Pub Health 4310Health Hazards in Industry: Pub Health 4310 Health Hazards in Industry John Flores Lecture 25 Minerals


Lecture 25:Minerals: Lecture 25: Minerals Chapters 25-32 Minerals Quarrying Mining Smelting Asbestos Products Asphalt Products Abrasive Products Glass Products Ceramic Products


Minerals – Glass Products: Minerals – Glass Products Introduction: Nearly every element of the periodic table has been used in glass-making but the principle component is still silica sand Major constituents, besides silica sand are limestone, soda ash, salt cake, lead oxide, boric acid, and crushed glass Minor constituents include arsenic, antimony, fluoride salts, silicafluoride, rare earth elements, and salts of chromium, cobalt, nickel, cadmium, and selenium, Some common glasses are Soda-lime-silica glass Represents 90% of the glass made in the US Lead-potash-silica glass Borosilicate glass Since the major portion of glass is sand, it would seem that there would be a serious silicosis hazard, but this is not the case Washed sand is usually used, which removes a substantial portion of the fine particles It is common to find that the airborne dust when mixing batches contains only 1-5% of crystalline silica Although silicosis is rare in modern glass plants the methods used to handle certain types of sands can still present a dust hazard


Minerals – Glass Products: Minerals – Glass Products Introduction (cont.): In the manufacture of optical glass and certain decorative glasses, lead is an important source of employee exposure The material is usually in the form of lead oxide and requires special handling procedures and the use of local exhaust ventilation with appropriate air cleaning The other major constituents o glass do not normally present a health hazard, although dermatitis is a possibility Many of the minor constituents have caused adverse health effects in the past Arsenic as an example at one time caused perforation of the nasal septum or severe skin effects Highly alkaline batch constituents commonly cause skin effects and irritation Modern methods of handling and ventilation control have eliminated most the the health problems


Minerals – Glass Products: Minerals – Glass Products Glassmaking Modern glass is made by 2 processes The pot process The pot process is still used but mainly for making high quality glass such as optical and mirror glass, and for small quantities of specialty glass At one time this process was the major cause of silicosis in this industry from the refractory dust created by hand shoveling and hand filling the pots Optical and specialty glass usually contains heavy metals like lead, barium, or manganese The tank method (more modern) Allows for continuous feeding of batch ingredients through an enclosed system, thereby reducing dust Using the more efficient tank melting system, the hazards of the pot melting have been eliminated Furnace blocks and bricks Refractory blocks and bricks used in making the furnaces contain free silica Silica bricks contain tridymite as the principle constituent During furnace install and repair, work should be monitored for airborne tridymite, cristobalite and quartz In the past furnace blocks and parts were cut to fit at the installation site with little dust control, now large well ventilated mechanized shops are used to prefabricate the refractory furnace parts The prefab parts are shipped to the furnace site for installation, requiring only occasional cutting


Minerals – Glass Products: Minerals – Glass Products Glassmaking Making glass objects Glass may be formed by blowing, pressing, casting, rolling, drawing, and floating After forming, all glass objects must go through a form of annealing to reduce internal stresses in the formed object This is accomplished by putting the glass objects into long, continuous annealing chambers called “lehrs” Because of their size and quantity of heat generated, furnaces create major heat issues After glass is formed and annealed it is usually finished by labeling, smoothing of rough edges, and other processes Grinding is usually done as a wet process using abrasives such as silicon carbide Polishing is also done wet using synthetic abrasives as polishing agents Abrasive blasting of glass is sometimes done in enclosed exhausted cabinets (glove box) using non-siliceous abrasive materials Application of decorative enamels can be done by spray or silk screen Hazards of glass making Glass dust itself is not hazardous because the silica is in the combined or silicate form Dust exposures occur when handling bulk materials in specialty or small pot processes Furnaces and the handling of molten glass create heat stress hazards from the worker radiant load Many workers in this industry have high exposures to IR, which impairs vision by forming cataracts


Minerals – Glass Products: Minerals – Glass Products Fibrous Glass Health Effects In general, studies of fibrous glass workers have not shown interstitial fibrotic changes nor decrement in pulmonary function An ongoing study of a large population of fibrous glass workers has demonstrated a slight increase in lung respiratory cancers (Enterline, 1991) A European cohort study of man made mineral fibers has shown and increase in lung cancer in rock and slag wool workers, but not in fibrous glass workers (Simonato et al., 1987) IARC classifies fibrous glass as a Group 2B, meaning it is possibly carcinogenic to humans Because of federal mandates on wastewater discharge, fibrous plants now re-circulate cooling and wash water This resulted in high concentrations of gram negative bacteria in these waters, which also created exposures to endotoxins in any of the mists generated from these waters In other industries where workers have been exposed to endotoxins concentrations in excess of 10 ng/m3, there have been outbreaks of “mill fever” and “humidifier fever” which have flu-like symptoms with chest tightness and pulmonary function decrement There is also evidence that exposures to endotoxins may cause chronic obstructive lung disease and emphysema A study in the early 1990’s has determined that respiratory disease is present in the fibrous glass worker population and that it is probably due to the endotoxin exposures (Milton et al., 1993)


