Design, Implementation, and Evaluation of Industrial Ventilation Systems and Filtration for Silica Dust Emissions from a Mineral Processing Company.

OBJECTIVE: Silicosis as an incurable occupational disease is common in industries and processes that contain silica dust. Since engineering controls can reduce the risk of silicosis, the goal of this study was to design, implement and evaluate industrial ventilation systems and filtration for silica dust, which is emitted from hydrocone crusher and screener units in a mineral processing company.
METHODS: In this project, local exhaust ventilation (LEV) system was designed and installed using the standard and valid guidelines. The dust concentration was measured in two stages before and after installation of the ventilation system in the workplace, silica emission sources and also in the workers' inhalation area. Finally, the efficiency of the system was determined.
RESULTS: The efficiency of LEV system in reducing workplace dust concentration and dust emission sources was 79.8% and 84.92% respectively. Furthermore, the efficiency of system in reducing the Respirable Crystalline Silica (RCS) at the worker's inhalation area was 92.13%. The collection efficiency of filtration system for total particles was 99.67 %.
CONCLUSION: The results indicate that with designation and installation of the local exhaust ventilation (LEV) system and also installation of bag filter to collect dust, the concentration of dust in the workplace and in the inhalation area of workers has decreased significantly. As a result, this system can be used to control dust in similar industries. Copyright:

INTRODUCTION

Inhalation of chemical pollutants, such as particles in the air, causes various diseases. Epidemiological studies have shown that exposure to inhalable particles not only exacerbates respiratory illnesses in patients, but also increases the mortality rate in the elderly with cardiovascular and respiratory diseases.[1]

Silica is one of the compounds that is found in various industries and its adverse effects on the health of workers and the environment has been proven. Silica is known to be one of the harmful chemical agents in industrial environments and breathing its particles causes lung disease. It has the potential to cause silicosis and lung cancer, and the International Agency for Research on Cancer (IARC) has classified silica in group 1 of carcinogenic chemicals.[2] National Toxicology Program (NTP) has classified silica dust in the form of quartz or cristobalite as a carcinogen.[3] Silicosis is a disease in which lung tissue is damaged and its ability to exchange oxygen with the air is reduced. The damage cause to lungs is irreversible, often progressive (even if contact with silica is stopped), potentially fatal, and there is no effective treatment for it; therefore preventing and controlling exposure is vital.[4] This disease is observed in many industries including mining, silica granulation industry, and ultimately industries directly using silica. According to article 29 CFR 1910.1053 of the OSHA (Occupational Safety and Health Administration) regarding Respirable Crystalline Silica (RCS) hydraulic fracturing companies must protect their workers against inhalation of crystalline silica in work environment, by June 23, 2021.[3]

A stage of silica processing is the crushing operation that produces different silica granules. The ore is brought to the factory from mine and fed to screener by conveyors and then transferred from screener to the crusher. In this process, particles break into smaller pieces and are finally transferred to the silica production section to produce silica powder, with desired diameters. This process releases silica particles into the workplace. Inhaled silica particles have an aerodynamic diameter of less than 10 μ, and are part of airborne crystalline silica which can enter the lungs.[56]

The National Institute of Occupational Health and Safety (NIOSH) has provided engineering and management guidelines regarding how to use general and local exhaust ventilation (LEV) systems to reduce silica exposure in workplace and prevent occupational diseases. Dust control systems include different equipment such as cyclones, electrostatic precipitators, and bag filters. One of the high-efficiency dust collecting systems is bag filter which consists of several fabric bags. As the gas containing the dust passes through the inlet valve toward the outlet filter, dust is trapped on the filter surface. The baghouse, if designed properly, can achieve high efficiency in removing particles with approximately 0.01 μm in diameter. The fabric filter removes micro size particles significantly as much as 99.95%.[7]

Today, mostly pulse-jet systems are used to clean up the filters.[8] Generally, the use of bag filter is highly efficient in removing particles and can significantly lower environmental pollution. In this study, with designing and implementing this system, occupational exposure to silica particles is expected to drop; besides, the system's outlet particles should be less than the allowed environmental limits of Iran.

