For many years the gold industry has employed the rotary kiln to reactivate activated carbon. With the years, some others kilns have appeared in the market to perform the regeneration process. One of them is the multi-tube kiln, which is a good option for small gold operations where small budgets are important. Basically, kilns can be heated by gas or fuel oil, but when electric power is available such as some places in Canada and the States, kilns are electrically heated. When there are high inventories of activated carbon such as 6 t, one or two rotary kilns are employed and cover any necessity. In South Africa, some gold operations employ the Rintoul kiln that uses the resistive heating produced when an electric current is passed through a packed bed of activated carbon. This kiln requires a pre-drier stage to eliminate partially the moisture and can process approximately 3 t batches of carbon. Other type of kiln employed in South Africa is a counter current apparatus that uses superheated steam to dry and heat activated carbon in a semi fluidized bed to around 750-800 oC.
The capital and maintenance cost of rotary kilns are usually high and some losses of activated carbon are expected. When the Carbon-in-Pulp process was adopted by several gold operations, it was noted the importance of frequent reactivation. This consideration led to a search for cheaper less aggressive reactivation kilns. Some types of kilns are indirectly heated by gas, fuel oil or electrical systems; and the retort is a variety of stainless steel or high temperature alloy. Vertical kilns developed for reactivation uses especial alloys. Lurgi designed a fluidized bed kiln and Shirco developed a moving grate furnace. Thermex FBR developed a kiln that uses superheated steam that heats the activated carbon, fluidized the bed, removes char and provides relatively inert atmosphere.
The addition of steam at the reactivation temperature promotes high activation of the activated carbon. Normally the steam is introduced at the discharge end of the kiln. In vertical multi-tube kilns, water injection in the discharge section help to cool the activated carbon before it contacts the air and the resulting steam percolates upwards through the hot descending carbon. It is important that only dry superheated steam be employed with Rintoul kiln to reduce any change in conductivity of the packed activated carbon bed brought about by excess moisture. It is recommended that acid washing only be performed on loaded activated carbon to reduce and minimize chloride corrosion of vertical kilns with metal tubes or rotary shell liners. Eluted carbon must be dewatered thoroughly in order to ensure effective kiln operation in terms of carbon reactivation temperature and throughput. Identification of the type and concentration of adsorbed contaminants and the most appropriate treatment methods can be employed using several analytical procedures.
Effect of temperature and steam on reactivation