Minerals – Glass Products: Minerals – Glass Products Fibrous Glass (cont.) Processes Making glass fibers To make glass fibers a gas or oil-fired regenerative furnace is used to melt and refine the batch materials The molten glass is then fed to a forehearth and through a bushing or orifice plate to make a fiber of a given size The organic binders or “sizes” applied to the fiber prevents fiber abrasion and act as bonding agents The product is then cured or annealed in a furnace then wound onto a spool as a continuous fiber or yarn Making fibrous glass wool used for insulation and acoustical treatment Raw materials are batched then melted and refined in a furnace Fibers are formed by air, steam, or flame blowing or by centrifugal forming technique The binder is applied to the hot product and then it is collected on a flat moving bed and conveyed to a curing oven Borosilicate glass wool Is conveyed pneumatically from storage silos to mixers, minor components are added by hand to the batch Mixed materials are charged to the furnace and the glass is formed at 1370 ºC (2500 ºF), then fed to a refractory tank, or forehearth where it is held at 1230 ºC (2250 ºF) In the flame jet process The molten glass flows through a bushing plate at the bottom of the forehearth forming a large diameter filament fiber


Minerals – Glass Products: Minerals – Glass Products Fibrous Glass (cont.) Processes (cont.) Borosilicate glass wool (cont.) In the flame jet process (cont.) This primary fiber is heated by gas flame while under tension to establish the fiber diameter, at completion the fiber bundle is cooled with water, and a phenol-formaldehyde binder is added The fibers are collected as a blanket on a moving grate If the fibers are to be used for home insulation, it is cured at 200-260 ºC (400-500 ºF), as other product lines may not require curing In the centrifugal process Fibers are formed by feeding the glass from the forehearth to centrifugal spinners where the fiber is forced out of small orifices in the spinner head The fiber is cooled by blasting it with cold air or cooling water Binder is sprayed on the cooled fibers and the mat of fibers are collected on a conveyor grate and sent to an oven for curing at approximately 260 ºC (500 ºF)


Minerals – Glass Products: Minerals – Glass Products Fibrous Glass (cont.) Exposures Obviously there are glass fiber and particulate exposures but other exposures exist in this industry Dusts are created during the batch mixing process during material transfer and during de-bagging Depending on the composition of the binder and the spraying technique various organic vapors and gases may become airborne during the curing process Fluoride can be released from fluxing compounds Combustion products from the furnace depend on the type of fuel, but often include SO2 and CO When fibrous glass is sprayed with wash water and the phenol- or urea-formaldehyde resin solution there are major releases of wash water mist and binder mist These mist contain resin components, formaldehyde, bacteria, and endotoxin Noise levels in furnace areas reach levels between 90-100 dBA from high velocity air and gas flow Heat stress is a concern in furnace rooms and other product forming areas Controls Principle control at the front end process is to have local exhaust ventilation at all material transfer points and de-bagging stations Local exhaust ventilation for particulate control is used at the forming station, it is used at the curing stations to remove phenol and formaldehyde, and to control particulate in areas where molded and formed products are manufactured


Minerals – Ceramic Products: Minerals – Ceramic Products Introduction Ceramic consumer and construction products ranging from common architectural tile to elegant dinner china are made from a variety of ceramic raw materials and processes In the US about 35,000 to 40,000 are employed in this industry The major health hazards of this industry are generated from dusts and airborne mists containing silica Three product lines are discussed to illustrate manufacturing techniques and industry health hazards Brick Sanitary Ware Pottery


Minerals – Ceramic Products: Minerals – Ceramic Products Brick The most common products made in the US are ceramic tile and brick, clay pipe and other construction materials are also made and have similar exposure issues Brick manufacturing process The principle raw materials are clay and shale, which are usually quarried in an open pit Although the bulk clay and shale are usually crushed at the mining site, additional crushing and size classifications are done at the brick plant Raw materials are formed into brick using either a dry or wet process Dry process The fine granular material is slightly moistened and injected into molds at pressures of 3.5-12.4 MPa (500-800 psig), the resulting formed brick is then fired in a kiln Wet process, also called the “stiff-mud” process Water and clay are added in a “pug” mill to make a thick mud The mud paste is extruded through a rectangular die, then cut to size The cut pieces are air dried, then fired in a kiln The kiln Is fired by gas or oil depending on the least costly fuel available Firing brick at 1090 ºC (2000 ºF) for 50-100 hrs evaporates free water, dehydrates the mass, oxidizes available material, and vitrifies the product


Minerals – Ceramic Products: Minerals – Ceramic Products Brick (cont.) Exposures or potential hazards Quartz content of the clay and shale can range from 20-40% During secondary crushing and screening operations dust is created at each transfer point Local exhaust ventilation and a continuous preventive maintenance program is used to control these dust exposures Dust exposures occur during mill charging, but once water is added the dust is manageable Major hazard found around the kiln are: Noise from the kiln burners, Heat stress from the radiant heat, and Exposure to CO and sulfur dioxide from furnace fuel combustion


