CIL (Carbon in Leach) process is the most efficient method to leaching and recovering gold from gold ore, The most protruding features is that leaching and adsorption are carried out at the same time, which greatly saves the cost of construction and operation. It has become the first choice of modern gold mining and processing plant design.
CIL process is widely used in the newly built leaching plants recent years, and a lot of plants are changing into CIL as well.CIL process is suitable for the treatment of oxidized gold ore with low sulfur content and mud content. It is unsuitable for the gold ore with high grade silver. Generally, the proportion of gold and silver should not exceed 1:5. The best conditions for CIL process in China: PH=10-12, sodium cyanide concentration is not less than 0.015%, the particle size of activated carbon is 1.0-3.35mm, the type of carbon is coconut shell activated carbon, and the slurry concentration is 40-45%.
CIL Plant Consists of Following Steps:
1. Crushing and Screening
After the crushing and screening by the crusher and vibrating screen, the bulk ore （up to 650 mm） can be broken to small size （about 10 mm or even smaller).
Usually, small or medium scale mineral processing plants can meet the requirements after two stages of crushing and one stage of screening. However, large scale mineral processing plants need three or even four stages of crushing to get qualified products.
2. Grinding and Classification
After grinding and classification, the ore material can meet the requirements of leaching (85~95% through 200 mesh).
In the classical process, the grate ball mill and the spiral classifier constitute the first stage grinding and classification unit, and the overflow ball mill and the hydro-cyclone constitute the second stage grinding and grading unit.
If the ore contains some particle gold, the gravity separation process should also be set in the first stage of grinding and grading unit.
The overflow slurry of classification is only 18-25%, if directly leaching will result in excessive energy cost and reagent consumption. Therefore, thickening is a necessary step. In order to accelerate the settlement, a certain amount of flocculants will be added to enhance the thickening effect.
The overflow will flow to recycle water system, and the underflow will be pumped to the leaching tank.
4. Leaching and Adsorption
Leaching and absorption are simultaneous is the most outstanding feature of CIL process. Generally CIL has 6-10 stages of tanks. The sodium cyanide(or some other leaching reagent) solution is added into the first tank, and gold starts to be leached, most CIL plant will add the activated carbon from the second tank to the last one. Carbon segregation screen is set in every absorption tank’s outlet to separate the carbon and slurry. Slurry flow automatically from the first tank to the last one based on the height difference of each stage of tank, while the activated carbon flow reversely through the air lifter. It means that the activated carbon is put into the slurry at the last absorption tank and gold-loaded activated carbon is taken out at the first absorption tank. After screening and washing, the gold-loaded activated carbon will be sent to the desorption operation. After absorption, the gold grade in the tailing solution will reduce to 0.01-0.03g/m³.
5. Desorption of Gold-loaded Carbon and Electro-deposition
Gold-loaded carbon and slurry are lifted by carbon pump or air-lifter to separation sieve (the most popular sieve is liner vibrating sieve). Gold-loaded carbon and slurry are separated by water on the sieve surface, and then gold-loaded carbon is sent to carbon storage tank and the slurry and water flow into the first absorption tank.
There are several desorption methods. In China, the general application methods are Zadra desorption method, high temperature and high pressure desorption method (hereinafter referred to as HTHP method) and integral pressure desorption method (hereinafter referred to as IP method).