Pulp Carbon

A very important development, within the use of the activatedcarbon, is the process of Carbon in Pulp or CIP, which consists, as its name indicates in performing the contact directly of the carbon with the lixiviated pulp. The first commercial application, in a small scale, dates beck to the year 1954, but it is only in the year 1973 when it has its optimized application in the plant of Lead of Homestake in South Dakota, previously to this date, Mintek of South Africa performed important betterings.

In this present time, around 70 plants like this exist worldwide which use this method.

The typical scheme of the CIP process includes the cyanidation by agitation in 4 to 6 reservoirs vigorously agitated and aired, with grinded mineral at – 100# and with 40 – 45% of solids. The agitator which is most used is the Dorr type with airlift or the pachucas. The small plants use open air agitator reservoirs, and the large ones use agitators with draft tubes. Recent designs, worthy of being mentioned, are the ones introduced by The Mixing Equipment Co., Denver, Mixtech in South Africa and Davy McKee Ltd., which has developed a contact which can use concentrations which are higher than the ones of carbon, in countercurrent with the pulp. Afterwards the contact must separate the carbon from the pulp to desorb the precious metals.

The critical aspects of the CIP process are:

• a) the remotion of the virutes and fibers, which cause problems to the sifting of the absorption stage and in the stage of elusion; and
• b) the separation of the carbon from the cyanized pulp.

For the solution of the first problem, there has been a lot of success with the lineal Delkor sifter, more than 50 installations in South Africa and 4 in the United States, instead of the conventional vibratory sifters.

For the sifters of of interstage transferrence important advances have been reached from the first, which were the basket type or external vibratories. The principal betterings have been in the design of sifters submerged in the reservoirs. Among the most successful we can find the horizontal stationary sifters, with mechanical cleansing, Kambalda type, recently introduced in some countries. These sifter Kambalda type are capable of passing 100 – 180 m3/m2/hour and one of 240 m3/m2/hourhas been anounced. Another important aspect is the type of pump to use in the transferrence of the carbon, in minimal conditions of contraction. Some have been derivated from the ones used in the passing of vegetables in the piscicultures.

Due to the fact that the reaction of lixiviation requires more time of residency than the absorption, it is possible to reduce the requirement of equipment in circuit, using the agitated reservoirs both for the cyanidation as for the absorption with carbon, in a simultaneous form. This is the process called Carbon in Leach or CIL. it is effectuated in a similar form to the CIP, moving the carbon and the pulp in countercurrent. The largest CIL plant in the world is the East Brand Gold and Uranium Co. Ltd. (ergo) plant which works with nearly 20 million metric tons per year of rewashings in the two circuits of CIL.

Both in the processes CIP as in the CIL the regeneration of carbon is essential. It requires to remove the deposits of carbonate of calcium through acid washing and the thermic reactivation to remove the organic species absorbed. One of the biggest problems is the attack to the tubes of the oven, including the stainless steels when the carbon has absorbed sulphurs. A recent solution is the superficial treatment of the steel with the aluminum. The traditional reactors are the rotational horizontal ovens. Presently there exists a vertical electric oven of a lower cost, operating in South Africa since 1986. various designs exist and they continue bettering in this line.