Minerals – Ceramic Products: Minerals – Ceramic Products Sanitary Ware The raw materials used in sanitary ware are: Clay (hydrated aluminum silicate) Feldspar (alkaline aluminum silicate) Flint (crystalline silica) The major steps in sanitary ware are: Glaze and slip preparation Liquid clay slip is formed by mixing water and the granular materials (clay, feldspar, and flint) in a blunger or mixer, when fully mixed it is pumped to the casting shop Casting and molding The slip is poured into plaster molds, allowed to harden an hour or so, then removed from the castings Fettling or trimming and smoothing The castings are then trimmed and smoothed, while the molds are reassembled and dusted with a parting compound for the next pouring Green casting – If trimming is completed within a few hours, the product is still moist, and will release very little dust during trimming and smoothing White casting – If the trimming is done after an extended period (2-3 days), the casting dries out and is identified as white casting, the longer the drying period the dustier the trimming operation


Minerals – Ceramic Products: Minerals – Ceramic Products Sanitary Ware (cont.) The major steps in sanitary ware (cont.): Spraying of glaze Parts are allowed to dry a day or two, then several coats of glaze are applied in a spray booth Kiln Final product is fired in the kiln Hazards and control In an article by Cooper et al., in the AIHA journal In a plant survey by NIOSH, respirable crystalline silica exposures exceeded the OSHA PEL in 95% of all area and personal samples NIOSH made some specific recommendation which the plant implemented A resurvey 3 yrs later showed the respirable concentrations to 1/5th and 1/10th of the original exposures The glaze department was the only area in which samples exceeded the PEL The recommendations included: Raw material transfer was changed from using a front end loader to transferring materials from the silo to the mixer using a pneumatic transfer systems (reduced exposures by 92%


Minerals – Ceramic Products: Minerals – Ceramic Products Sanitary Ware (cont.) Hazards and control (cont.) Cooper et al., article (cont.) The recommendations (cont.): The original dusting compound which contained 20-25% crystalline silica, was replaced with a non-silica compound, white dry castings were wet down before trimming, and housekeeping was improved resulted in an 89% reduction in exposures Respirable crystalline exposures were reduced by 79% in the glaze spray areas by increasing face velocity of the spray booths, patching ventilation leaks, and eliminating man cooler fans which caused air-flow disruption at the spray booths Although the glaze spray area had significant reductions, 50% of the re-surveys exceeded the OSHA PEL, this was due to a failure of plant personnel from ventilating a bag dumping station and not improving the pebble mill charging technique The case study validates the effectiveness and feasibility of using good procedures and good ventilation controls


Minerals – Ceramic Products: Minerals – Ceramic Products Pottery Raw materials used in pottery manufacturing also include: Variety of clays that may contain free silica, Feldspar Flint which is 100% quartz Pottery Process Clay, feldspar, flint, and water are mixed in a blunger (mixer) to make a “slip” The slip is screened to remove any foreign objects and is sometimes passed over magnets to remove iron particles Pottery parts are formed by throwing, casting, spreading in a mold, dry pressing, or extrusion, then allowed to dry After drying the parts are trimmed or “fettled” using knives, sandpaper, or rags The underside of the ware is sprayed with a mineral oxide coating and dried before being fired The part is either fired in a box kiln or tunnel kiln The part is then cleaned mechanically or by sandblasting A final glaze is sprayed on the part which is made from frit (crushed glass), clay, and metals The part is then fired in the kiln again Finishing the piece include grinding the base, adding a decorative decal, and then a final firing


Minerals – Ceramic Products: Minerals – Ceramic Products Pottery (cont.) Hazards and controls The primary hazard of the clay industry is pneumoconiosis from silica dust exposures Major exposures occur during: Crushing, screening, and clay preparation Secondary cleaning of the parts Spraying of the slip and glazes Most of the airborne dust is due to re-suspension of dust from: Equipment Floors Clothing Control measures for silica include: Reducing percentage of quartz in raw materials Substituting non-siliceous material Maintaining and handling materials in a wet state Providing local exhaust ventilation Good housekeeping practices Use of appropriate PPE


Minerals – Ceramic Products: Minerals – Ceramic Products Pottery (cont.) Hazards and controls (cont.) Preparation and application of glazes May be hazardous depending on the use of lead and other heavy metal based glazes The UK has specified that glazes may not contain more than 5% soluble lead and requires: good housekeeping, clothing changes, locker and shower facilities, no smoking or eating in the workplace, and local exhaust ventilation at application locations The UK conducted a study to evaluate the impact of 1970’s improved dust control in the kaolin clay pottery industry and concluded: Because of improvements in dust control, workers who entered the industry after 1971 would not be expected to develop Category 1 pneumoconiosis in a working lifetime