Blog of Mineral Processing (Metallurgy) Articles

Regeneration of the spent alumina was accomplished by using 0.25% NaOH and 0.5% NaOH. Figure 11 shows the release of selenium, silica, and aluminum when regenerating the adsorption columns that had received the mine leachate. In this case, the regeneration was performed in an upflow mode using 30% bed expansion. As expected, the losses of aluminum during regeneration

increased with increasing NaOH strength. These results indicated that the removal of silica and selenium from the spent material was possible by using a NaOH concentrations as low as 0.25%. More selenium was released from fixed-bed columns receiving synthetic selenite solutions than from columns receiving synthetic solutions. Batch regeneration tests indicated that similar release of selenium could be accomplished with one hour of contact with a 0.25 % NaOH solution.

Pulps of 20% solids is prepared from -200 mesh pyrite concentrate (after froth flotation), and is inoculated with 5% (V/V) of adapted bacterial solution. The bioleaching tests is carried out in 250 ml Erlenmayer flasks, each contained about 220 ml pulp. The pulps is aerated with air bubbling at one liter per minute air flow rate, and is continuously agitated with a magnetic stirrer while the temperature was kept at 30 degrees C.

The bioleaching tests should be started at pH 2.0. The pH left to decrease during the tests, but not less than 1. Partial replacement of the leaching solution with water done by decantation at the rate of 25% by volume per day i.e., about 55 ml/flask/day. Distilled water is added to the leaching solution to compensate for the replaced solution and evaporation. Only partial re-inoculation with 1% (V/V) of adapted bacterial solution every third day is needed to compensate for lost bacteria.

Samples are withdrawn from the replacement leach solutions and analyzed to determine the dissolved metals by a Perkin-Elmer computerized atomic absorption spectrophotometer model 3030. At the end of the bioleaching test, the residues are collected, dried and weighed before use in cyanide leaching.….Read more

Extended freezing conditions exist in many parts of the world in the winter because of latitude or high altitude or both. Such conditions, coupled with the sprinkling techniques commonly used in heap leaching, result in formation of layers of ice which inhibit leaching by tying up solution inventory on heap surfaces and shutting off solution percolation. Loss of leaching availability can be serious, cutting total available leaching time to as little as 8 months per year in many cases. To increase leaching time and precious metal production, several methods have been developed and others can be considered.

Some feel that heating of solutions might provide protection against glazing the heap surface with ice. A submerged combustion heater was tried because of its high thermal efficiency. This approach heated the circulating solutions economically but had the undesirable side effect of dissolving large amounts of carbon dioxide in alkaline liquors. This carbon dioxide in combination with the calcium ion already present made existing pipeline scaling problems much worse. A heat-exchanger can be installed in place of submerged combustion and has been used successfully ever since.

Nevada has pioneered the large scale application of buried drip irrigation for precious metal leaching. The installation was ….Read more

Some gold and silver ores are difficult to successfully heap leach because of the high percentage of clay, and/or fines which are intrinsic in the material or generated by the crushing or ore preparation. This type of ore can result in extremely slow percolation during heap leaching. Channeling can result which promotes dry areas within the heap, leach solutions may run off the sides instead through the bed, and extraction of precious metals is low. Agglomeration of the crushed ore has proven to be the answer in treating certain of these ores. Rotating inclined tubes, concrete mixers, slanted grooved rotating plates or discs, inclined rocking chutes or conveyor belts which move up an incline as the ore tumbles counter currently all have been used to produce competent agglomerates.

Some operators use measured additions of lime, cement or mixtures of these two components to insure permeable agglomerates. At some properties, milk of lime, strong cyanide solution, or barren cyanide liquors are used to supply the required moisture for agglomeration.

In most instances, the key to successful formation of a durable agglomerate is curing. The individual agglomerates require sufficient time to dry and harden. In the tropics, air drying of agglomerates is difficult. Consideration ….Read more

It is feasible to conduct cyanide heap leaching at high altitudes as has been demonstrated by a number of pilot and commercial operations in the altiplano of the Andes Mountains. Operations at elevations in excess of 3000 meters (9800 ft) must be conducted with care to insure that heaps are fully permeable and the lixiviant solution does not become depleted in oxygen during transit through the heap.

There are locations on the west coast of South America, in Australia, and in the California internal desert where fresh water is practically unavailable. It has been found that precious metal dissolution can be accomplished in sea water or in strong brine provided the leaching environment is adjusted to compensate for the buffering characteristics of the brine solvent and equipment is built to stand this corrosive environment. Unprotected mild steel construction cannot be used. Merrill-Crowe zinc dust precipitation is the most logical recovery method for the gold in saline environments since carbon absorption has proven to be ineffective in strong electrolytes. Any data on the successful use of carbon absorption under saline conditions would be welcomed by the authors.

The previous tests demonstrated the use of facultative and extreme thermophiles for refractory sulfide gold ores and flotation concentrates. The following tests were conducted on carbonaceous gold ores to evaluate response to bioleaching.

Facultative Thermophile Bioleaching of Carbonaceous Gold Ore

The facultative thermophile S1I was used to pretreat a carbonaceous gold ore under CSTR conditions. This ore is very active, i.e., has a high preg-robbing potential. It contains approximately 1% marcasite and has a head grade of 13.1 mg/kg gold. Pulps were prepared in mineral salts media at either 5% or 15% pulp density and adjusted to pH 1.6-1.7 with sulfuric acid. Samples were inoculated with starter cultures containing S1I and incubated at 50°C.

Because the ore only contained 2% iron, it was leached fairly rapidly; in two days, 50% of the iron was extracted in the 15% pulp and 75% of the iron was extracted in the 5% pulp (Table 3). For the 15% pulp, gold recovery was increased from 8.8% to only 35.9% following bioleaching with the facultative thermophile. However, when the bioleached residue was cyanided in the presence of activated carbon (CIL), gold recovery increased to 93.6%. This indicates that bioleaching had little effect on the preg-robbing characteristics of the ….Read more

A few representative cycles from the first 20 oxidation/reduction cycles for a pyrite electrode in 0.1 M sodium borate are presented in Figure 3. The first scan was made in the cathodic direction starting from the rest potential of the mineral which typically had a value of 0.18 V. The large reduction peak observed in the first cycle shows the presence of oxidation products on the pyrite surface. These oxidation products are formed during the polishing of the pyrite electrode prior to its insertion into the solution. The anodic current in the first cycle is considerably smaller than the cathodic current which implies that the species reduced during the first cathodic cycle are not formed in the duration of the anodic scan. This is confirmed by a considerable decrease in the current in the second cathodic cycle. The reduction peak potential also shifted to less cathodic potentials. Such a shift in potential is associated with a change in the type of species being reduced.

Several investigators have studied the oxidation of pyrite and proposed that several solid products Fe(OH)3, FeOOH, Fe3O4, Fe2O3, etc., may form (Hiskey ana Schlitt, 1981; Nordstrom, 1982), Reduction of ….Read more

The Gunnison, Colorado, tailings provide an example of the chemical reactions across the interface with soils rich in pyrite but low in calcite. In this case, the retardation of trace components is determined primarily from the Eh controls and less from the pH (Johnson, 1985). The compositions of groundwater from wells in the vicinity of the tailings were used to determine the reactions that occur between the tailings seepage and the groundwater/soil.

The water samples from wells around the Gunnison tailings showed four types of chemically distinct groundwater corresponding to different locations in the aquifer (Table 3). Type 1 groundwaters are from wells upgradient of the tailings and have a near-neutral pH range, a wide range in iron concentrations, and low concentrations of uranium and sulfate. Type 2 groundwaters are from monitoring wells in the upper 15 feet of the aquifer directly downgradient of the tailings. These waters have a pH slightly more acidic than the background waters, a wide range of iron concentrations, and consistently high concentrations of uranium and sulfate. Type 3 groundwaters are from wells in the same location as Type 2 waters, but at a depth of about 45 feet. The uranium and iron concentrations in the ….Read more