Characterisation of nickel in platinum refining process dust

Abstract

Objectives: The objective of this mini-dissertation was to determine the physical (particle size) and chemical (species) form of the nickel present in airborne particulate matter at a precious metals smelter. Methods: After a site walk through at the smelter facility and technical suggestions from the on-site occupational hygiene department, the following areas were selected for inclusion in this study: Paste floor of the Furnace building, Paste floor of the Slag cleaning furnace, Slow-cooling and Crusher area at the Converting process. Sampling was conducted by means of the Institute of Occupational Medicine (IOM) sampler with the aid of GilAir-Plus® (Sensidyne, United States of America) sampling pumps and flexible connecting tubing. The sampling pumps were calibrated at a flow rate of 2.0 l/min by a calibrated Defender 510 DryCal calibrator (BIOS, United States of America). The IOM reusable filter cassette was installed with 25 mm mixed cellulose ester (MCE) filters, with a pore size of 0.8 μm. Where applicable, MultiDust Foam inserts® (SKC, United Stated of America) were used to determine the respirable fraction. Three IOM samplers were used to determine the inhalable dust fraction (without foam insert), respirable dust fraction (with foam insert) and the particle size analysis (without foam insert). Area sampling was conducted by mounting the three IOM’s on a sampling station 1.5 m from the floor surface. The height of 1.5 m represents the height of an average person’s mouth and nose from ground level. Pre- and post-weight of the three IOM’s filters determined the mass concentration of the inhalable and respirable size fractions. The particle size was determined through analysing with a dynamic light scattering instrument to indicate the presence of submicron particles. A direct reading instrument namely the AeroTrakTM Airborne Particle Counter (APC) (TSI, United States of America), with a particle size range of 0.3 – 10 μm, was also used to determine the concentration of airborne submicron particles (0.3 – 0.5 μm). The background levels (ambient) of submicron particles were measured in the vicinity of the office area of the smelter. This indicated the concentration of submicron particles present in a non-production/process environment. The background levels were subtracted from the levels measured in the smelter production areas. To determine the nickel content of airborne particulate matter, sample filters were analysed through X-ray powder diffraction to determine individual nickel species. This process did not destroy the samples which made it possible to analyse for insoluble and soluble nickel content. For the interpretation of results, basic descriptive statistics were applied e.g. the mean, minimum and maximum. T-tests were used to compare the dust concentration between the inhalable and respirable fractions. A one-way ANOVA was performed to establish whether a significant difference between nickel concentrations at the four sampling areas existed. To determine if the particle size differed between the four areas, a one-way ANOVA was performed. A p-value of ≤ 0.05 was considered to be statistically significant. Results: The highest mean dust concentration exposure, for both the inhalable and respirable particle fractions, was recorded at the Slag cleaning furnace. The mean dust concentrations for the inhalable and respirable particle size fractions were 63.09 ± 49.64 mg/m3 and 5.73 ± 3.02 mg/m3 respectively. The inhalable and respirable dust concentrations at the Furnace building (p = 0.001), Slag cleaning furnace (p ≤ 0.001) and Slow cooling area (p = 0.03) statistically differed from each other. The mean aerodynamic diameter of particles at the Furnace building was 0.25 ± 0.18 μm, Slag cleaning furnace was 0.31 ± 0.34 μm, Slow cooling area was 0.47 ± 1.0 μm and Crusher area was 3.27± 1.48 μm. Statistical differences were observed between the particle sizes at the Crusher area and Furnace building (p ≤ 0.001), Crusher area and Slag cleaning furnace (p ≤ 0.001), and Crusher area and Slow cooling area (p ≤ 0.001). The Slow cooling area was the only production area where the submicron concentration (0.3 μm) exceeded that of the ambient concentration. The Furnace building and Slag cleaning furnace had the highest concentration of particulates with a diameter of 0.5 μm. Statistical differences were observed between the Crusher area and Furnace building for particulates with a diameter of 0.3 μm (p = 0.015) and 0.5 μm (p ≤ 0.001). A statistical difference was detected for particulates with a diameter of 0.3 μm between the Crusher area and Slow cooling area (p ≤ 0.001). The concentration of particulates with a diameter of 0.5 μm showed a significant difference between the Crusher area and Slag cleaning furnace (p ≤ 0.001). The mean insoluble and soluble nickel concentrations, for both the inhalable and respirable particle size fractions, were the highest at the Crusher area. The insoluble nickel present in the inhalable fraction was 0.66 ± 1.07 mg/m3 and for the respirable fraction 0.09 ± 0.12 mg/m3. The soluble nickel concentration in the inhalable fraction was 0.32± 0.41 mg/m3 and 0.05 ± 0.06 mg/m3 in the respirable fraction. With regards to the mean insoluble nickel concentration for the inhalable particle size fraction, a statistical significant difference was found between Furnace building and Crusher area (p = 0.04). No statistical differences were found for the inhalable soluble nickel concentration. The XRD analysis successfully identified the presence of nickel subsulphide (Heazlewoodite – Ni3S2 and Polydymite – Ni3S4) and iron nickel sulphide (Fe9Ni9S16). The inhalable samples from the Furnace building (50%), Slag cleaning furnace (50%) and Slow cooling area (75%) were all successfully identified with Ni3S2. All the samples from the Crusher area were identified with Ni3S2 (100%). Conclusion: The presence of submicron particles (particles with a diameter of less than 1 μm) was found at the Furnace building, Slag cleaning furnace and Slow cooling area. A high insoluble nickel concentration was found at the Slag cleaning furnace, Slow cooling area and Crusher area. In addition, high concentrations of soluble nickel were found at the Crusher area and Slag cleaning furnace. It is recommended that nickel exposure is controlled by reducing dust levels in the areas through the installation of local extraction ventilation or a mechanical air mover. The current sampling strategy at the smelter is mainly focussing on the measurement of personal exposure to respirable dust. A monitoring programme is also in place for insoluble nickel as a total inhalable dust fraction, whereby only five samples are taken annually, as per the requirement of the South African Department of Minerals and Resources (DMR) which classifies it as Category C. This sampling strategy is based on yearly baseline measurements. The samples from this study were all of a static nature and, therefore, cannot be seen as a true representation of personal exposure within the production areas. However, the results are a good representation of the chemical composition of the airborne particulate matter and justify the implementation of a monitoring programme for insoluble and soluble. It is recommended that personal exposure to inhalable insoluble nickel is monitored for at the Slag cleaning furnace, Slow cooling area and Crusher area. In addition, soluble nickel exposure should be measured at the Slag cleaning furnace and Crusher area. Only after personal sampling, assumptions could be made regarding the compliance to legislative exposure limits. The further nickel speciation showed the presence of nickel subsulphide (Ni3S2) in airborne particulate matter at the Furnace building, Slag cleaning furnace, Slow cooling area and most prominently the Crusher area. The findings from this study indicate that employees at the various smelting processes are exposed to insoluble and soluble nickel particulates which may have an aerodynamic diameter of less than 1 μm. For future studies, the investigation of personal exposure to nickel species at the various production areas of the smelter may be considered, specifically nickel subsulphide. Further, an investigation with regards to the presence of nickel species at the refinery can also be considered. Such investigations may assist with the future development of preventative exposure measures at the smelter and refinery.