METHODS

The present study was carried out in both crusher and screener units of a mineral processing company. Initially, workplace environment was checked to see equipment layout and limitations for running the ventilation system. Then, the concentration of dust released from sources, dust in the workplace air and worker's exposure to silica particles were measured using the NIOSH 0500 method and NIOSH 7602 method, respectively. Extraction and analysis of silica samples according to NIOSH 7602 method was performed by FTIR method.[9]

Designation of the ventilation system was conducted based on the standards of the ACGIH industrial ventilation committee. The VS-99-01 standard was applied to design the (LEV) of the screeners. In order to design the (LEV) hydrocone crushers, the guidelines to design ventilation system for crushers was used.[10] Finally, the hoods of local ventilation system were installed on two hydrocone crushers and two screeners.

To assess the performance of the implemented ventilation system, velocity pressure method was used which is introduced by the American Conference of Governmental Industrial Hygienists (ACGIH). In this method, friction drops and dynamic drops of the ventilation system were applied as a coefficient of velocity pressure in the calculations. Given the necessary flow rate and minimum transfer velocity required for hoods and ducts diameter and cross section of all branches were calculated.[11]

Because of the great variety in particle size and density in the workplace, filtration refinery is used to collect released dust. To estimate required flow rate for the ventilation system, the number of fabric filters and the diameter and height of each were calculated.

After designation, construction, and implementation the ventilation system, its efficiency was evaluated by personal and ambient samplings.

Isokinetic sampling of particles in the ventilation ducts were performed according to Standard No. 5 of the EPA. To determine the number of sampling points across the ducts, the standard BS 3405 was followed.[9] To measure the distribution of particle size by the isokinetic method, 4 samples were taken from the upstream and downstream of the filtration system by the Portable Dust Monitor model Grimm-1.108.[13] The following equation was used to calculate the Dust collector performance efficiency in removing particles from the air.[14]

E: System efficiency in percent (%)

C1: Concentration of dusts in the upstream of bag filter

C2: Concentration of dusts in the downstream of bag filter

Ambient sampling, in the workplace and a source of air pollution in ON and OFF mode of the ventilation system was conducted according to the NIOSH 0500 method. A 37 mm PVC filter and close face filter cassettes were used in sampling pump (model 224-PCER3), which were manufactured by SKC Co. of England.

The NIOSH 7602 method was applied for personal sampling and measurement of respirable crystalline silica on 4 workers who worked in the unit per shift. A 37 mm PVC filter was installed with a 5 μm pore size with an aluminum cyclone, and a personal sampling pump with a flow rate of 2.5 L/min.[9]

The sampling was performed in both ON and OFF modes of the ventilation system. Totally, 8 environmental samples, 8 samples from the sources, and 8 personal samples were taken. Then, the efficiency of collecting pollutants was calculated, and finally the efficiency of the ventilation system was determined. All filters used were placed inside the desiccator for 24 h before and after sampling. A digital scale with a sensitivity of ± 0.0001 was applied for weighing filters.

After sampling, the filter samples were placed in a muffle furnace for 2 h at 600°C. From the extracted samples with KBr, pellets with weight of 0.0250 mg were made and analyzed by FTIR spectrophotometer (Perkin Elmer model). Finally, the analysis was compared with prepared standard silica samples. SPSS16 software was used for analyzing the data and the statistical analysis of data was conducted by paired t-test.

Ethical clearance

Ethical approval for this study (Dedicated ID: IR. UMSHA.REC.1396.195) was approved by the ethical committee of Hamadan University of Medical Sciences, Hamadan, on 27 May 2017.

RESULTS

The LEV was installed on the mentioned devices and the bag filter was used as a dust collector, with pulse jet cleaning. In Figures 1 and 2, the location of the industrial ventilation system and the bag filter are illustrated with the CATIA software. The layout of LEV system with duct segment identification is shown in Figure 3 and details of branch's dimensions are presented in Table 1. The filtration system can be seen in Figure 4.

