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1 iecent developments m Sts ssfr@I f respirable dust @iseisirafii$ feUwfif lis If dirfi By Bharath K Belle, HR Phillips School of Mining Engineering, University of the Witwatersrand, SA Abstract A directive was issued in 1997 by the South African Department of Minerals and Energy (DME) to reduce the respirable dust concentration level to below 5 mg/m3 for the sampling period at the operator's cab posi tion on continuous mining machines used in coal mines. Furthermore, the directive also set a 12 m rule, i.e., the maximum advance in any cutting sequence should be restricted to 12 m, to assist effective dust control and minimise the dust exposure of workers. The focus of this paper is to make a detailed analysis of the efficacy of the 12 m rule from the perspective of dust control and also to address the question of whether the mere restric tion of not mining beyond a distance of 12 m will on its own solve dust prob lems. Analysis of the results indicated that the application of a 12 m rule on its own does not solve the dust problems, but that meticulous application of available state-of-the-art dust control technologies, best workpractices, and the regular maintenance of the installed system combine to ensure that it works at all times. The authors strongly believe that the use of a cutting block (CB) distance is one of the most important parameter affecting work er exposure to coal dust. As an administrative control measure for effective dust exposure control, the mine environment officers, occupational hygien ists and mine management can effectively use this information in order to rotate CM operators during the working shift to ensure that a particular CM operator is not exposed to high levels of coal dust introduction A directive (1997) of the South African Department of Minerals and Energy (DME) was sent to all coalmines in South Africa instructing them to reduce the dust-concentration level to below 5 mg/m3 for the sampling period at the operator's cab position on contin uous mining machines. Further, the directive also specified a 12 m rule to assist effective dust control and mini mize the dust exposure of workers. The 12m rule required the following aspects to be considered (DME Directive, 1997): The directive stated as follows: 1. "No continuous miner (CM) head ing must be developed further than 12m from the last row ofpermanent support or from the point of auxiliary ventilation; and 2. Only ventilation systems that can ensure, at all times, a maximum dust reading of 5 mg/m3, measured at the driv ers position on the continuous miner (CM), must be employed.'' Background The interpretation of the DME direc tive of 1997 suggested that the continu ous miner (CM) cannot advance for a distance further than 12 m before the necessary support is installed and the auxiliary ventilation devices such as jet fan or force fan are advanced to the required distance behind the CM. Therefore, in a production section, if there is a normal lag distance between the face and the most forward point of auxiliary ventilation devices such as jet fan or force fan, that exceeds 12 m, then the operation must be stopped in order to obey the directive. In the event of a CM being operated by a remote control device the limit of the unsupported roof is then the length of the CM. From the perspective of roof hazard prevention, the rule is also favourable to the worker and prevents him from traveling under unsupported roof. The mining industry's initial reaction was that the 12 m rule would negatively impact on the operation of a CM sec tion, because the CM would be pre vented from advancing uninterruptedly for more than 12 m in the same head ing. However, a research study con ducted by Oberholzer (1998) conclud ed that the introduction of the 12m rule, although detrimental to production, would not have a significant enough effect to exclude its use based on pro ductivity constraint considerations. Aspects like the perceived increase of effort to do the necessary support and installation of secondary ventilation are seen as being necessary and part of the work required for extracting coal safely. The focus of this paper is to carry out a detailed analysis from the perspective of dust control after following the 12 m rule and also address the question whether the mere restriction of not min ing beyond a distance of 12 m will on its Journal of the Mine Ventilation Society of South Africa, July/September 2003 85 own solve dust problems. The 12 m rule would undoubtedly enhance roof sup port practice, but its efficacy in keeping dust levels down on its own is less cer tain. Since the introduction of the 12 m rule directive, it has drawn the attention of the industry's environmental officers to the problem of achieving the required dust concentration results. This paper describes the measurement and analy sis of results for the dust concentration levels in 30 m development headings in CM sections that comply with the 12 m cutting sequence rule. Test systems, data collection and analysis For the analysis of the 12 m rule with regard to its role in reducing dust levels, data was gathered from two bord and pillar CM production sections. For the purpose of