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Characterization of exposures among cemented tungsten carbide workers. Part I: Size-fractionated exposures to airborne cobalt and tungsten particles

Journal of Exposure Science & Environmental Epidemiology volume 19, pages 475–491 (2009)Cite this article

Abstract

As many as 30,000 workers in the United States of America are exposed to cemented tungsten carbides (CTC), alloys composed primarily of tungsten carbide and cobalt, which are used in cutting tools. Inhalation of cobalt-containing particles may be sufficient for the development of occupational asthma, whereas tungsten carbide particles in association with cobalt particles are associated with the development of hard metal disease (HMD) and lung cancer. Historical epidemiology and exposure studies of CTC workers often rely only on measures of total airborne cobalt mass concentration. In this study, we characterized cobalt- and tungsten-containing aerosols generated during the production of CTC with emphasis on (1) aerosol “total” mass (n=252 closed-face 37 mm cassette samples) and particle size-selective mass concentrations (n=108 eight-stage cascade impactor samples); (2) particle size distributions; and (3) comparison of exposures obtained using personal cassette and impactor samplers. Total cobalt and tungsten exposures were highest in work areas that handled powders (e.g., powder mixing) and lowest in areas that handled finished product (e.g., grinding). Inhalable, thoracic, and respirable cobalt and tungsten exposures were observed in all work areas, indicating potential for co-exposures to particles capable of getting deposited in the upper airways and alveolar region of the lung. Understanding the risk of CTC-induced adverse health effects may require two exposure regimes: one for asthma and the other for HMD and lung cancer. All sizes of cobalt-containing particles that deposit in the lung and airways have potential to cause asthma, thus a thoracic exposure metric is likely biologically appropriate. Cobalt-tungsten mixtures that deposit in the alveolar region of the lung may potentially cause HMD and lung cancer, thus a respirable exposure metric for both metals is likely biologically appropriate. By characterizing size-selective and co-exposures as well as multiple exposure pathways, this series of papers offer an approach for developing biologically meaningful exposure metrics for use in epidemiology.

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Acknowledgements

We thank K Teschke of the University of British Columbia and M Waters of NIOSH for critical review of this paper. We also thank B Tift and N Edwards at NIOSH for data management and graphics development, and W Miller at NIOSH for useful discussion of this work.

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  1. Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Morgantown, West Virginia, USA

    Aleksandr B Stefaniak, M Abbas Virji & Gregory A Day

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  1. Aleksandr B Stefaniak
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  2. M Abbas Virji
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Correspondence to Aleksandr B Stefaniak.

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Mention of a specific product or company does not constitute endorsement by the Centers for Disease Control and Prevention. The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health. The authors declare that they have no competing interests, financial or otherwise.

Appendices

Appendix A

The manufacturing process and the metallurgical characteristics of CTC are described in detail by Schneider (1989) and Kirk-Othmer (1995). The purpose of this appendix is to describe the work activities and potential sources of exposure variability encountered during manufacture of CTC at the time of the airborne (this study) and dermal and surface contaminations (see companion paper) surveys.

Metal separation facility

At the metal separation facility, tungsten, cobalt, and other metals (molybdenum, and so on) are recovered from machining fluid sludge (metal separation), hard metal powder is reclaimed from off-specification sintered material and/or worn and used product (reclamation), and resultant powders tested for conformance to quality control specifications (powder laboratory).

Metal Separation

The CTC manufacturing process begins in the metal separation plant with a worker loading tungsten-containing metalworking fluid sludge into an oxygen–atmosphere furnace and heating the material to oxidize the tungsten. The resulting tungsten oxide cake is crushed and passed through a rotary furnace to yield tungsten oxide powder, which is placed in a tank with caustic and heated to form sodium tungstenate solution. The solution is purified, converted into ammonium tungstenate, and heated to form ammonium paratungstate (APT) crystals. APT crystals are manually loaded into feed bins, then placed in a calciner (heat-treater) to form tungsten oxide powder, which is reduced (manually or using an automated process) in hydrogen atmosphere furnaces to form tungsten metal powder, and then placed in a blender to homogenize. In manual reduction, a worker scoops tungsten oxide powder into oblong heating containers referred to as “boats,” places the powder boats in the furnace, and then unloads the resultant tungsten metal. In contrast, during automated reduction, the tungsten oxide powder is augered into the boats, and a machine loads the boats into the furnace and unloads the boats.

