Cross-Sectional Silica Exposure Measurements at Two Zambian Copper Mines of Nkana and Mufulira

ABSTRACT

We measured the quartz content of 20 bulk settled dust and 200 respirable dust samples in a cross-sectional dust exposure assessment that is part of an epidemiological study to ascertain the risk of nonmalignant respiratory diseases among Zambian copper miners. Dust samples were collected from the copper mines of Mopani Copper Mine plc (Mufulira and Nkana Mines). Analytical measurements employed NIOSH Method 0600 for gravimetric analysis of respirable dust and NIOSH Method 7500 for quartz analysis in bulk and respirable dust samples. The measured quartz content of respirable dust showed that 59% and 26% of Mufulira and Nkana Mine samples, respectively, were above the calculated U.S. Occupational Safety and Health Administration permissible exposure limit. The mean intensities of respirable dust exposure at Mufulira and Nkana were 0.992 mg/m3 (range 0–7.674) and 0.868 mg/m3 (range 0–6.944), respectively while the mean intensities of respirable quartz at Mufulira and Nkana were 0.143 mg/m3 (range 0–1.302) and 0.060 mg/m3 (range 0–0.317), respectively. These results indicate weak dust monitoring at these mines which may increase the risk of nonmalignant disease in many miners. Since Zambian mining regulations do not have crystalline silica exposure limits, these results accord with the recommendation that Zambian mining houses and the government establish crystalline silica analysis laboratory capacity and adopt dust mass concentration occupational exposure limits for more protective dust monitoring of workers.

EXPERIMENTAL DESIGN AND METHODS

Reported dust exposure measurements at the two mines were of a cross-sectional design. At each mine, bulk and personal respirable dust samples were collected at scheduled areas. Scheduled areas are production areas of the mine where mining regulations of the Zambian mining regulatory authorities apply. In these areas, the main production activities are ore mining and its metallurgical processing. Sampling was restricted to sections of scheduled areas of each mine that are frequently associated with dust, namely; underground work areas of the mine and sections of the metallurgical works of the mine that deal with ore diminution (crushing and milling) and handle tailings.

In each scheduled area, personal dust sampling design required collecting samples from two primary groups of workers, namely; those doing jobs with low and high potential dust exposure. In underground work areas high potential exposure jobs included those of miners working at the ore face and those engaged in ore transportation while jobs in the ore diminution sections of the metallurgical areas with high exposure potential included those of crusher, milling and conveyor attendants. In both areas jobs with low dust exposure potential were those of workers engaged in the provision of mining and metallurgical support services.

Personal dust samples were collected on 5 μm PVC filters using Gil Air 5 sampling pumps with a nylon cyclone pre-selector for respirable particulates. Samplers were pre- and post-calibrated to ensure that pump flow rates during sampling were within 5% of the recommended pump flowrate of 1.7 litres/minute. Filters collected with samplers that did not meet this criterion due to pump failure were rejected.

Collected dust samples were sent to an accredited analytical laboratory (Bureau Veritas North America, Inc.) in the U.S. for analysis. The instrumental technique of xray diffraction spectrometry (XRD) employing NIOSH Method 7500 was used for analyses of crystalline silica in bulk and respirable dust while total respirable dust in personal dust samples was gravimetrically determined using NIOSH Method 0600.

RESULTS

All bulk and respirable samples were scanned for all three forms of crystalline silica that may be found in workplaces, namely; quartz, cristobalite and tridymite. The only form of crystalline silica detected in the samples was quartz. The mean bulk dust silica content of Nkana and Mufulira Mines were 20 and 57%, respectively.

At Nkana and Mufulira Mines, 102 and 101 personal dust samples were collected during eight hour shifts, respectively. The mean sampling times for Nkana and Mufulira Mines were 307 and 312 minutes, respectively. These respirable dust samples were analyzed for quartz content (%) and exposure concentrations of respirable dust and quartz (mg/m3).

Table 1 shows the number of samples in which each analyte was not detected (ND), was determined to lie between the limit of detection and the limit of quantification (LOD-LOQ) and was determined to be greater than or equal to the limit of quantification (≥LOQ). The LOD and LOQ for quartz were 10 μg/sample and 33 μg/sample, respectively while the LOD and LOQ for respirable dust were 30 μg/sample and 87 μg/sample, respectively.

Table 2 shows exposure descriptive parameters for each analyte at these mines. Results from the two mines are comparable with respect to gravimetric measures. Mufulira Mine had 77 (76 %) samples above the LOQ and a mean respirable dust concentration of 0.992 mg/m3 (range 0 – 7.674) while Nkana Mine had 69 (68 %) samples above the LOQ and a mean respirable dust concentration of 0.868 mg/m3 (range 0 – 6.944). Although Table 2 shows that the two mines have comparable mean respirable quartz content (Nkana, 13.6% and Mufulira, 17.5%), Table 1 shows that quartz was more frequently above the LOQ in personal dust samples collected at Mufulira Mine.

The understanding of what constitutes a safe exposure to a workplace contaminant has led to development of occupational exposure limits (OELs) by professional bodies and many national regulatory authorities. Table 3 shows OELs of respirable dust and crystalline silica for various U.S. institutions (namely, Occupational Safety and Health Administration – OSHA, Mine Safety and Health Administration – MSHA, National Institute for Occupational Safety and Health – NIOSH, American Conference of Governmental Industrial Hygienists – ACGIH) and the Zambian mining industry regulatory authority (Mine Safety Department - MSD) that are of interest to this study. The Zambian mass concentration OEL in the table is derived from the konimeter dust count OEL by applying a conversion factor given by van der Linde.

Figure 1 shows the percent of samples at each mine that exceed various U.S. OELs for respirable crystalline silica. The first pair of bars shows samples above the OSHA PEL for respirable dust containing quartz. The quartz content of each sample was used in the OSHA PEL equation to determine the OSHA PEL for that sample. The second pair of bars shows the percent of samples above the NIOSH Recommended Exposure Limit (REL) of 0.050 mg/m3 for respirable crystalline silica. The third pair of bars shows samples above the ACGIH Threshold Limit Value (TLV) of 0.025 mg/m3 for respirable crystalline silica.

Figure 1 shows that 26% and 59% of the samples at Nkana and Mufulira Mines, respectively, were above the OSHA (which in this case is the same as the MSHA) exposure limit for respirable dust containing quartz.
