SIDEBAR 5-1 Phosphogypsum

Disposal of phosphogypsum is a special problem. The production of phosphate generates 5 tons of phosphogypsum for every ton of phosphate product manufactured. In Florida alone nearly a billion tons of phosphogypsum have been generated. This by-product is generated when phosphoric acid is manufactured using sulfuric acid, an inexpensive reagent. The resulting phosphogypsum must be managed as a waste. Other acids could be used to manufacture phosphoric acid; however, the by-products of these processes are an insoluble calcium salt, which also raises disposal issues. Although potential uses for phosphogypsum have been identified, its use is prohibited by the Environmental Protection Agency because of the presence of radium at a concentration greater than 10 picocuries per gram present in Florida phosphogypsum. Potential uses of phosphogypsum include material for road bases, agricultural fertilizer to supply sulfur and calcium, and a supplement in landfills and sewage treatment plants to enhance the microbial decomposition of organic waste.

Research to identify or develop uses for phosphogypsum that contains radium could reduce the amount of material that must be managed as waste, as well as limit its potential environmental impact. Research should also focus on the development of a phosphate manufacturing technology that does not produce phosphogypsum as a by-product.

magnitude and severity of the problems, and the applicable solutions may not be. Nevertheless, all mining sectors would benefit from new technologies to reduce or eliminate adverse environmental impacts caused by mining operations.

Water-quality issues related to mine closures are often the most challenging and costly to address for all types of mining. They also present significant opportunities for research that could increase overall productivity. New technologies for managing other materials of environmental concern, such as innovative, cost-effective solutions to managing slimes, could also increase productivity in the mining industry.

Acid-Rock Drainage1

The most common water-quality problem associated with metal and coal mining is the release of metals, acidity, and sulfate from reactive rock surfaces, particularly when pyrite is present (University of California, 1988). Sulfide-containing rock is oxidized when newly fractured rock comes in contact with oxygen, water, and (very often) bacteria. The sulfides are converted to sulfuric acid, which in turn can mobilize a variety of metals (Alpers and Blowes, 1994; Doyle and Mirza, 1990). The most common metals released are iron, copper, zinc, cadmium, manganese, arsenic, antimony, lead, nickel, and mercury. The concentrations of metals vary dramatically with the type of deposit and the environmental factors surrounding the site. The pH of the water can also vary, depending on the amount and type of sulfide minerals oxidized, as well as the amount of neutralization capacity present in the rock. Highly acid-generating rock produces a drainage pH of less than 1; well buffered-rock (e.g., rock that contains calcite or dolomite) may have elevated sulfate concentrations indicative of sulfide oxidation but still retain a near-neutral pH. Metal concentrations, and thus the environmental and health impact of drainage water, depend on the pH. Low pH water (high acidity) has much higher concentrations of metals than near-neutral pH water (low acidity). Some potentially toxic elements, such as selenium and arsenic, are problematic at any pH.

Acid generation is an issue of concern for certain types of coal and hardrock mines (particularly mines that contain high concentrations of pyrite but low concentrations of pH-buffering materials). Problems are most common for reactive waste-rock dumps, although tailings and waste facilities, low-grade ore piles, highwalls, and precious-metal heaps can also generate acids under specialized conditions. Acidic drainage into receiving waters can result in severe impacts to the biological integrity of a stream and can change a diverse, healthy biological system into one in which only less-susceptible organisms can thrive or one devoid of higher organisms.

Prior to planning a mining operation, the potential for acid generation is generally estimated based on cores, drill cuttings, and bulk samples used to characterize the orebody. The characterization of acid-generation potential is critical for constructing waste dumps in a way that minimizes the release of contaminants by oxidation of sulfidic rock. For sites where acid generation is a potential problem, characterization of the rock for contaminant release should be conducted concurrent with characterization of the ore potential. Common tests for acid-drainage potential include acid-base accounting and humidity-cell tests. Both

1  

Additional information on acid drainage can be obtained from Webbased information sources, including the Acid Drainage Technology Initiative (www.mt.blm.gov/bdo/adti), the International Network for Acid Prevention (www.inap.com.au/inap/homepage.nsf), and the Mine Environment Neutral Drainage (MEND) program (mend2000.nrcan.gc.ca).



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement