New Mexico Geological Society Annual Spring Meeting — Abstracts

The Hanover Mill Tailings were produced by Empire Zinc Mine at Hanover, Grant County, New Mexico. Empire Zinc Mine had been in production from the early 1900's through early 1970's except for some periods when the demand for zinc was low. The ore minerals were smithsonite, sphalerite and galena, the gangue minerals were garnet, pyroxene, calcite and magnetite.

Hanover Mill Tailings are located east of Silver City, southwest New Mexico, along Hanover Creek and consist of five ponds. Four small ponds are located at west side of Bull Hill, and the main pond is east side. Tailings contained in four small ponds will be removed from the banks of the creek, placed on the main pond and stabilized. In this study, possible recovery methods for garnet and sphalerite from these tailings ponds have been investigated in order to offset the cost of remediation. Total material to be removed is about 160,000 tonnes with 20% to 40% garnet (mainly andradite and small amounts of grossular) in different size fractions. The tailings contain approximately 4.3% zinc, mainly in oxidized form. Iron, lead and cadmium sulfides and oxides are also present, in addition to zinc.

The major end uses for garnet are abrasive (sand blasting, water jet cutting, and polishing) and filtration media applications. Garnet does not contain free silica, and the heavy metal content is much less than copper and iron blast furnace slags used as abrasives. It is cheaper than diamond and aluminum oxide compounds used for coated abrasives, and it can be recycled up to 5 times in sand blasting and water jet cutting applications. Because of the above advantages over its competitors, the demand for garnet has increased in recent years.

The recovery methods investigated in this study were desliming, grinding, sulfide and oxide minerals flotation, gravity concentration, magnetic and electrostatic separation. A garnet concentrate has been obtained with approximately 45 % recovery by gravity concentration during preliminary tests. Before gravity concentration, sulfide and oxide bulk flotation concentrates were produced in order to remove lead, zinc and iron minerals from the gravity circuit feed and to improve the garnet grade. A sphalerite concentrate, with 50+% zinc, was obtained with approximately 26% zinc recovery after selective flotation from the bulk' sulfide concentrate. Both flotation and gravity separation processes need to be further optimized.

Another objective ofthis study was to reduce the heavy metal concentration in the reprocessed material. Coarse gravity tailings and the concentrates satisfy the above purpose and they can be classified as non-hazardous material.

The garnet concentrate contains approximately 400 ppm lead, 1.20% zinc, 5.50% iron and 29 ppm cadmium. Its specific gravity is 3.53 g/cm³. The concentrate passes the EPA Toxicity Characteristic Leaching Procedure (TCLP) test, however it contains 6.70% acid soluble matter (mostly calcite) which can be further reduced by recleaning and optimization of the gravity circuit feed.