Figure 1

Locating industrial ventilation system and the bag filter

Figure 2

Layout of local exhaust ventilation (LEV) system (Plan view)

Figure 3

Layout (LEV) system with duct segment identification

Table 1

Detail of Branch’s Dimensions

No.	Duct Segment Identification	Length (ft)	Diameter (in)	No.	Duct Segment Identification	Length (ft)	Diameter (in)
1	3-A	3.3	6.5	9	D-E	49.2	1
2	2-A	9.8	7	10	3-F	3.3	2
3	A-B	1.6	12	11	2-F	9.8	3
4	1-B	39.3	8	12	F-G	1.6	4
5	C-B	131.2	14	13	1-G	9.8	5
6	1-C	36	11	14	G-E	8.3	6
7	C-D	29.5	21	15	E-FAN	34.5	7
8	1-D	36	11	16	FAN-STACK	26.3	8

Figure 4

Filtration system

According to the calculations, the designed flow rate is 20,404 cfm and depending on the workplace condition, 220 polyester fabric filters with a density of 550 g/m2, a height of 2.5 meters, and diameter of 0.16 m were used. The pressure drop of the bag filter and LEV were found to be 6 and 7.24, respectively. A centrifugal fan with radial blades and absorption power of 87.6 hp was used to supply air flow and its static pressure.

The results show that the average of total mass concentration of the particles in the inlet bag filter is 995.7 mg/m3, and in outlet bag filter is 3.2 mg/m3. The collective efficiency is 99.67% which is considerably higher than studies in similar industries that used other dust collectors, such as cyclones and etc., This proves that the filtration system is high efficient in trapping and removing the dust. The concentrations of total dust and RCS in both of on and off modes are shown in Tables 2 and 3.

Table 2

Comparison of local exhaust ventilation (LEV) efficiency for controlling air pollutants

Sampling type	Number of samples	LEV OFF mode		LEV ON mode		P
		Mean value (mg/m3)	Standard deviation (mg/m3)	Mean value (mg/m3)	Standard deviation (mg/m3)
Total dust concentration	8	23.5	1.6	4.73	1.2	0.061

Table 3

Comparison of LEV efficiency for controlling respirable crystalline silica (RCS)

Sampling type	Number of samples	LEV OFF mode		LEV ON mode		P
		Mean value (mg/m3)	Standard deviation (mg/m3)	Mean value (mg/m3)	Standard deviation (mg/m3)
Personal sampling	8	3.05	0.8	0.24	0.04	0.019

The concentration of RCS has dropped from 3.05 mg/m3 to 0.24 mg/m3. The concentration of total particles in the workplace air was reduced from 23.5 mg/m3 to 4.73 mg/m3, and the concentration of release by pollution sources decreased from 39.6 mg/m3 to 6.9 mg/m3. The efficiency of the LEV for RCS is 92.13%, for total particles in the workplace is 79.9% and finally for the removal of particles released by sources is 84.92%.

The amount of particles released into the environment before the bag filter was installed for PM2.5 and PM10 are 768.3 mg/m3 and 949.6 mg/m3, respectively, and after installation these figures dropped to 2.8 mg/m3 and 3.2 mg/m3. Efficiency of bag filter for removal of particles released into the environment for PM2.5 and PM10 was estimated to be 99.63% and 99.66%, respectively. In Figure 5, bag filter efficiency to remove PM1.0, PM2.5 and PM10 particles is depicted.

Figure 5

Efficiency of bag filter system to reduce particles

DISCUSSION

Many studies have been conducted on controlling particles released in the mineral industry. They have identified that jobs such as operators of crusher in closed environments are more exposed to dust.[15] The results of this study show that dustmen compared to the other operators who work longer hours in the unit are more exposed to workplace pollution while other workers often work in the control room which leads to a significant reduction in exposure to pollutants. It's recommended that the control room uses a positive pressure ventilation to keep out the contamination.

In this industry mainly silica particles are released which, in long run, increase the risk of developing silicosis in workers; it can lead to ulcer in lung tissue and disrupt oxygen exchange. Silicosis has both acute and chronic forms. Acute silicosis appears a few weeks up to 5 years after exposure to silica particles.[16] The best way to reduce silica exposure in the workplace and prevent occupational diseases is using LEV. This system is compatible with all processes and can be used for controlling almost all chemicals, including dust released from industrial processes.[17]

In this study, as mentioned above, LEV was used to control silica particles. The results show that the total concentration of silica particles was much higher than the exposure limits defined by Occupational Exposure Limits (OEL) of Iran and ACGIH, and NIOSH.[9]

According to the Iranian OEL booklet of 2016, the occupational exposure limit of respirable silica dust is 0.025 mg/m3 for 40 h of weekly work. In the present study, the work shift is 12 hours and also rotational which amounts to averagely 56 h. Weekly work time exposure limits after correction based on the unconventional work programs for these workers was 0.018 mg/m3.

