8.3 PILLAR MINING
8.3.1 Economics
Technical Services performs an economic evaluation of every pillar extraction area in Order to provide a pillar plan that both profitable and technically feasible. The evaluation is done in the following manner.
1. The mine first provides Technical Services with tonnage and grade information for all pillars in a designated pillar removal area. To ensure that the grades are a accurate as possible, Doe Run has a database on pillars constructed with information about previous face grades, jackhammer grades, and diamond-drill-hole grades. Where data are lacking, a pillar-washing program provides additional information. In pillar washing a high pressure water spray containing sand and mounted on a boom lift truck is used to wash a strip down each side of the pillar. A visual estimate of grade is then made.
2. Technical Services inputs the tonnage and grade data into a spreadsheet to determine profits and losses based on current metal prices and production costs. The pillars are then sorted and ranked by valuc.