Treatment & value recovery from mining wastewater rich in sodium chloride and sodium sulphate
Date
2024-03
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The platinum metal recovery (PMR) process of platinum group metals (PGMs) produces effluents with high concentration of NaCl and Na2SO4, and lower yet variable concentrations of selenium and tellurium. The goal of this study was to develop a process to firstly remove impurities (selenium and tellurium oxyanions) and to subsequently recover valuable components from these effluent streams. An in-depth literature study was performed on selenium and tellurium removal technologies and on sustainable value recovery methodologies from the remaining concentrated NaCl and Na2SO4 solution. Acidic precipitation of selenium and tellurium using SO2 was selected as a treatment strategy due to high overall recoveries, fast kinetics, and no impurity addition to the process. Conversion of aqueous NaCl to HCl and Na2SO4 using MgSO4 catalyst was also chosen as value recovery strategy due to its affordability and insensitivity to impurities in feed. Two stages of experimentation were carried out. In the first stage, precipitation was carried out by blowing excess SO2 into 1 L of industrial PMR effluent within a stirred glass reactor for a duration of 3 hours. Temperature was varied between 75 °C, 85 °C and 95 °C, and concentration of acid hydrogen ions ([H+ ]) was varied between 1.65 mol/L, 2.25 mol/L and 2.85 mol/L. The type of acid was varied between HCl and H2SO4. In the second stage, 110 mL of solution collected from first stage was added to 2 g of MgSO4 catalyst and stoichiometric amounts of H2SO4 in a still-pot distillation set up and run for 6 hours at boiling temperatures to distil HCl solution. In the first stage, the use of HClto acidify the PMR effluent yielded higher recoveries and faster kinetics when compared to that of H2SO4. When HCl was used, the overall recovery of selenium and tellurium increased with increase in both temperature and [H+ ]. Optimum conditions for impurity removal were found at 95 °C and [H+ ] of 2.85 mol/L where selenium and tellurium recovery were 97% and 58% prior to addition of sulphur dioxide, and 100% and 99% once sulphur dioxide was added for 3 hours. Elemental sulphur and silica also co-precipitated during the experimentation. In the second stage, HCl solutions with concentrations of 5.2 mol/L, 1.7 mol/L, and 4.1 mol/L were distilled from the HCl treated solution, H2SO4 treated solution, and from a 3.5 mol/L NaCl control solution respectively. Na2SO4 and MgSO4 crystals were also produced as waste. The overall proposed process was determined to require HCl to increase [H+ ] of PMR effluent to 2.85 mol/L. Heating the solution to 95 °C would precipitate 97% of selenium and 58% of tellurium. Addition of SO2 in excess for 10 minutes would then precipitate over 95% of Se and Te. The precipitate would then be filtered for further treatment and the filtrate would proceed to a distillation process where chlorides in solutions are recovered as 5 mol/L hydrochloric acid. Lastly, Na2SO4 and MgSO4 would be crystallized as waste. Further research into the recycling of MgSO4, concentrating the HCl product and recovering Na2SO4 is necessary to produce marketable products for reuse/resale.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (MEng)--Stellenbosch University, 2024.
Keywords
Selenium precipitation; Tellurium precipitation; Mining effluent valorization; Hydrochloric acid production; Wastewater treatment