Real Del Monte Finds: Low Base Metal-High Silver Ores Give Better Smelter Returns With Pre-Cyanidation Treatment

SINCE the first applications of cyanidation to silver ores about 1906, treatment of ores in the Pachuca district has been entirely by straight cyanidation. Until about the year 1921, Real del Monte removed a sulphide concentrate (largely pyrite) by tabling in the cyanidation grinding circuit. The ores milled prior to the year 1935 were dry ores, ideal for cyanidation, and yielded 90 pct of the silver and 92 pct of the gold. Since 1934 there has been a notable increase in the refractivity to cyanidation of the average ores received by the mill. Tonnages of the former amenable ores have gradually been replaced by tonnages of more refractory ones, so that present average recoveries have fallen to 83 pct of the silver and 90 pct of the gold. During this period there has also been an increase in the content of base metal sulphides' in many of the ores although this increase is not always directly related to the increased refractivity. Since March 1950, the Cia. de Real del Monte has been developing a flotation treatment for some-of its ores before cyanidation. Suitability for flotation is judged not only by the base metal content, but also by the amount of additional recovery of silver and gold made possible by the combined treatment. Selection and segregation of ores most suitable for flotation before cyanidation is a difficult problem for the mines, and the best that can be done is a compromise that leaves much to be desired. Development of Pre-cyanidation Flotation at Real del Monte Much laboratory work has been done on flotation of ores from the Pachuca district, however, the data could not supply reliable information on the many factors connected with the economics of low-base metal-high silver ores. Pilot plant operations would be required. Pilot plant facilities were made available in March 1950, when it became necessary to close down Loreto's 100 ton per day selective flotation plant because of exhausted ore supply. The results of the 100-ton pilot plant tests indicated that a large part of our tonnage of cyanidation ores could be profitably treated by flotation, and a tonnage of 1100 tons per day was selected as the maximum tonnage of ore that the mines were likely to deliver, and because this tonnage fitted with the grinding capacity of this circuit. Equipment was ordered for the production of a single flotation concentrate. The conception of flotation before cyanidation, at that time, was the production of a single concentrate to be shipped to the lead smelter. The 100-ton pilot plant was immediately increased to 350 tons by installing additional flotation cells.. This. plant was operated continuously until the 1100ton plant was put in operation in January 1951. In July 1950, an increase in the quotation for zinc called for an investigation- of the possibilities of producing a zinc concentrate, and laboratory tests on the tailings from the 350-ton plant indicated that a