this analysis individual head ings and splits were divided into various 12 m cutting blocks as shown in Figure 1. The CM followed the cutting sequence in the ascending order of cut ting block numbers (designated 1 to 14) as shown in Figure 1. We can observe that the total length of the split was 21 m, while the heading was 30 m. During the underground trials four different dust control systems were evaluated under the 12 m cutting sequence rule. The four different dust control systems were: - half-curtain sys tem, retrofitted hood system, double scrubber system and integrated hood system. Fiereafter these systems will be referred to as System 1, System 2, System 3 and System 4 respectively. The four different dust control systems were analysed using the results obtained from over 60 underground sampling shifts, for which the real-time dust concentration data was available. collects only the respirable dust (< 10 mm) fraction. In each underground shift, data on cutting distance block (CB) either in the heading or the split (12m or 24 m), cutting direction (head ing [H] or split [S]), and dust control system type were collected. order to analyze the dust concentration level at the operator during each cutting block (CB) use was made of real-time dust concentration data. From the dust concentration data for the sampling period, the real-time dust concentration data was adjusted using a correction factor. Also, from the time-study data of individual cutting block (CB) during each shift, the dust concentration levels for each CB scenario were determined. The results of the dust concentration levels during individual cutting blocks (1 to 14) were classified into cutting block (12 m and 24 m) data and cutting direc tion (FI and S) data for four different dust control systems. Figures 2 to 5 show the relationship between cutting blocks (12 m or 24 m) and cutting direction (H or S) on dust concentration levels for four different dust control systems. Figure 1. 12 m cutting sequence rule in a development underground bord and pillar section Data analysis The dust samples collected under ground were weighed and the proce dure for determining the particulate mass was followed according to DME guidelines (DME, 1995). Using the mass of dust collected on the filters, the dust concentration in mg/m3 was obtained at the operator's location. In Results and discussions Sampling results Tables 1 and 2 provide a summary of the experimental results and show the dust concentration levels for the four dust control systems tested under ground. The results were tabulated according to cutting block (12 m and 24 m) and cutting direction (heading [FI] and split [S]) respectively. The influence of the test parameters (cutting block and cutting direction) on Statistic System 1 12m 24m System 2 12m 24m System 3 12m 24m System 4 12m 24m Overall 12m 24m Mean 4.199 6.371 1.022 1.457 5.464 7.446 8.560 12.632 4.970 6.829 Median 3.420 4.240 0.820 1.110 3.210 6.150 6.320 11.400 3.380 4.680 Variance 7.704 5.566 0.661 0.548 24.608 23.068 50.136 56.299 27.407 40.333 Minimum 0.970 1.240 0.460 0.670 1.390 3.060 2.430 2.070 0.460 0.670 Maximum 12.230 23.450 3.760 3.340 16.510 20.990 31.440 24.240 31.440 24.240 Sample Size 24 27 15 13 12 11 20 13 71 64 Dust monitoring Table 1. Summary statistic of cutting block (CB) dust concentration levels The dust-monitoring instruments were deployed at the CM operator's cabin position. The respirable dust con centration levels were determined by gravimetric respirable dust samplers along with Hund tyndallometers. The sampling set-up contained two gravi metric samplers and a real-time dust monitor. The gravimetric samplers con sisted of an air pump that draws 2.2 LVmin of air through a mini-cyclone, which separates the airborne dust and Statistic System 1 HS System 2 HS System 3 HS System 4 HS Overall HS Mean 5.869 4.849 1.400 1.072 7.204 5.685 11.065 8.587 6.807 4.822 Median 3.460 4.340 1.110 0.880 5.410 4.230 9.320 6.660 4.205 3.740 Variance 38.227 4.474 0.800 0.487 26.074 22.722 71.417 25.779 49.280 16.280 Minimum 0.970 2.520 0.460 0.460 3.060 1.390 2.070 3.890 0.460 0.460 Maximum 23.450 10.300 3.760 3.340 20.990 16.510 31.440 17.970 31.440 17.970 Sample Size 25 26 13 15 11 12 21 12 70 65 Table 2. Summary statistic of cutting direction (H and S) dust concentration levels 86 Journal of the Mine Ventilation Society of South Africa, July/September 2003 20 @12 m 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 20 2m 16 o' i3C<tDT128 4 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 Figure 2. Influence of cutting block (both 12 m and 24 m) and cutting direction (both H and S) on dust concentration ievels for dust control System 1. 12 m ....:........................................... 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 20 H 16 o 312 20 fe|24 m 16 o' 3C0)T12 it 8 4 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 4 0 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 Figure 3. Influence of cutting block (both 12 m and 24 m) and cutting direction (both H and S) on dust concentration levels for dust control System 2. 12 m . 