Engineering Controls Database

Control of Silica Dust Exposure in Pottery Manufacturing

Problem:	Health department surveillance data indicate the largest number of silicosis cases in the state of New Jersey exist in the sand mining and processing, foundry, and pottery (sanitary ware) industries. This disease is caused by exposure to crystalline silica in these industries. The operation described in the primary and follow-up reports summarized here manufactures vitreous china products, such as lavatories and toilet bowls. The work force at this plant consists of approximately 150 employees, including 30 salaried and 120 hourly workers. Vitreous china is a specialized type of ceramics. Raw materials are combined to form a clay slip. The slip poured into a plaster mold to form a casting, and the casting is then shaped, dried, glazed, and fired. The principal raw materials in the slip are clay (hydrated aluminum silicates), feldspar (alkaline aluminum silicates), and flint (crystalline silica). One or more glaze coats (containing crystalline silica) are sprayed onto the molded greenware prior to firing in a tunnel kiln. All production work in the subject operation is carried out in one building. After the pieces have been fired, they are inspected and packaged. Pieces having a blemish are sent to the Refire Department to be repaired and refired. Production areas having a potential for silica exposure include the Slip House, Cast Shop, Mold Shop, Spray Area, and the Kiln, Glaze, and Refire Departments. The cast shop consists of approximately 50 workstations with one worker per station. Casters start the shift by pouring slip from a hose into several rows of plaster molds, which had been set up the previous day. After casting, the slip is allowed to set (typically an hour or more) before the molds are removed. After the molds are removed, they are dusted with a parting compound. The pieces are dried for several hours, smoothed and trimmed using an assortment of hand tools, and then dried further for one to two days. Because of limited storage space, the castings are stored on carts in and around the casting area while they are drying. At night, waste heat from the kilns is directed to this area to enhance drying. The entire casting area, therefore, may be considered a large drying area. In addition to drying greenware, dust and pieces of scrap clay on the floor become thoroughly dried, presenting an opportunity for reentrainment into the air and adding appreciably to the likelihood of a high background dust level. The dried pieces are carted from the Casting Department to the Spray Department, where nine to twelve employees are involved in piece inspection and glaze spraying. One or more coats of glaze (depending on the type of fixture) are applied in ventilated spray booths by means of a compressed-air spray system. The liquid glaze is prepared in the Glaze Department by two workers mixing Supersil powder (crystalline silica), feldspar, whiting, Pyrax, China clay, talc, and other color producing compounds. Bags of material are dumped manually into one of several pebble mill blenders. In dumping the bags of material, the operator climbs up a portable ladder. The bags of material are placed on a pallet and elevated behind the operator by a fork lift. The operator turns around, takes one bag at a time, and holds each bag near the manway to the pebble mill (which is at shoulder height and even with the ladder). He cuts the bag with a knife, manually dumps it into the pebble mill and manually compresses the bag. (Visible dust is forced from the bag.) The mix is milled overnight and the glaze is then pumped to holding tanks for final adjustment and storage prior to use in the Spray Department.
What this means to you:	The principle material investigated in this study was crystalline silicon dioxide often referred to as silica. Silica may be present in crystalline and noncrystalline (amorphous) forms. Amorphous silica, usually considered to be of low toxicity, may produce changes in the lung, visible by X-ray, without disability. Crystalline silica (quartz) has been associated with silicosis, a pulmonary fibrosis of the lung caused by the deposition of fine quartz particles in the lungs. Symptoms usually develop slowly, with cough, shortness of breath, chest pain, weakness, wheezing, and nonspecific chest illness. Silicosis usually occurs after years of exposure but can appear in a shorter time if exposures are very high. The NIOSH RELs for respirable quartz is 50 mg/m3, as time-weighted averages. NIOSH considers quartz to be a potential human carcinogen. NIOSH recommends that occupational exposure be controlled so that no worker is exposed to a time-weighted average (TWA) exposure concentration of quartz greater than 50 micrograms per cubic meter of air (0 .05 mg/m3) as determined by full-shift respirable dust sample for up to a 10-hour work day, 40-hour work week. The OSHA PEL is 0 .1 mg/m3 as a TWA for respirable quartz in an 8-hour day.
