by Dave Neibauer, Quadna

Engineering got underway for the modern Cresson Mine, located between Cripple Creek and Victor, Colo., in 1993. (The historic underground mining operation for the Cresson Mine was first developed in the 1890s.) Today, rock containing gold is removed from the mine and hauled to a crushing facility. Once crushed, the ore is taken to a fully lined valley leach facility where a dilute sodium cyanide process solution dissolves the gold. The solution is captured in the bottom of the lined leach valley and piped into the plant. The gold is recovered through a carbon absorption process before being smelted into a gold button called dore (pronounced doh-rey), or semi-purified gold.

The Cresson Mine carbon absorption process requires two trains of five tanks each (a train is a set of five tanks). The tanks are carbon beds in which the gold cyanide solution from the leach valley is absorbed into active carbon. The gold must be recovered by redissolving it from the activated carbon (elution) through an acid system and/or a hot concentrated cyanide solution, followed by electroplating the mixture, which ultimately extracts the cyanide and leaves gold cake. The cake, which resembles potting soil with tiny flecks of gold, is placed into a smelter and, approximately 20 minutes later, is poured as gold dore. The cooled resulting "button," nearly 75 percent pure gold and weighing the same as the potting soil-like cake that went into the fire, is now ready for the refinery.

Angled piping is used to transfer carbon to the plant's A & B train tanks.

Only one part of the process, carbon absorption, requires several stages. These stages occur as the cyanide process solution, pregnant with gold, is pumped through the two trains of five tanks each.

Getting There From Here

An innovative, efficient solution was needed to improve the carbon absorption process. Cripple Creek previously used 10 sump pumps to carry the pregnant solution from one tank to the next. The mine, however, had once experimented with a Hydrastall pump and found it to be far more efficient. The new pump caused less damage to the carbon in the solution, allowing for increased carbon reuse. It was also more efficient, saving valuable time. However, there was not enough room to install 10 pumps to replace the sump pumps. 

Figure 2Pumps #2 and #3 are centered on concrete pads, within the plant.

We looked at the configuration and decided a solid solution was possible. By using one pump for two sets of tanks, the team was able to cut the number of pumps needed from 10 to five. However, Y piping that was needed to feed both sets of tanks from one pump required that the new pumps be staggered and set onto their own foundations. In addition, a special platform had to be created to raise the operator's instrumentation to a level above the pump platform and create more space. With this configuration, the enhancement would provide for safer and more convenient access to larger control valves.

Figure 3Dave Neibauer holds an 80-point gold dore.

Our team knew the new equipment would fit in the small space available. To install the components and perform welding and assembly, several team members had to use full body harnesses so they could be dangled from the ceiling. Working with the mine's electrical supervisor Jody Keel, we installed control boxes with each new pump to enable operators to switch the pumps from one five-tank train to the other five-tank train.

Figure 4Start location of the gold ore leach pad.

This solution cut the carbon fine transfer time at the Cripple Creek plant from 30 to 15 minutes. The enhanced system also saves Cripple Creek money because it decreases the amount of carbon destroyed in the pumping process.