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 20 224 m 16 o u3OC_D1' 28 0 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 ||H ":3 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 20 IS 16 o 3O<1)1' 2 0 1 2 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 Figure 4. Influence of cutting block (both 12 m and 24 m) and cutting direction (both H and S) on dust concentration levels for dust control System 3. the dust concentration levels at the operator's position was examined by drawing scatter diagrams for the four different dust control systems. The combined plot of the influence of cutting block and cutting direction on dust concentration levels from four dif ferent dust control systems is shown in Figure 6. From the plots the following conclusions may be deduced: - For each of the dust control system types, the mean dust concentration levels in the 24 m cutting blocks was higher than the 12 m cutting block Similarly, for the various dust control system types, the mean dust concen tration levels in the headings were greater than the split However, for dust control system 2, the scatter was narrow and the mean dust concentration levels were comparative ly lower than the other system types. As can be expected (see above state ments) the overall plots show there is a difference between the mean dust concentration levels when the CM was cutting either headings or splits and 12 m or 24 m cutting blocks. In order to come to a conclusive rela tionship and rank the significant factors that reduce the dust levels in an under ground section, statistical analysis (twosample t-test and ANOVA) was subse quently carried out Statistical analyses . An analysis of frequency distribution of the dust concentration values for the four different dust control systems resulted in a set of histograms. The sample distributions of the histograms were not normally distributed. The his togram plots of the loge-transform of the dust concentration data lead to the con clusion that the dust levels were logenormally distributed. Since there was an uneven number of concentration values for pair-wise statistical comparison, a two-sample ftest was performed on the set of all the sample data combinations to determine if there was a statistical difference in the loge-transformed (normally distributed) concentration levels. A two-sample ttest of hypotheses was developed to compare the mean concentration level Journal of the Mine Ventilation Society of South Africa, July/September 2003 87  12 m 20 16 o D12 CT CD ' LL 8 24 m , .!mm , 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 0. ,, 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 20 16 >o 12 <OD ' it 8 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 Figure 5. Influence of cutting block (both 12 m and 24 m) and cutting direction (both H and S) on dust concentration levels for dust control System 4. ting direction (heading and split) for individual dust control systems as well as the overall system. Results of the two-sample f-test sta tistical analyses are given in Tables 3 and 4. The p-value represents the prob ability of making a Type 1 error, which is rejecting the null hypothesis when it is true. In this study a cut-off p-value of 0.05 was used (95 % confidence level). From the analysis table, we observe that with various degrees of freedom, the large p-value (> 0.05) suggesting that the measured mean concentration levels are consistent with the null hypothesis, H0: (xA = jxB, that is, the dust concentration levels between the test parameters are not affected at 95 % level of confidence. 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 5 10 15 20 25 30 40 50 Dust concentration, mg/m3 Figure 6. Combined plot of the influence of cutting block (both 12 m and 24 m) and cut ting direction (both H and S) on dust concentration levels for various dust control systems. From Table 3, it can be observed that, for individual dust control systems as well as for overall dust control sys tem, f-statistic C12-C24 (concentration C at 12m-concentration C at 24 m) was negative confirming that the mean dust concentration levels during 24 m cut ting block were generally greater than the mean dust concentration level from the 12 m cutting block. However, the null hypothesis was rejected (small pvalue, < 0.05) for system 2 and this does not hold true for the system 1,3 and 4, where there was no significant difference between the concentration values obtained for the 12 m and 24 m cutting blocks at the 95 % confidence level. measured between the test parameters (uA and jxB). The null and alternative hypothesis for the tested sample pairs were: Ho: Aa. = Rb HT- In the two-sample f-test, hypothesis H0 states that the mean dust concen tration levels between test parameters (jaA and lib) are equal. On the other hand, the alternative hypothesis states that the test parameters in fact have dif ferent mean concentration levels. It is therefore necessary to use hypothesis testing to accept or reject H0. For the analysis, a standard 95% confidence level was chosen. As the hypothesis stated were |iA = jaB and (xA ^ p,B, all analyses were two tailed to account for both conditions fiA<nB and uA>|iB. Hypothesis tests were carried out on each of the test parameters in terms of cutting block (12m and 24 m) and cut A two-sample f-test was performed on all the data to determine if