March/April 2009

Metallurgy

The strategic mineral that became an industrial nuisance (Part 2)

By F. Habashi

A history of the changing fortunes of pyrite II

The recovery of elemental sulphur from pyrite

It was realized long ago that a method to get elemental sulphur from pyrite would be ideal for acid manufacturers because it would yield great savings in transportation costs. In the period between the two World Wars, the introduction of flotation technology to concentrate pyrite contributed to the necessity to devise such a method.

Elemental sulphur can be obtained from pyrite by heating in a controlled amount of oxygen such that the heat required by the endothermic reaction

FeS2 Æ FeS + S

is compensated by the heat generated from the reaction:

3 FeS + 5 O2 Æ Fe3O4 + 3 SO2

Ferrous sulphide was obtained as a molten phase. A plant based on this idea was designed by Noranda in 1954 and another similar one operated in Finland for some years.

Heating in a sulphur dioxide atmosphere leads to elemental sulphur formation as follows:

3 FeS2 + 2 SO2 Æ Fe3O4 + 8 S

The reaction is ideal since required sulphur dioxide can be generated from part of the sulphur recovered. The reaction was studied extensively by Duisburger Kupferhütte in Germany, who found that the reaction kinetics were very slow.

Heating in a chlorine-oxygen atmosphere also leads to the formation of elemental sulphur as follows:

2 FeS2 + 2 Cl2 Æ 2 FeCl2 + 4 S
2 FeCl2 + 3⁄2 O2 Æ Fe2O3 + 2 Cl2

Based on this process, a plant was operated in Niagara Falls, Canada in 1936 using two rotary kilns. Named after its inventors, this was called the Comstock-Wescott process.

Pyrite and the steel industry

Noting the success of Duisburger Kupferhütte, European, Japanese and American producers of sulphuric acid who used pyrite as a source of sulphur dioxide became interested in marketing the iron oxide they generated to the steel industry. But, the presence small amounts of sulphur and some nonferrous metals made it unsuitable for steel industry use. Although the iron oxide pelletization process had been invented in Sweden in 1912, it was not used in the iron ore industry until 30 years later, when the need for pelletized feed arose with the introduction of the electric arc furnace. This gave an incentive  to sulphuric acid manufacturers to upgrade their pyrite cinder for the steel industry. As a result, more processes were developed to deal with this problem.

Kowa-Seiko Process: In this process developed at Kitakyushu, Japan, cinder was mixed with calcium chloride, pelletized and heated in a rotary kiln at 1,100 degrees Celsius to volatilize nonferrous metal chlorides. These were scrubbed in water from the exit gases and the solution treated for metal recovery. The process was also used in Portugal, among other countries.

Bethlehem Steel Process: At the Sparrows Point plant in Maryland, cobalt from the cinder was recovered by this process, based on careful temperature control during the roasting of pyrite. If the temperature was kept at 550 degrees Celsius, cobalt in the pyrite was converted to sulphate, which was leached directly from the cinder with water. The hot pyrite cinder was quenched with water to obtain a slurry con­taining six to eight per cent solids. When the solids were filtered off, the solu­tion containing 20 to 25 g/L Co was processed further for metal recovery. In the cinder, the iron:cobalt ratio was 50:1; in solution it was 1:1. This plant supplied the only domestic source of cobalt in the United States.

Outokumpu Process: Developed at the Outokumpu plant in Pori, Finland, in 1979, in this process the sulphated pyrite cinder contained 0.8 to 0.9 per cent cobalt and other nonferrous metals. It was leached with water to get a solution with a pH of 1.5 analyzing 20 g/L cobalt, 6 to 8 g/L nickel, 7 to 8 g/L copper, 10 to 12 g/L zinc, and trace amounts of iron, which was treated for metal recovery.

The decline of the pyrite industry

During the Napoleonic wars, Spanish pyrite competed with Sicilian sulphur in many markets. The production of pyrite, however, declined gradually towards the end of the 19th century after the discovery of an economic method for the recovery of sulphur from the sulphur domes in the Gulf of Mexico. The great rise in petroleum refining activities after World War II resulted in the emission of large amounts of sulphur-containing refinery gases. The sulphur from these had to be recovered to avoid environmental pollution. Also, the availability of large volumes of natural gas containing hydrogen sulphide at Lacq in southern France in 1950s and in Alberta, Canada, in 1970s contributed to the decline of pyrite demand.