The amount of RCS before installing the ventilation system was 169 times more than the exposure limit, while afterwards the samples indicated that the amount of RCS decreased by about 12 times.

Nevertheless, the new figures are still higher than the occupational exposure limit, which requires management to take steps to reduce the work hours. Calculating the permissible working hours of exposure showed that daily work hours must be reduced to 2 h and 10 min. This is an attainable goal since workers spend all working time in the control room, supervise production process just through surveillance cameras, and visit production unit only for repairing machines.

In the present study, the efficiency of the LEV to remove total particle was 79.9%. The results are consistent with previous studies too. In a study by Farasati et al. on pollution control in mining factories, it was found that the performance of the local system to remove particles was 91.83%.[10] Aliabadi et al. investigated the effect of LEV on workers' exposure to silica dust in the crushing plants in Hamedan Province. Their findings indicate the extraordinary performance of LEV in reducing the exposure of workers to silica dust, so much that the concentrations of respirable particles was below the occupational exposure limits and risk of occupational lung diseases was removed.[14] A study by Cecala A et al. indicated that in the drilling unit at surface coal operations, the concentration of dust prior to the installation of the ventilation system was 0.64 mg/m3 which dropped to 0.05 mg/m3 after installation.[18]

Moradi et al. in 2015 conducted a study to minimize the release of silica, coke and silicon carbide particles in a crushing unit. The ventilation system used cyclones and a new design of the scrubber. The implemented system reduced the average personal exposure to pollutants by 93.01% and the environmental releases by 64.64%.[19] In a study conducted by Ghorbani et al. in a mining company, the efficiency of removing total dust in the workplace was reported to be76% by LEV and for respirable particles was 73%.[12] In 2013, Mehrizi et al. carried out a study to cut down workers' exposure to silica and formaldehyde contaminants in a casting plant by implementing LEV. Results revealed that the average concentration of silica particles before pollution control was 0.2183 mg/m3, while after the implementation of LEV fall to 0.043 mg/m3.[20]

The dispersion of pollutants from the industry to the environment leads to environmental pollution. In the present study, due to the high concentration of pollutants released from the sources, using a filtration system was identified as the best option for solving environmental problems. Sampling revealed that the concentration of PM10 and PM2.5 in the bag filter was lower than the environmental standard recommended by the Iranian Environmental Organization (150 mg/m≥). Furthermore, the total concentration of particles at the entrance of the bag filter is 995.7 mg/m3, and 3.2 mg/m3 at the next parts. Particle collecting efficiency is 99.67% which is significantly higher than other dust collectors.

The concentration of particles in the stack without the bag filter was 995.7 mg/m3 and after its installation reached 4.24 mg/m3, which is 234 times less. It is 47 times less than the environmental standard that indicates high efficiency of filtration system to trap and remove dust. It can be concluded that in order to prevent environmental pollution in similar industries, use of the bag filter can be excellent.[21]

In 2014, K. Rahimi et al. demonstrated that bag filter has been relatively successful in reducing the concentration of suspended particles in the output section of the Shot-Blast and induction furnaces.[22]

Yun Zhou et al. (2013) concluded that filtration system is suitable for cement industry, dam, and road construction activities; however, it isn't effective in trapping heavy metals.[23]

One of the factors affecting the ventilation system is attending to the leakage problem. To curb it, possible leaking of LEV and bag filter should be inspected periodically.[24]

The results show that considering the huge amount of dust emissions in the mining, filtration is an appropriate option for trapping pollutants in this industry. If properly designed, the bag filter can remove most of the particles smaller than 0.01 μm.

CONCLUSION

Considering the high-efficiency of local exhaust ventilation (LEV) and filtration system, it is expected that the present project can be practical in solving the health and environmental problems of similar industries.

Financial support and sponsorship

This work was supported by the vice chancellor for research and technology, Hamadan University of Medical Sciences (Number of thesis: 9603091591).

Conflicts of interest

There are no conflicts of interest.