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Bismuth as an additive in free-machining brasses.

Stringent regulations on lead have resulted in a search for effective substitutes. Bismuth may be the answer for lead-containing brasses.

Bismuth appears to be an attractive candidate to replace lead in free-machining brasses, especially those used in potable water systems where lead contamination may pose health risks.

Metallurgical studies and limited manufacturing trials have shown promising results, but questions remain as to bismuth's supply and toxicity.

Preliminary examinations, however, have found there are adequate bismuth resources to meet this added demand, and that bismuth presents minimal health and environmental risks.

Health Considerations

Good machinability in copper and its alloys is traditionally conferred by lead additions, which are present as a finely dispersed, insoluble and soft second phase. These dispersed lead particles lubricate tools and minimize abrasive wear while providing good chip-breaking characteristics that allow high machining speeds and an excellent surface finish.

These qualities, together with good mechanical properties, corrosion resistance and moderate cost, resulted in the widespread use of free-machining brasses in many areas, including faucets and other fixtures used in drinking water supply systems. Research shows, however, that lead leaching from such fixtures can contribute to lead contamination in drinking water at the tap and to health hazards associated with lead.[1]

Concern with lead contamination in potable water supplies, regardless of its source, is prompting stringent regulations. In the U.S., the EPA adopted a rule that limits the lead contamination at the tap to 15 ppb (effective in November 1992).[2] This rule requires a substantial reduction in the lead content of free-machining brasses for plumbing fixtures and potable water supply use, which now ranges from 3-7 wt% in the most widely used grades.

This growing level of concern about lead's toxicity and its presence in drinking water has created a pressing need for a free-machining additive that works as well as lead, but poses little or no potential health hazard.

Bismuth as an Additive

Although many alternatives to lead in free-machining brasses have been investigated over the years, bismuth is emerging as an attractive candidate for alloys used in potable water systems. While bismuth enhances the machinability of copper alloys with few health problems, it has also been known to embrittle copper when used alone at the levels needed for good machinability.

Fortunately, recent work shows this embrittlement can be overcome by controlled additions of a third element (primarily phosphorus, indium or tin) that retains all of the other desirable properties of these free-machining brasses.[3] Thus, while a Cu-1 wt% Bi alloy in the study cited showed extensive cracking after a single cold-rolling pass of 10% reduction in thickness, this same alloy with 0.15 wt% P would be cold-rolled 60-70% without any evidence of cracking.[3]

The promising results reported in the literature[3] were obtained only on laboratory-sized samples. More recently, these results were confirmed at the foundry level, and appear to demonstrate the feasibility of commercially producing both wrought and cast versions of these alloys.

Bismuth Supply & Demand

Bismuth is usually produced as a byproduct from the treatment of ores of other metals, principally lead and copper, but also tungsten, tin, zinc, silver and gold.[4] In Bolivia, however, it is commercially feasible to recover bismuth directly from bismuth ores. The sole U.S. producer of primary bismuth obtains it from refining domestic lead ores. About 90% of the U.S. requirements for bismuth come from imports.[4]

The latest estimates by the U.S. Bureau of Mines for world mine and refined metal production are given in Table 1.[5] These current production levels reflect a reduced world demand for bismuth. The average price of $3.56/lb in 1990 is insufficient to bring higher-priced supplies, such as those of Bolivia, into the market.
Table 1. World Mine and Refined Production and Reserves (1991 Estimate in Metric
[Tons.sup.5], 1MT = 1000 Kg)
Country Mine Output Refined Reserves Reserve
 Bi Content Metal Base
U.S. [100.sup.4] 100 9000 14,000
Mexico 6.50 475 10,000 20,000
China 900 750 20,000 40,000
Peru 600 550 11,000 42,000
Australia 400 - 18,000 27,000
Canada 200 280 5000 30,000
Japan 130 450 9000 18,000
Rep. Korea 40 90 4000 5000
USSR 100 80 5000 10,000
Bolivia 70 - 5000 10,000
Belgium - 800 - -
Other 110 145 14,000 34,000
World Total 3300 3720(*) 110,000 250,000
(*) Higher level of refined metal production due to additional, stockpiled ore s

It has been estimated that an additional 1000-2000 metric tons of bismuth could be made available from world sources to augment the mine production at a price of $5/lb.[6] This price would still be well below the average bismuth price of $5.22/lb in 1985 and $5.76/lb in 1989.

Less than half of the world's estimate production capacity for mined and refined bismuth was needed to meet 1990 production levels (Table 2). Nearly all of these capabilities should be brought on-stream to meet additional bismuth demand if the need arises and if the price were about $5/lb.[6]
Table 2. World Annual Bismuth Production
Capacity, Dec. 31, 1990[13]. (Metric Tons)
Country Mine Refinery
U.S. 700 500
Australia 1800 -
Mexico 1100 1000
China(*) 1000 1000
Peru 900 800
Japan 700 1200
Canada 700 300
Bolivia 700 300
Rep. Korea 250 250
USSR(*) 100 100
Belgium - 1100
U.K. - 400
Other 250 750
World Total 8200 7700
(*) estimated

Uses and Consumption

The major uses for bismuth are in pharmaceutical, cosmetics, electronics and applications that depend on the ability of bismuth to impart fusibility and machinability to a variety of alloys.[4,7] Bismuth's use in medicine and in fusible alloys and solders has a long history, while that in electronics and as a free-machining additive in steels and aluminum alloys are more recent. While there are substitutes for bismuth in many of these applications, they are not competitive with bismuth in quality or cost.

