The Climax ore body is basically a sulfide deposit capped by an oxidized zone. No secondary enrichment is present. As oxidation occurred the molybdenum from the molybdenite immediately reprecipitated in place with the iron minerals. The molybdenum-bearing minerals in the oxidized zone were identified as geothite, jarosite, ferrimolybdite, and a molybdenum isomorph of the wolframite-ferberite series. The goethite, containing from 1.3 to 9.6 percent molybdenum, is, the most important of these minerals. It is estimated that the goethite accounts for approximately 92 percent of the non-sulfide molybdenum in the oxidized zone.
The slimes containing 0.25 to 0.35 percent molybdenum after being pumped to the leach were screened on a vibrating screen at 35 mesh to remove wood chips. The slimes were then stored in a repulp tank that allowed one half hour surge capacity. The leach operator by watching the level in this surge tank could adjust leach feed rate and reagent rate to keep up with the thickener.
In leaching the sulfuric-sulfurous acid solubilizes the hydrated ferric oxide, releasing the molybdenum. Under the conditions of leaching, the molybdenum forms “molybdenum blue”, a molybdenyl molybdate. The composition of this compound is complicated by the fact that the anion part of it, the molybdate, can polymerize into a range of species depending upon the pH of the solution.
The pH of the slurry was adjusted to between 1.0 and 1.2 with sulfuric acid. Originally the residual sulfur dioxide concentration in the last tank was held at between 10 to 12 grams per liter, but after the sulfur dioxide recovery circuit was shut down, the residual sulfur dioxide concentration was dropped to one gram per liter without any noticeable loss in recovery.
An essential step in the process as conceived was the desorption and recovery of sulfur dioxide from the leach tails. In the initial test work a residual concentration after leaching of ten to twelve grams of sulfur dioxide per liter of solution was considered necessary.
After desorption the residual sulfur dioxide was removed from the pulp by blowing it with air. Two wood tanks twelve feet in diameter by twelve feet high were used in series. Air was introduced in a sparge ring located below the impellers. The combined retention time in the two tanks was thirty minutes. The air flow was 1000 scfra at 25 psia. Blowing tended to oxidize the pulp and the E.M.F. was changed from minus 200 to a minus 270.
Part of the overflow from the last co-current adsorption tank was screened on a 4 feet by 10 feet vibrating screen. The oversize, the charcoal, was washed on the lower four feet of the screen deck. The undersize from the first four feet of screen deck was returned by gravity flow to the first counter-current adsorption tank. The underflow from the wash section was sent to a thickener. The thickener underflow was pumped to the first counter-current adsorption tank. The thickener overflow was re-used as wash solution. The wash charcoal was fed by gravity into one of two surge tanks. These surge tanks would hold 3.8 tons of charcoal.
The charcoal stripping was automated, but in rare cases could be run manually. Manual control was not easy, since it involved the off-on positioning of about 120 valve. When one of the surge tanks became full it would set off the automatic controls. The six stripping column were operated in a manner similar to that normally used in column ion exchange. The char was brought into contact with an ammonia-air mixture in the columns. The amnonia neutralized the acid and the air oxidized the molybdenyl molybdate.
The charcoal tended to become “poisoned” with continued use. This poisoning would decrease the charcoal efficiency to adsorb a given amount of molybdenum in a given period of time. The exact cause or nature of the “poisoning” was never determined. It did not appear to be due to the loading of organics on the charcoal. It may have been due to the loading of sulfur compounds on the charcoal, but efforts to identify them failed. Another theory was that the pores became blocked with slimes or inorganics.
Evaporation and Crystallization
A standard Struthers-Wells type evaporator and crystallizer were used to recover the molybdenum from the solution. About one half of the water was removed in the evaporation step.
The feed to the evaporator contained 60 to 70 grams per liter molybdenum and 6 to 7 grams per liter sulfur. Most of the sulfur was present as ammonium sulfate, but a small amount was present as reduced sulfur compounds.
In the crystallizer the concentration of sulfur was maintained at about 90 grams per liter. At this high concentration of ammonium sulfate the solubility of molybdenum was reduced from 220 grams per liter to 90 grams per liter. Under the above operating conditions about 90 percent of the molybdenum in the feed was crystallized out as ammonium dimolybdate with no crystallization of ammonium sulfate.
The crystals from the crystallizer and from the acid precipitation step were filtered on two separate belt filters. The crystallizer belt filter was one foot wide by 10 feet long. The filtrate from the crystallizer belt filter was either returned to the crystallizer or bled off to the acid precipitation step.
The acid precipitation step filter was one foot wide by four feet long. The filtrate from this filter was returned to the leach. In the original flowsheet installed in the plant, the filtrate from the acid precipitation was sent to solvent extraction. It was feared that this liquor might contain compounds that would poison the char, but this proved not to be the case. An amine solvent extraction circuit was built into the plant, but was never used.
The two cakes from the filters were combined in a bin ahead of the calciner. Very little washing of the cakes was possible so that the solution contained in the cakes to the calciner were high in ammonium sulfate.