Assessment of Concentration of Airborne Heavy Metals in the Workplace and of Operational Efficiency of a Scrap Iron and Steel Smelter
Energy and material audit, measurement of concentration of suspended particulate matter (SPM) and elemental analysis of the SPM were carried out in a scrap iron and steel smelter in Lagos, Nigeria. This was with the view to assessing the operational efficiency and the occupational exposure of workers in various production processes of the industry to toxic heavy metals. Data were obtained through questionnaire served on operation engineers on mass and energy usage, and from purposively selected workers on exposure to toxic heavy metals in the workplace. Estimate of the input and output was computed using equation (1): Σ Mi =Σ Ms+ Σ Mf and Σ Mo =Σ Mp + ΣMsl +Σ Mw………… (1) (where Σ Mi,Σ Ms,Σ Mf,Σ Mo , Σ Mp,Σ Msl and Σ Mw were total weight input, total scrap weight used, total output weight, total weight of the product, total slag weight and total weight of the waste rod respectively). Size segregated suspended particulate matter–PMIO (respirable fraction), PM2.5 (inhalable fraction) and TSP (Total suspended particulate matter) were collected at the two electric arc furnace sections (EAF-1, EAF-2), the continuous casting section (CC), the rolling mill (RM), a mechanical workshop,(MW), a quality control laboratory (QL) and air-conditioned administrative office (AQ) using Gent PM10 and Negretti 1000 samplers once a month from April, 2003 to March, 2004. A combination of Total Reflection X-Ray Fluorescence (TXRF), Polarized Energy-Dispersive X-ray Fluorescence (EDXRF), Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) and conventional EDXRF techniques were used for the elemental analysis of the collected samples. The result showed that the average energy input per ton of scrap iron processed in EAF-1 was 15500 + 150 MJ, while the average energy consumed in melting a ton of the scrap was 8060 ± 57 MJ with an average energy efficiency of 52 %. The average efficiencies of EAF-2 and CC were similarly estimated to be 59 % and 63 %. On the average 527.9+ 28 MJ of energy was used in rolling one tonne of rod at the RM section. Average mass efficiencies of 91 % (EAF-1), 89 % (EAF-2), 91 % (CC) and 97 % (RM) were obtained using equation 1. The mass concentration values for TSP were 2306 (EAF-1), 3335 (EAF-2), 2534 (CC) and 646 (RM) μg/m3; for PM2.5 were 190 (EAF-1), 153 (EAF-2), 148 (CC) and 93 (RM)μg/m3 for PM2.5 and for PM10 were 2051(EAF-1), 4570 (EAF-2), 3319 (CC) and 687 (RM) μg/m3. The highest concentrations of toxic heavy metals measured were Cr 6.4 μg/m3 at EAF-2, Ni 1.8 μg/m3 at EAF-2, As 0.4 μg/m3 at EAF-1 and Cd 2.3 μg/m3 at EAF-2. The average concentrations of Zn measured in the PM2.5 samples were 38.0 ± 2.8 (EAF-1), 21.0 ± 1.6 (EAF-2), 40.0 ± 3.1 (CC) and 3.7+ 0.09 (RM) μg/m3 while the average concentration of Pb were 5.2 ± 0.13 (EAF-1), 5.9 ± 0.17 (EAF-2), 9.7 ± 0.27 (CC) and 1.4 ± 0.03 (RM) ,ug/m3. Also, the average concentrations for Zn measured in the PM10 samples were 312 ± 4.2 (EAF-1), 2348 ± 28 (EAF-2), 482.0 ± 5.1 (CC) and 39 ± 1.9 (RM) μg/m3 while the average concentration of Pb were 59.0 ± 2.9 (EAF-1), 160 ± 5 (EAF-2), 104 ± 3.8 (CC) and 11.0 ± 0.9 (RM) μg/m3. Exposure concentrations per hour measured at EAF-1 and EAF-2 sections for Zn were 87 μg/m3 and 6.4 mg/m3 while for Pb were 15.6 and 178 μg/m3 The study concluded that TSP was above the WHO guidelines of 40 μg/m3 and the Federal Ministry of Environment (FMEnv) Nigeria for industrial environment at all location. Also, average elemental concentrations for Pb and Zn were above the United States Environmental Protection Agency limits of 1.5 and 50 μg/m3 .The study also concluded that workers' exposure to heavy metals was higher than Occupational Safety and Health Administration (OSHA) limits.