Questions exist on what amount of bismuth is required to meet its potential use in free-machining brasses for potable water applications and whether there are adequate supplies to meet this need. Current U.S. brass rod mill production of leaded brasses is about 318,000 metric tons/yr, and about 180,000 metric tons/yr for foundries.[1] Since a very significant proportion of this tonnage is melted from turnaround scrap, net consumption will be substantially lower.

The net alloy consumption can be estimated from the average scrap recycle time, which is about one month for wrought products and six months forcastings. Since thescrap recycle time for brass mills is only about one month, then only 26,500 metric tons/yr are required to fill this pipeline.

Similarly for foundries, the amount required to fill the pipeline would be 90,000 metric tons/yr. The net total estimated tonnage requirement is then about 116,500 metric tons/yr. Since global use is estimated[8] to be three times more than the U.S., the net estimated global tonnage requirement is then about 350,000 metric tons/yr.

Only a fraction of this tonnage will be consumed in potable water applications. The size of this application can be estimated by considering that about 30 million faucets were sold in the U.S. in 1991.9 If the average finished weight of each faucet is 1.5 lb, then 20,000 metric tons (45 million lb) of the 116,500 tons (U.S.) can be considered to be directed toward the potable water market.

The average lead content of the leaded brasses produced in brass mills is about 2.9 wt% compared to the 5 wt% in foundry alloys.[8] This reflects the fact that the predominant free-machining brass in the rod mill is alloy C36000, which nominally contains 3.1 wt% lead. Foundry alloy C83600 nominally contains 5 wt% lead.

If bismuth were to replace-lead one-for-one along with an equal distribution (cast/wrought) for faucet manufacturing, the bismuth required would be 790 metric tons/yr for the U.S. and 2370 metric tons/yr for world demand.

There is some indication that bismuth may be more effective in improving machinability[3] on a weight percent basis, which would reduce the net average levels of bismuth. If this is the case, these values may be on the high side.

Bismuth Toxicity

Bismuth is among the least toxic metals in an industrial setting, with no evidence linking it or its compounds with industrial poisoning.[10,11] An internal review of bismuth toxicity in 1991[12] found no reports of adverse effects from occupational, dermal or inhalation exposure to bismuth compounds.

In fact, bismuth compounds have various therapeutic uses in the pharmaceutical industry, where they are used chiefly in the treatment of gastro-intestinal disorders.[10-12] Preparations based on bismuth salts are taken as an indigestion remedy and bismuth compounds are also used for external applications.

Accounts of neurotoxic effects in French and Australian patients administered insoluble bismuth salts in doses above one gram/day were found to be typically reversible on cessation of exposure and urinary elimination of accumulated bismuth.[10,12]

Overall, bismuth appears to present minimal risks to human health when used as an additive in free-machining brasses. In fact, one U.S. company now offers a line of lead-free plumbing fixtures that contain bismuth as the free-machining additive.[14] Similarly, bismuth is of minimal concern in regards to its environmental toxicity.

Bismuth is attractive because it is essentially nontoxic and, with the recent discovery of ductilizing additives, it offers desirable properties without posing significant health hazards.

There are additional costs involved in replacing bismuth for lead in free-machining copper alloys, but this added cost could be significantly offset by a reduced cost of manufacturing. Foundries must now meet the increased costs of disposing of lead-contaminated casting sands and are under increasing pressure by the EPA to monitor lead levels in their plants and in workers' blood.


[1.] R.G. Lee, W.G. Becker, D.W. Collins, "lead at the Tap: Sources and Control," JAWWA, vol 81, pp 52-62 (1989) [2.] Federal Register, p 26460 et seq, (June 7, 1991). [3.] J.T. Plewes, D.N. Loiacono, "Free-Cutting Copper Alloys Contain No Lead," Advanced Materials and Processes, vol 140, no. 4, pp 23-27 (1991). [4.] "Mineral Facts and Problems, 1985 Edition," U.S. Bureau of Mines Bulletin 675, U.S. Government Printing Office, Washington, D.C. [5.] "Mineral Commodity Summaries 1992," U.S. Bureau of Mines, Washington, D.C. [6.] Y.V. Palmieri - Private communication, Bismuth Institute, Grimbergan, Belgium. [7.] C.A. Hampel (ed.), Rare Metals Handbook, 2nd Ed., pp. 58-68 (1961). [8.] Estimate provided by the Copper Development Assn., Inc. [9.] D. Martin - Private communication, Plumbing Manufacturing Institute, Arlington, Virginia. [10.] L. Triberg, G.F. Nordberg, V.B. Vouk,(eds.), Handbook on the Toxicology of metals, chapter 20, (1979). [11.] M. Sittig, "Hazardous and Toxic Effects of Industrial Chemicals," NoyesData Corp., Park Ridge, New Jersey, pp 74-76, (1979). [12.] L. Brooks, Unpublished report, ATT Bell Laboratories (June 1991). [13.] "Bismuth in 1990. Mineral Industry Surveys," U.S. Bureau of Mines, Washington, D.C. [14.] NIBCO, Inc., Elkhart, Indiana.
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Title Annotation:Health, Safety & Environmental Issues Facing Foundries
Author:Schlabach, Thomas D.
Publication:Modern Casting
Date:Feb 1, 1993
Previous Article:Practical ways to improve safety, reduce workers' compensation.
Next Article:Understanding the Material Safety Data Sheet.

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