With the growing environmental pressures on the use of cyanide in certain regions and the unsuitability of cyanide for certain complex ores, various reagents have been evaluated. Some of these alternative reagents are summarized in Table 4.
It appears that complexation of copper with ammonia lowers the consumption of CK Similarly, present work on leaching copper-molybdenum sulfides indicates that carbonate also complexes with copper, which depresses the amount of copper leached with CI2. This may have applications to copper-gold ores. Then there are various other processes including the K-process using undisclosed reagents However chlorine and thiourea have received the most attention and have specific applications.
As shown, the advantage of chlorine and thiourea is that they dissolve gold much faster than CN- and sometimes give higher recoveries; but unfortunately greater reagent quantities are often required (Table 5).
The problem with chlorine is that it reacts with sulfide minerals leading to high CI2 consumption and acid production (equation 3).
With thiourea, it is the dimerisation and degradation of thiourea that leads to high consumption (equation 4). With other reagents there is the problem of cost, or stability, or how gold is recovered.
Of all the reagents examined so far, thiourea offers the best prospects for the difficult ores which cannot use cyanide. It is particularly suited to the acidic residues from bacterial leaching or pressure leaching of refractory sulfides because it complexes gold in acid media. A recent examination of a number of Australian oxidised gold ores identified some other promising applications. Furthermore, over the last 3 years there have been developments which could make thiourea even more attractive. Recent work has snown that SO2 inhibits dimerisat’on and degradation of thiourea by controlling the Eh and that a modified thiourea is even more stable. Thus certain types of ore which consume base or CN- could be economically treated with modified thiourea reagents. Furtner research in this area is currently being undertaken.
In the past few years much progress has been made in understanding and developing the C.I.P. process. However, there is still much fundamental worK to be aone ana mprovements to be made with the treatment of refractory ores. No two ores are exactly alike and each needs to be evaluated with a range of options at our disposal.
By understanding the characteristics and properties of carbon, its activity and performance has been improved. Whilst no immediate threat to current C.I.P. practice exists, alternatives to carbon and alternatives to CN- may develop in a few years time.