Case 1: Contamination by Cobalt in the Vicinity of a Cemented Tungsten Carbide Tool Grinding Plant

Background

During 20 years of operation of a tool grinding plant large quantities of fine particulate cemented tungsten had been lost to the local environment. Work practices at the plant had been such that debris and dust produced by machining operations were swept out of the back door onto the factory grounds, and the plant's ventilation fans simply exhausted dust to the exterior of the building.

Cemented tungsten carbide, or "hard metal", is composed of finely divided (0.5-14 µm) tungsten carbide in a matrix containing up to 25% cobalt with small quantities of other metals also present. Inhalation of significant quantities of fine particulate cemented tungsten carbide can result in a debilitating condition called "hard metal disease". Machine grinding of hard metal tools results in the aerosolization of respirable cemented tungsten carbide particles which require appropriate control measures.

Aims of the Study

the purpose of this study was to investigate the contamination of the local environment which had come about as a result of plant operations.

Materials

Various material samples were collected from deposition sites in and around the factory. these included: interior wall wipe samples, exterior waste heap samples and garden soil samples approximately 20 m from the plant. Small quantities of dust and soil were later deposited on membrane filters for computer controlled SEM analysis.

Study Results

Tungsten (W) bearing particles were found in each of the samples. the types of element associations found with W, and the proportions of each particle type, found at each sites are set out in the table below. Most of the internal wall dust particles were composed exclusively of W, with a small proportion also containing Co (W-Co-bearing). An example of a large aggregate of W (with Co) particles from the interior wall is shown.

the waste debris heap located immediately outside the plant contained a greater proportion of W bearing particles containing W plus other elements. Significantly, a large number of the W particles from this site also contained Ca, which was not seen on the indoor sample. A typical example of a large particle from this site consisting of silicon, iron and aluminum associated with W, Co and Ca is shown.

the compositional differences between the two source samples (the interior and debris heap materials) suggest that a substantial proportion of the exterior debris heap particles contain quantifiable amounts of Ca while few of the indoor dust particles contained Ca in addition to W. these two samples would appear to represent two types of material processed at the plant. In contrast, the W-bearing particles identified in the nearby garden soil were, in general, not associated with Co or Ca. this lack of an association with Ca might suggest that the material in the soil is derived from the plant interior (exhausted to the external environment and atmospherically dispersed) rather than originate as fugitive emissions from the debris heap. However, the possibility exists that particle disassociation may have taken place in the soil environment. the lack of any association with Co in the soil site W-bearing particles is likely to be the result Co dissolution in the chemically active soil environment. It is re cognized that Co is readily dissolved even in a mildly acidic soil environment.

Discussion

Contamination of the environment in the vicinity of a cemented tungsten carbide tool grinding plant has been documented. Neighborhood soils contain elevated levels of tungsten and cobalt which have been derived from plant operations. this contamination can be attributed to material which has either been expelled through the plant's exhaust fans and atmospherically dispersed, or has been re-entrained by wind action from an extensive mound of factory waste at the rear of the plant. Cobalt levels in nearby garden soil, as determined by Atomic Absorption Spectroscopy, range from 100–1800 µg g-1. this far exceeds the norm for U.S. soils which is 7.2 µg g-1. Computer controlled SEM analysis has shown that proportionally only a small number of tungsten particles (as compared to the factory site samples) contain cobalt. this might suggest that some re partitioning of the cobalt had taken place in the soil. the dissolution of cobalt and the subsequent re-adsorption by other soil phases would likely make the coba lt more available for crop uptake which may pose a threat to landowners who cultivate vegetables and fruits on this land for private consumption.