During the recent expansion of precipitate copper production at the Chino Mines Division of Kennecott Copper Corporation, Santa Rita, New Mexico, local studies of the dump leaching process were intensified. As these studies progressed it became apparent that a reliable method for locating leach solutions within the dump was required, since the contact between leach solutions and the copper-bearing material would determine, in great part, the recovery of values. In the past, the problem of determining leach solution contact has been attacked by various methods, including destructive testing (dump dissection), indirect observation (such as chloride or dye tracers), and mathematical or laboratory model studies. These methods have not proved completely satisfactory for the studies now being developed.
Only three of several available radiation logging techniques are currently in use at Chino.
Natural Gamma Ray Logging
Isotopes of certain elements occurring in nature are inherently unstable, and undergo spontaneous transformation to more stable forms by emission of energy in the form of a ray or particle. The most common naturally occurring isotope which decomposes by readily detectable gamma ray emission is potassium-40. Potassium tends geologically to be concentrated in clay minerals (such as illite), their parent minerals (orthoclase feldspars and micas), and potash minerals.
Neutron-Neutron or Moisture Logging
Neutrons used in logging are emitted from a source as “fast” neutrons with a velocity of about 10 4 km/sec. As these neutrons pass through a volume of material they interact or collide with the resident atoms. The collisions are elastic, and since most atoms have a mass considerably greater than the mass of a neutron, the neutron loses very little energy. I
Gamma-Gamma or Bulk Density Logging
Gamma rays emitted from a source bombard the material surrounding the source and are reflected by collisions with the atoms of the material. The number of reflected gamma rays is statistically related to the density of atoms in the material surrounding the source.
Equipment and Operation
The equipment purchased by Chino Mines Division is slightly modified oil field logging equipment manufactured by Gearhart-Owen, Inc., Fort Worth, Texas. Components of the system are:
- a. Downhole detectors and sources;
b. Surface electronic analyzer and power supply;
c. Hoist and transportation.
The surface electronics are mounted on a Widco Model X electrohydraulic hoist unit with measuring sheave. The entire system has been fitted into a suburban truck, forming a self-contained and very mobile unit.
The unit can be used with other downhole instruments to provide caliper, tracer, and water flow logs, and to perforate the casing. The system is compatible with open hole or electric logging, although significant modification would be required for a complete range of determinations.
Chino Mines Division has drilled and cased approximately 100 drill holes in waste dumps as a part of the study of leaching processes. The drill cuttings were continuously sampled; the samples assayed, and reference samples of the cuttings retained for future examination. Logging in these drill holes has provided a great deal of information on the physical aspects of leaching.
First efforts in logging were directed toward location of leach solution within the dump. Zones of high moisture content are clearly shown. The existence of solutions in the indicated zones was confirmed by perforating the casing and recovering solutions with a bailer.
Causes of Non-uniform Distribution of Solutions
- a. Concentration of Clay Minerals
- b. Compaction of Dump Materials
- c. Iron Precipitates
Studies of solutions in the dump were initiated to determine the contact between leach solutions and copper-bearing material. A hole was drilled in the middle of a production leaching pond and logged to determine the initial moisture content. Water was then turned into the pond, and the hole was periodically relogged to determine the change in water content. The results are shown in Figure VI.