Carburization

Tungsten metal powder and carbon black powder are ball-milled to homogenize constituents and the resulting powder is manually loaded onto powder boats by a worker, heated (carburization) to form aggregated tungsten carbide (WC) “bricks,” which are automatically dumped into a coarse crusher and then further processed by ball milling and/or jet milling to break the material into discrete WC particles. The WC powder is either packaged and sold to external customers or sent to the powder handling facility.

Reclamation

The reclamation process is housed in two buildings (A and B). CTC scrap is recycled into powder with a composition that matches the parent material. Beginning in building A, a worker dumps several hundred kilograms of sintered scrap (e.g., worn drill bits) into furnace trays using a hydraulic lift to assist with picking up and dumping the scrap-laden buckets; this tray-loading operation generates visible dust plumes. The trays are placed in a furnace using an overhead chain hoist, heated using a proprietary process, and the resultant cake mechanically crushed and screened (these jobs are visibly dusty). The screened powder material is moved to building B, manually placed in trays, the trays inserted in a furnace and heated, and the resultant cake mechanically crushed (a dusty job) and milled to form hard metal powder. This hard metal powder is placed in bins and loaded on top of a screener, sized, and the final product powder blended to homogenize, if necessary, and then placed in storage containers for sale to external customers or shipment to the powder-handling facility.

Powder Laboratory

In the powder laboratory, small (in grams) quantities of powders are tested to ensure that the formulations meet customer and manufacturer quality control specifications. Powders include raw materials, in-process materials, and finished powder for use in production.

Maintenance

Maintenance workers at this facility work throughout the work areas and also maintain a maintenance crib (separate building from main production facility), where contaminated equipment may be brought for repair.

Administration

Administrative workers at this facility are mainly accountants and other white-collar professionals. Administrative offices are located in a building that is physically separated from the production buildings; however, employees occasionally walk through the production areas.

Powder-handling facility

In the powder-handling facility, powders from the metal separation facility are received (inventory control) and processed (powder mixing, wet milling, spray drying, screening) into hard metal powders that are sent (shipping) to the forming and machining facility.

Inventory Control

Tungsten carbide and reclaimed hard metal powders received from the metal separation facility and feedstock powders (cobalt, chromium, nickel, tantalum carbide, etc.) purchased from external suppliers are inventoried by a receiving clerk and then distributed for processing. Inventory control clerks may open and seal several containers during a work shift that contain WC, reclaimed hard metal, and feedstock powders. Each container may weigh tens to hundreds of kilograms. From the inventory control, powders are sent to the powder-mixing work area, which is collocated with a soft (unsintered) scrap-reprocessing operation.

Powder Mixing

In powder mixing, powders are formulated by manually scooping several hundred kilograms of cobalt feedstock powder and lesser quantities of additives (including chromium, nickel, titanium carbide, tantalum carbide, and vanadium) and pouring the powders into large hoppers that contain WC and reclaimed hard metal powders. Adjacent to the powder-mixing area, pre-sintered compacts of hard metal that fail quality control specifications are reprocessed by crushing, screening, and blending in batches containing several hundred kilograms, which generates visible airborne dust. The powder-mixing and reprocessing work areas are among the most dusty work areas in this facility and are physically isolated from the remainder of the facility; workers wear protective disposable coveralls and respirators.

Milling (Wet)

Each hopper containing hard metal powder is connected to a ball mill, filled with an organic solvent and other liquids, and then wet-milled into a slurry; each charge consists of several hundred kilograms of hard metal powder.