there was a statistical difference in the concentra- Mean Std Deviation Sample Size P - Value t - statistic Hypothesis System 1 12m 24m 1.241 1.570 0.646 0.743 24 27 0.100 -1.680 Accept H0 System 2 12m 24m -0.149 0.275 0.546 0.455 15 13 0.036 -2.220 Reject H0 System 3 12m 24m 1.364 1.882 0.830 0.484 12 11 0.085 -1.810 Accept H0 System 4 12m 24m 1.916 2.313 0.657 0.766 20 13 0.120 -1.590 Accept H0 Overall 12m 24m 1.158 1.512 0.981 0.943 71 64 0.035 -2.130 Reject H0 Table 3. Results of two-sample t-test hypothesis (on transformed values) Mean Std Deviation Sample Size P - Value t - statistic Hypothesis System 1 HS 1.325 1.501 0.939 0.390 25 26 0.380 -0.880 Accept H0 System 2 HS 0.173 -0.061 0.588 0.491 13 15 0.260 1.150 Accept H0 System 3 HS 1.816 1.425 0.549 0.830 11 12 0.200 1.320 Accept H0 System 4 HS 2.112 2.003 0.806 0.556 21 12 0.680 0.420 Accept H0 Overall HS 1.420 1.219 1.030 0.907 70 65 0.220 1.220 Accept H0 Table 4. Results of two-sample t-test hypothesis (on transformed values) 88 Journal of the Mine Ventilation Society of South Africa, July/September 2003 tion levels between two parameters (cutting direction and cutting blocks) for the overall dust control system. The result of the two-sample f-test was a test statistic with 133 degrees of free dom, and p=0.035 confirming that there is a significant difference between measured dust concentration levels when the CM was cutting the 12 m and 24 m cutting blocks. From Table 4, it can be observed that except for System 1 (half-curtain sys tem), f-statistic CH-CS (concentration C in heading [H] - concentration C in split [S] was positive indicating that the mean dust concentration levels in the heading (H) were generally greater than the mean dust concentration lev els in a split (S). Further, the null hypothesis was accepted (large pvalue, >0.05) for the individual dust control systems as well as for the over all system confirming that there was no significant difference between the mean concentration levels in a heading (H) or a split (S) at 95 % confidence level. Analysis of variance (ANOVA) The dust concentration data were also used to perform an analysis of vari ance (ANOVA). In order to statistically quantify the influence of cutting blocks (CB), cutting direction and type of dust control systems on the dust concentra tion levels in the section, a factorial analysis was carried out. Essentially the airborne dust concentration at the operator's data that was used was in the form of Cjjk (mg/m3). The subscripts have the following definitions: i = cutting block (CB), i = 1 is a 12 m cutting block, i = 2 is a 24 m cutting block j = cutting direction, j = 1 is a heading (FI), j = 2 is a split (S) k = dust control system, k = 1, 2, 3, and 4 respectively indicate the half-cur tain dust control system, retrofitted hood system, double scrubber and integrated hood system respectively. The results of analyses of variance (ANOVA) on the data are summarised in Table 5. The main factors of the sta tistical analysis were: dust control sys tem type; cutting direction; and cutting block (CB). The ANOVA table gives, for each term in the model, the degrees of freedom (Df), the sums of squares (SS), the adjusted means squares (MS), the F-statistic from the adjusted means squares, and its p-value. In the ANOVA table, some p-values were less than 0.05, indicating that these factors are significant. From the results of ANOVA, the fol lowing conclusions can be deduced: Effect of dust control system type on the dust concentration levels is high ly significant with a mean square (MS) value of 407.43. There is a slight evidence (p - value of 0.017) of the effect of cutting block (12m and 24 m) on dust levels recorded at the operator's position during the coal cutting operation for various dust control systems (the MS value for the cutting distance is 147.78). The effect of cutting direction (head ing or split) on the results of dust con centration levels at the operator's position is insignificant with a p-value of 0.236. From the magnitude of each test parameter the dust control system and cutting block can be placed in a descending order of importance. As we note from the table, the dust con trol system has a pronounced effect on the dust concentration levels (pvalue of 0.000 and highest MS value of 407.13). Sources of Variation A (Cutting Distance: 12m or 24m) B (Cutting Direction: Heading or Split) C (Dust Control System Type) A*B A*C B*C A*B*C Error Total Df 1 1 3 1 3 3 3 119 134 SS 147.78 35.97 407.43 15.78 63.53 23.83 15.50 3022.67 Table 5. Results of Analysis of Variance (ANOVA) MS 147.78 35.97 407.43 15.78 21.18 7.94 5.17 25.40 F value 5.82 1.42 16.04 0.62 0.83 0.31 0.20 Pr > F 0.017 0.236 0.000 0.432 0.478 0.816 0.894 The following two-factor interactions viz., cutting distancexcutting direc tion, cutting distancexdust control system type, cutting directionxdust control system type in the analysis are statistically insignificant. Also, three-factor interaction effect, dust control system typexcutting directionxcutting distance do not have significant effect on concentra tion levels at the operator's position. Finally, the main factors, such as dust control system type and cutting dis tance (to some extent) influencing the dust