The very survival of the U.S. copper industry is presently tied to the leach SX-EW process, a technology which 15-20 years ago was considered too radical and far fetched to be accepted by the industry. It came to life rather dramatically with the Ranchers Bluebird Mine and shortly thereafter with the Cyprus Bagdad plant. All of a sudden, it was a viable technology and as we have seen has developed into an excellent technology.
In order to understand the design of a tankhouse we have to start with same basic premises. The first is that Faraday’s Law will have a role. The law can be stated as
Wt = I’ t’Mβ/Z’F
Where Wt is the wt of copper produced in a period of time
I is the current in amps
t is the time in seconds
M is the molecular wt
Z is the valence
F is the Faraday
β is the current efficiency
The most significant feature to be included in the Bagdad tankhouse was the interlocking capping block used to provide positive electrode spacing and separation . This capping block or a version of it has been used in every plant since, and while it was not original equipment, it was eventually installed in the Ranchers plant.
This capping block system has done its job well but has suffered from being fragile, and once damaged, difficult to replace. These faults have led to an improved design known as the Dremco Capping Board system. In this system, a base board with vertical extensions rests on the cell wall and holds the buffer sheets in place. Individual, virtually indestructible blocks key into place on the base boards to position the electrodes and bus bar.
The early SX plants adopted the use of starter sheets grown on a stainless steel mother blank as used in refineries. The most notable feature of the Bagdad plant cathode was the use of long loops.
Anodes can be distinguished by their composition and their shape. The first plants used antimonial lead because of its favorable experience in direct EW plants. Unfortunately, Pb-Sb could not tolerate the organic entrainment of the early plants and so was replaced with Ca-Pb. Despite the difficulties in casting the calcium alloy, anodes made from it gave good service; low cathode lead and long life. Casting problems which developed have generally accounted for failures of 10-20% of the entire plant inventory.
If there is a distasteful feature of an EW plant, it is the obnoxious acid mist. Fortunately the mist can be minimized with floating plastic balls.
Copper plants will follow the pattern set in the zinc industry, where electrodes are pulled by computer operated cranes and taken to automated stripping machines.
One of the basic premises mentioned at the start of the paper was current density. Present day technology is based on 15 ASF operation and it is apparent that this value is used because it is safe. In fact, it is not really a very conservative value.
By installing an air agitation system, it is possible to get the iL up to 400 ASF so good deposits can readily be obtained at 60 or 100 ASF since both are less than 40% of iL. In fact the process is less critical at 60 or 100 ASF with air agitation than the present plants are at 15 ASF.