Description of control:	This facility has fully or partially implemented all but one of the recommendations made by NIOSH in 1988. The recommendations (shown in italics; limited to engineering controls) and the actions taken: 1. Install an enclosed automatic material handling system in the Slip House. The company had been planning this change in 1988 and had completed installation of the new system prior to the 1991 study. The new system is designed to reduce atmospheric dust dispersion during silo loading and transfer to the blungers. A programmable logic controller (PLC) automatically weighs and pneumatically conveys the raw materials for making the slip through an enclosed system to four receiving bins above the blungers. 2. Substitute a noncrystalline silica mold release for the glaze overspray material used as a parting compound. Perlite, an amorphous silica is now used . 3. Where possible, cleaning of "green" castings~ should be done rather than cleaning "white" castings. If feasible, wiping the white pieces with a damp sponge or misting them with a water sprayer prior to cleaning should reduce dust generation. While it is not possible to completely eliminate "white" casting cleaning, the "white" castings are now wetted with a non-abrasive sponge prior to cleaning. 4. "Man-cooler" fans in the Glaze Spray Department reduced effectiveness of the local exhaust ventilation into the inspection booths and can also reentrain settled dust and should be eliminated. These fans are no longer used in the Glaze Spray Department. 5. Reduce the pressure in the spray guns at the glaze spray booths and increase the ventilation in these booths to at least an average of 100 fpm air velocity at the face to contain the sprayed-on glaze. The average face velocities were increased approximately 100 fpm; they now average 145 to 260 fpm. This meets and exceeds the guidelines set in the ACGIH Industrial Ventilation Manual which recommends a minimum of 100 to 150 fpm. No changes were made in the pressure to the spray guns. 6. Conduct scheduled maintenance and repair of dust control systems, including exhaust ventilation systems. Holes in the spray booths had been repaired and the glaze spray ventilation system is balanced at least once each year. One manual spray booth has been eliminated. Ventilation to this booth was blocked off resulting in increased face velocities at the remaining two booths. (Note: If it is necessary to use a third booth, additional ventilation must be installed.) 7. Reduce cross contamination from other activities in the plant as well as reentrainment of dust from scrap and waste on the floor and other surfaces. Curtains have been hung in one area of the Casting Department. There is also now a wall separating the crusher at the Slip recycler from the Casting Department. 8. Improved housekeeping throughout the plant. Dry sweeping should be eliminated and improved methods for cleaning up scrap material such as using a HEPA equipped vacuum cleaner are needed. Any HEPA vacuum used must be of large enough capacity to handle the volume of dust. Dry sweeping with brooms is being eliminated by the use of squeegee-type brooms. One worker is permanently assigned to cleaning. Additional employees, those assigned to light duty, also assist in cleaning.
Engineering and Physical Hazard Report Numbers:	171-11B; 171-11C;
NAICS Codes:	327112
Keywords:	ceramics ceramics glazing glazing pottery pottery respirable dust respirable dust Silica Silica silicosis silicosis
How well does this control work?	The environmental evaluations for the 1991 survey, after engineering controls had been implemented, included the collection of six bulk/material samples, analyzed for crystalline silica content, and 53 personal and 33 area air samples, analyzed quantitatively for respirable crystalline silica and respirable dust. Respirable crystalline silica dust exposures averaged 0.04 mg/m3, approximately a 78 percent reduction since 1988. Respirable dust exposures averaged 0.34 mg/m3, approximately a 65 percent reduction. Area respirable dust levels average 0.12 mg/m3, a 70 percent reduction. Of the four departments, there was a statistically significant difference at the 95 percent confidence level in the average respirable dust exposure in the Casting Department (48 percent decrease), and the Spray Department (85 percent decrease). There was an increase, although not statistically significantly different, in the average dust exposures in the Glaze Preparation Department (37 percent). There was a statistically significant difference in the average respirable dust levels in the Slip House (79 percent decrease). There were reductions, although not statistically significant, in the average respirable dust levels in the Spray Department (79 percent), the Casting Department (54 percent), and the Glaze Preparation Department (86 percent).