M nitric acid, the sludge was treated hydrothermally with the addition of glycol to precipitate Fe effectively. Without the addition of glycol, only 52% Fe was hydrothermally precipitated in the form of hematite aggregates. With the addition of glycol at the optimal Mglycol/Mnitrate molar ratio of 0.4, nearly 100% Fe was removed in the form of hematite nanoparticles with an average diameter of 50 nm, whereas over 98% of Sr was retained in the leachate. The generated hematite was highly purified with an Fe2O3 content of 95.23%. Sr was present at a high concentration of 3.9 g/L in the treated leachate and further precipitated in the form of strontianite with a purity of 96.8% through Na2CO3 addition. Tertiary butanol (TeB) exhibited a similar Fe removal rate as glycol even though its optimal MTeB/Mnitrate molar ratio was 0.1, which was approximately one-fourth the optimal Mglycol/Mnitrate molar ratio. Fe removal involved spontaneous Fe3+ hydrolysis under hydrothermal conditions and was promoted by increasing the pH of the redox reaction between nitrate and glycol and/or TeB. The method reported here successfully enabled the resource recycling of Sr-bearing sludge to generate high-purity strontianite and hematite products without producing any secondary waste.

Polyaluminium chloride (PAC) is a common flocculant that precipitates heavy metals from electroplating wastewater but is involved in the mass production of hazardous sludge. The conventional method of recovering heavy metals from sludge is tedious and heavily consumes extractant. Herein, we developed a facile strategy to effectively recycle heavy metals from electroplating wastewater effluent. The strategy included five steps. Firstly, wastewater with 87.4 mg/L Zn was collected from the effluent of an electroplating workplace and subjected to PAC coagulation to produce a brownish sludge. Secondly, the sludge was leached by 2.4 M nitric acid and 2.2 M hydrochloric acid to produce an acid solution with 18.4 g/L Ca, 2.7 g/L Al and 6.3 g/L Zn, where Ca and Al were the major impurities from the commercial PAC product. Thirdly, more than 97% Ca was precipitated as gypsum prior to the separation of Al and Zn by adding 0.6 M Na2SO4 with Al/Zn loss l -ascorbic acid. However, Al removal efficiency reached only 41.2% without ascorbic acid. Fifthly, 99% Zn was recycled as simonkolleite with a composition of 60.3% ZnO, 0.1% Al and 0.5% Ca after the remaining solution was adjusted to pH 7.3. The developed route effectively enriched Zn from electroplating wastewater and recovered Zn as simonkolleite with gypsum and hydrobasaluminite as the by-products. Accordingly, the route reactivated the PAC coagulation process in the treatment of electroplating wastewater.

Water treatment sludge, which is mechanically dewatered and landfilled as solid waste, is considerably generated in water plants for potable water production. Herein, a novel route to hydrothermally convert this sludge into magnetic particles (MPs) is demonstrated. The sludge comprised amorphous aggregates with a relatively high Al/Si ratio of 3.7 and low Fe content of 8.5 wt%. After hydrothermal treatment, the Al/Si ratio of the MPs was approximated to 1, which was unaffected as the NaOH concentration increased from 2 M to 4 M or 6 M. The amorphous sludge was converted to MPs in the following order: spherical sodalite with a diameter of 3–5 μm, large spherical sodalite with a diameter of 5–10 μm and crystal dendritic cancrinite. Dendritic cancrinite was generated by recrystallisation of amorphous Al/Si oxides with spherical sodalite as the intermediate. With the addition of ascorbic acid, magnetisation of the weakly magnetised sludge increased from 0.11 emu g−1 to 3.6 emu g−1 and 14.8 emu/g by raising the NaOH concentration from 2 M to 4 M and 6 M. The magnetic property was related to the magnetite generated from the reduction of ferrihydrite and hematite in the sludge by the added ascorbic acid. Dendritic cancrinite exhibited an optimal surface site concentration of 0.31 mmol g−1 and desirable adsorption capacity of tetracycline (TC) (482.6 mg g−1), which were twice those of spherical sodalite prepared with 4 M NaOH. This study not only highlights the resource recovery of wastewater treatment sludge for MP preparation but also presents a new and effective adsorbent for treatment of TC-containing wastewater.

