Long-term leaching characteristic study of coal processing waste streams.

A study of the mobility of major and potentially hazardous trace elements from coal processing waste materials was conducted using two types of leaching tests. The baseline leaching test simulates stable waste storage under water, whereas the kinetic test models the storage of waste under more variable conditions including intermittent exposure to air and variations in humidity. Coarse and fine refuse materials were obtained from three commercial coal preparation plants that were being used to upgrade US bituminous run-of-mine coal containing low-to-high amounts of pyritic sulfur. X-ray diffraction analyses revealed a large variation in mineralogy between the coarse and fine refuse streams due to the mineral fractionation that occurs in the processing units and plant. The coarse refuse samples contained higher pyrite contents while the fine refuse samples had high clay content and a minor amount of calcite. This variation in mineralogy resulted in relatively large difference in the leaching characteristics of the waste streams. The most acidic pH and highest release of trace elements were observed in the leachate of coarse refuse containing medium-to-high amounts of coal pyrite, while the fine refuse samples released lower amounts of trace elements in their circumneutral leachate. The least amount of trace elements was observed in the leachate of low pyritic refuse streams. The test data suggested that the most effective disposal practice for coal waste material is segregation and isolation of the coal pyrite and co-disposal of the coarse and fine refuse streams.

Development of an electrical conductivity screening test for mine waste assessments.

An environmental concern at mining operations is the potential leaching of trace elements from overburden and byproduct streams of processing plants. To provide a timely assessment of this concern, electrical conductivity of the leachate emanating from the plant waste streams can be measured as an indicator of the trace element content levels using the USGS Field Leach Test (FLT). However, the research reported in this publication revealed the need to modify the FLT procedure to improve the precision of the test results. The primary issue involved the importance of leachant volume-to-particle surface area ratio in the assessment of the leaching potential for a given source. To determine the key factors impacting leachability of a given material, a statistically-designed parametric study was performed. The experimental program evaluated the effects of particle surface area, the leachant volume-to-surface area ratio, and the amount of oxidant used to expedite the leaching rate during the test. The results revealed that the significant parameters are leachant volume-to-solid surface area ratio and the amount of oxidant. The findings were used to recommend a modification to the conductivity screening test.