Table of Contents
In-Situ mining at San Manuel, initiated in 1986, has evolved from a system utilizing an underground collection gallery to recover gravity drainage pregnant leach solution to a system primarily utilizing well-to-well leaching and recovery. This shift in production methodology began in 1989 after a study of the feasibility of well-to-well leaching. This study led to the construction of the first well-to-well recovery system at San Manuel.
In-situ leaching of the oxidized copper mineral resource via well-to-well recovery has proven to be a technically and economically successful hydrometallurgical method of copper extraction. Although this leaching method has been adequately modeled to predict future production capabilities, the anticipated magnitude of the ultimate recovery from any portion of the currently mined mineralized zone is not fully understood.
The in-situ resource recovery test co-exists in an oxide orebody with an active open pit mining operation and is based on collection of information from a segment of a wellfield on a temporary bench within the open pit mining sequence. The test segment consists of three cells or hydropads within the 2460 wellfield. The configuration of the cells is a modified seven-spot pattern capable of delivering a high volume of injection flow per unit area of mineral resource.
The pre-operational phase consisted of drilling, sampling, logging, and assaying 16 boreholes which were drilled on approximately 7 meter centers with rotary reverse circulation methods. These boreholes were drilled with a 12.7 centimeter bit and cased as injection wells with 3.8 cm schedule 80 PVC containing rows of 0.51 millimeter slots as screens. The borehole cuttings were collected and composited on 3.05 meter intervals.
The resource recovery test design for this phase placed emphasis on flow, fluid level, and chemical characteristic information. Detailed instantaneous and totalized flow measurements of the barren leach and pregnant leach solutions coming into and out of the test area are collected by electromagnetic flowmeters. Each flowmeter installation has a companion time integrated sampler for 24 hour composite fluid samples of the barren leach and pregnant leach solutions.
Concomitant with the flow measurements are chemical analyses of the barren and pregnant leach solutions coming into and out of the test area.
The post-operational test design consists of drilling, sampling, logging, and assaying the cuttings from a set of yet to be determined series of boreholes. The number of, and location of, these boreholes will be based on geostatistics generated from the initial mini-block model (variogram analysis) of the test area and from operational experience gained during the experiment.
Results and Discussion
The wellfield for the resource recovery test was established on a temporary bench within the open pit mining sequence. Previous mining activities had constructed a bench with an effective width of 19.8 meters from toe to crest. Drill cutting samples were collected from the surface to total depth of the borehole on 3.05 meter intervals for the 16 injection wells and from approximately 56 meters below land surface to total depth of the borehole for the recovery wells.
Solution Flow and Balance
The resource recovery test was placed into operational production on 25 June 1992 and is expected to remain in operation through December 1993. During the past 159 days the barren leach injection and pregnant leach recovery rates have averaged 1,570 and 1,230 liters per minute respectively.
Pregnant Leach Solution Grade
The pregnant leach solution grade has averaged 1.45 grams per liter copper the first 159 days, whereas the average copper grade of the barren leach solution has averaged 0.19 gram per liter.
Sulfuric acid usage has remained relatively stable during the test period. Acidified barren leach solutions have averaged approximately 24 grams per liter sulfuric acid and pregnant leach solutions in the recovery wells have averaged 15 grams per liter sulfuric acid.
Copper Production and Recovery Rate
The decreasing pregnant solution flowrates are a direct result of lower solution levels which increased the dynamic head on the submersible pumps which consequently lowered the pump efficiencies and pumping flowrates.
Future Resource Recovery Testing