Zn2+ is a heavy metal ion, and hazardous sludge from the electroplating and alloy industry is rich in Zn and impure Al and Ca. Such sludge is commonly recycled by dissolution in strong acid and then selective extraction to recycle Zn2+ by a special extraction reagent. In this process, impurity Al3+ is dissolved and then participates in the extraction of Zn2+, so Al3+ should be removed first. Here, a new strategy was reported for the effective removal of Al3+ and recovery of Zn2+ from a simulated Al/Zn-bearing sludge via an improved acid solution–precipitation route. The sludge was simulated by coagulating Zn-bearing waste water (290 mg/L Zn2+) with 600 mg/L polyaluminium chloride. The sludge was dissolved in sulphuric acid and nitric acid to form an acidic solution with Al3+ and Zn2+ concentrations of 3.2 g/L and 5.7 g/L, respectively. The solution was treated directly by hydrothermal method at 270 °C, in which 55% Al3+ was precipitated as boehmite and Al hydroxide. After the addition of 0.2 mL of ethylene glycol, the removal rate of Al3+ dramatically increased to 99.8%, with Zn2+ loss of 1.5%. The residual Zn2+ was 5620 mg/L in the treated solution and further directly precipitated by adjusting the pH of the solution to pH 7.5 with NaOH. The precipitated Zn2+ was in the form of simonkolleite with ZnO content of 63.1%, with Al content of only 0.8%. In the hydrothermal precipitation, the removal rate of Al3+ increased with the temperature and extension of reaction time. Al3+ was hydrolysed and precipitated as aluminium oxonium sulphate hydroxide and then recrystallised in boehmite form. Then, H+ was generated and consumed in the redox reaction of nitrate and ethylene glycol, accelerating the Al3+ precipitation. This method provides a way to efficiently separate Al3+ from a Zn-bearing solution and can be applied in the recycling of Al/Zn-bearing sludge.

Electric-arc furnace (EAF) slag is a waste generated in large quantities by the steelmaking industry. This waste contains high levels of Ca, Fe, Mn and Si. In the present study, an improved hydrometallurgical process was demonstrated for efficient recovery of Mn from EAF slag. Firstly, the EAF slag was dissolved in a mixture of nitric and hydrochloric acids to remove the insoluble Si-containing mixture. The generated leaching acid contained 6 g/L Mn, 26.9 g/L Ca, 15.9 g/L Fe and small amounts of other impurities. Secondly, Ca precipitated into anhydrite when concentrated sulphuric acid was added, and the residual Ca ion concentration was 213 mg/L. Thirdly, after Ca was precipitated, nearly 100% Fe was removed as hematite nanoparticles with 67.7% of Fe during the hydrometallurgical processes by adding methanol at an Mmethanol/Mnitrate ratio of 0.75, the retention rate of Mn was higher than 98.5%. Without adding methanol, only 54.8% of Fe was precipitated as an irregular hematite block. Fourthly, the residual Mn content is 5.8 g/L, which was recovered as hausmannite with Mn content of 60.3% when solution pH is adjusted to 8. Hydrolysis of Fe was not observed at 80 °C but accelerated when temperature was higher than 140 °C. Ethanol exhibited removal efficiency of Fe similar to that of methanol, but the optimal Methanol/Mnitrate molar ratio was 0.3, which was 2/5 that of methanol. By using the improved method, high-purity hausmannite product and hematite by-product were effectively recovered from the EAF slag without generating secondary waste.

Flocculant overdose has been considered an inefficient technique for precipitating heavy metals from wastewater at low levels due to the high yield of hazardous waste sludge that should be treated properly before it can be disposed of safely in landfills. This problem was effectively solved in this study via a novel method that recycles sludge separately into high-purity hematite and heavy metal-bearing products. The wastewater, which contained 10.3 mg/L of Co and 4.8 mg/L of Sr, was coagulated by adding ferric salt to generate Co/Sr-bearing sludge. The sludge was dissolved in HNO3, followed by hydrothermal treatment with the addition of organic matter (e.g. methanol or isopropanol). Without the addition of organic matter, only 56.5% of total Fe was removed as irregular hematite particles, whilst Co/Sr remained unchanged in the acid. Over 99.5% of total Fe was eliminated as hematite nanoparticles with 97.7% Fe2O3 content, but more than 98% Co/Sr remained in the acid when methanol with a molar ratio (Mmethanol/MFe) of 5 was added. Nearly 100% Co was precipitated by adjusting the pH of the acid to 8 to generate Co hydroxide with 83.9% purity. Meanwhile, the residual Sr was further precipitated by adding Na2CO3 to generate SrCO3 with 96.8% purity. Isopropanol achieved total Fe removal similar to that of methanol. The optimal molar ratio (MIsopropanol/MFe) was 1, which corresponded to the removal of 98.7% total Fe. Methanol and isopropanol can react with NO3− in acid to reduce NO2− concentration and improve acid pH, promoting hydrolysis followed by the crystallisation of ferric Fe with hematite as the final product. This paper is the first report on an environment-friendly method for enriching Co/Sr without generating any waste.