concentration levels at the operator's position are well demon strated. Conclusions Although recent international analy ses of the incidence of pneumoconio sis show favorable trends, the latest studies by British scientists and by NIOSH indicate that the risk of devel oping the most serious form of CWP at the current exposure level standard (2 mg/m3) is higher than had been ' previously believed. Flowever, Australian researchers have reported that they have no evidence of CWP at levels greater than the 2.0 mg/m3 stan dard (US Federal Register, 1995). Previous studies have shown that the risk of progression to a higher cat egory of pneumoconiosis due to coal dust exposure increases with increas ing intensity of the exposure, repre sented by the mean dust concentra tion (Jacobsen et al., 1970,1971), and with increasing cumulative exposure, represented by the product of intensity and duration (Jacobsen, 1973, 1979). Therefore, from this study we can indi rectly deduce that a worker positioned inside the cabin of a CM during the cutting of a 24 m block is usually at higher exposure risk than the worker when cutting a 12 m block. Based on the mean dust concentration results, the miner who is operating in a head ing is exposed to more dust than when operating in a split position. From this study, a dust exposure level chart based on dust levels during coal cutting operations in a section can be shown pictorially (Figure 7) and an index of approximate exposure of workers present in the working face are summarized in Table 6. The plot Journal of the Mine Ventilation Society of South Africa, July/September 2003 89 Oc) TD 11 D CC CD I B CL CD 0 12 Cutting distance, m 24 Figure 7. Dt/sf exposure level index chart based on cutting direction and cutting distance Exposure Parameter Split-12 m Split - 24m Heading -12 m Heading - 24 m Code A B C D Descriptor Good Unhealthy to Hazardous Good Unhealthy to Hazardous Table 6. Dust exposure level index based on cutting distance and cutting direction shows in the x-axis cutting distance for a length up to 24 m and y-axis repre senting cutting direction (split or head ing). The cutting distance was limited to 24 m as face ventilation becomes totally ineffective beyond 24 m hence dust level will be very high. Finally, the analysis of the dust con centration results at the operator's position indicated that the mere appli cation of the 12 m rule on its own does not solve the dust problems, but that meticulous and regular mainte nance, together with the application of available state-of-the-art dust control technologies, effective dust control strategies and best practices, ensures the reduced workers exposure. The introduction of remotely controlled cut ting operation in the section may effec tively lower the duration, severity and intensity of workers' exposure to coal dust. The authors strongly believe that the use of the cutting block (CB) dis tance is one of the most important and easily controlled parameters in assess ing the worker exposure to coal dust. As an administrative control measure for effective dust exposure control, the mine environment officers, occupa tional hygienists and mine manage ment can effectively use this informa tion by rotating the CM operators dur ing the working shift so that a particu lar CM operator is not constantly exposed to the higher levels of coal dust. Acknowledgements The research conducted in the preparation of this paper was part of a doctoral research program conducted by Mr. Belle. The authors are indebted to the Safety in Mines Research Advisory Committee (SIMRAC), CSIR's Division of Mining Technology (Miningtek) and the University of Witwatersrand for the financial support of this research. References DME, 1995, "Guidelines for the Gravimetric Sampling of Airborne Particulate for Risk Assessment in Terms of the Occupational Diseases in Mines and Works Act # 78 of 1978, " Parent Document, 3rd Edition, South Africa. Department of Minerals and Energy, Government of South Africa, Pretoria, 1997, "Directive on the Maximum Cutting Distance During Mechanized Mining of Coal." Jacobsen, M., Rae, S., Walton, W. H. and Rogan, J. M., 1970, "New Dust Standards for British Coal Mines," Nature, Vol. 227, pp 445-447. Jacobsen, M., Rae, S., Walton, W. H. and Rogan, J. M., 1971, The Relationship between Pneumo coniosis and Dust Exposure in British Coal Mines, In: Walton W.H., ed, Inhaled Particles III, Vol. 2, Surrey, UK, Unwin Brothers Ltd, Gresham Press, pp 903-919. Jacobsen, M., 1973, Progression of Coal Workers' Pneumoconiosis in Britain in Relation to Environmental Conditions Underground, In: Proceedings of Conference on Technical Measures of Dust Prevention and Suppression in Mines, Luxembourg, October 11-13, Luxembourg: Commission of the European Communities, pp 77-93. Jacobsen, M., 1979, "Effect of Further Dust Exposure among Men with Early and More Advanced Signs of Simple Pneumoconiosis," Edinburgh Institute of Occupational Medicine, Final Report on CEC Contract 6244-00/8/107, Report No. TM/79/16. Oberholzer, J., 1998, CSIR-Miningtek Report, South Africa. US Federal Register Notes, 1995, Vol. 60, No. 20, January 31, Washington, USA. 90 Journal of the Mine Ventilation Society of South Africa, July/September 2003