Spray Drying

After milling, the slurry is transferred to a pumping container and dispersed in a spray dryer to evaporate the liquids to form homogenized hard metal powder, which is discharged onto a conveyor belt, and, depending on the grade of powder, either dropped into a storage container or passed across a coarse screen and dropped into a drum for further processing. In spray drying, workers may fill several containers each with tens of kilograms of hard metal powder per work shift, which requires the worker to be in close proximity to the spray dryer powder discharge and the powder conveyor belt.

Screening

If necessary, powder from the spray dryer is sent for screening to attain a customer-specified size; at the time of this survey, the cowls on each screener used to contain discharged powder as it dropped into the storage containers were in poor repair or were not used properly. Prior to shipment, gram quantities of each powder were sent to the powder laboratory at the metal separation facility for quality control testing.

Shipping

The final product hard metal powders are shipped to the forming and machining facility for compacting or to external customers.

Maintenance

Maintenance workers at this facility work throughout the work areas and also maintain a maintenance crib work area, where cobalt-contaminated equipment may be brought for repair.

Forming and machining facility

At the forming and machining facility, hard metal powders are received (production control) and distributed throughout the facility for compacting, sintering, machining to final dimensional specification, and then packaged and shipped to customers.

Production Control

Hard metal powder is received from the powder-handling facility and distributed by an inventory control clerk for compaction via pneumatic pressing or extrusion; clerks may open and reseal several containers of hard metal during a work shift.

Pressing

At this facility, 95% of the hard metal powder received is pressed mechanically or pneumatically into “green” (i.e., unsintered forms) customer-specified compact shapes; at each press, the hard metal powder is manually scooped into a hopper and gravity fed into the die.

Extrusion

The remaining 5% of hard metal powder received from the powder-handling facility is augmented with a plasticizer to form a clay-like material and extruded to form “green” compacts.

Shaping

Some green compacts may first be rough shaped (machined), for example, using high-pressure water to cut pieces. All compacts are sintered (heat-treated in vacuum furnaces) on graphite-coated trays to form CTC.

Breakdown

Sintered compacts are removed from their sintering trays by manually flipping the trays over and dumping the parts on a table (i.e., breakdown); at the time of this survey, a ventilated table had been purchased, but not yet installed, by the company to replace the current unventilated table used for breakdown.

Grinding

During sintering, the compacts shrink, necessitating grinding to precise final dimension; most sintered compacts are wet-ground, although dry grinding is performed as necessary.

Sandblasting

After grinding, about one-third of sintered compacts are sandblasted in an enclosed glove box to remove burrs.

Product testing

All finished products are sent for product testing (quality control) to ensure that the parts meet customer specifications.

Shipping

All products that pass quality control inspection are packaged manually and sent (shipping) to customers. During packaging, finished product parts are stacked and organized, which may generate dust.

Maintenance

Maintenance workers at this facility repair machinery throughout work areas and, if necessary, take cobalt-contaminated equipment from work areas to their maintenance crib for repair.

Administration

Administrative workers at this facility include clerical staff responsible for filing production records, photocopying, and record keeping. At the forming and machining facility, administrative offices are located within the same building as the production work areas and separated only by a short hallway. “Shop packets” (tracking sheets that accompany all orders from the metal separation plant through to shipment of final product to customer) are filed and maintained by the clerical staff at this facility.

Appendix B

The table below presents the correction factors for inlet efficiency and interstage particle losses (Rubow et al., 1987); particle size range and midpoint; and factors applied to each impactor stage to estimate the respirable, thoracic, and inhalable particulate mass concentrations.

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Stefaniak, A., Virji, M. & Day, G. Characterization of exposures among cemented tungsten carbide workers. Part I: Size-fractionated exposures to airborne cobalt and tungsten particles. J Expo Sci Environ Epidemiol 19, 475–491 (2009). https://doi.org/10.1038/jes.2008.37

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