As a sulphur dioxide source for sulphuric acid manufacturing, elemental sulphur came to be preferred over pyrite because of the purity of the gas generated and the elimination of the need for dust recovery equipment in the plant. Sulphur became available using the following processes:

  • Sulphur deposits discovered in the Gulf of Mexico were being exploited economically since 1895 by Herman Frasch (1851-1914), who used superheated water to melt the sulphur and floated it to the surface by compressed air. The Farsch process is useful only when sulphur is stratified in impervious rock formation, which was not the case in Sicily. The success of Frasch’s process ruined the Sicilian sulphur industry.

  • Hydrogen sulphide-containing natural gas or petroleum refining gases became an important source of elemental sulphur. Recovering sulphur from them also solved an environmental problem — the large amounts of sulphur dioxide generated by the burning of these gases. Hydrogen sulphide must first be separated from the gases by an absorption-desorption process. It is then oxidized by a controlled amount of oxygen at 400 degrees Celsius on an aluminum oxide or bauxite bed, using the Claus reaction, which was discovered in 1883 by German chemist Carl Friedrich Claus:

    H2S + 1⁄2 O2 Æ S + H2O

Liquid sulphur produced by this process contains some dissolved hydrogen sulphide, which presents odour problems and potential toxic and/or explosive hazards during storage and transportation. Degasification is carried out in a pressurized vertical vessel that counter-currently contacts liquid sulphur with pressurized air at a controlled temperature to accelerate the oxidation of the residual hydrogen sulphide and polysulphides (H2Sx) to sulphur.

As soon as these new sources of elemental sulphur became available, pyrite roasters for sulphuric acid manufacture were dismantled and replaced by sulphur burners and the decline in pyrite production was accelerated. Acid plants based on pyrite roasting were expensive because they included bulky equipment for dust separation.

Pyrite and the gold industry

A problem in gold metallurgy was the treatment of gold that, being locked up in pyrite or arsenopyrite crystals, was unresponsive to cyanidation. Roasting followed by leaching is an expensive and polluting proposition but is used under certain circumstances. In 1971, researchers at the erstwhile U.S. Bureau of Mines developed an aqueous chlorination process in which the oxidizing conditions in situ can be achieved by passing an electric current in a pulp prepared from a solution of finely ground ore and sodium chloride. The sodium hypochlorite generated oxidizes the pyrite (as well as organic matter) to sulphate.

Pressure leaching

A hydrometallurgical approach was developed in 1985 and proved to be a successful solution for this type of ore. Bar­rick Goldstrike now treats a pyrite ore-water slurry in autoclaves at high temperature and oxygen pres­sure. Horizontal autoclaves are used, each 30 metres long and five metres in diameter, operating at 160 to 180 degrees Celsius and 2,000 kilopascals, with a retention time of 20 minutes. The autoclaves, made of eight centimetre-thick car­bon steel, are lined with a six-millimetre lead membrane and two layers of acid-resisting brick of 22.5 centimetres total thickness. After this treatment, the ore is suitable for cyanidation. Table 1 gives data on some aqueous oxidation operations for refractory gold ores.

 

Bacterial leaching

Bacterial leaching has been successfully applied for heap leaching of copper ores. It was extended in 1986 to treat auriferous pyrite concentrates to liberate gold and render it amenable to cyanidation by a process known as BIOX.

Pyrite and the coal industry

One of the problems of using coal as a fuel is the presence of small amounts of pyrite in the coal, which on combustion results in sulphur dioxide in the stack gases. One way to solve this problem is by scrubbing the gases to remove sulphur dioxide in a variety of forms, e.g. as gypsum (CaSO4.2H2O), which may be marketed to the construction industry. Another option is to upgrade the coal by removing the pyrite prior to marketing it. Standard mineral beneficiation methods were applied with some success. Bacterial leaching of pyrite was also tried by many researchers but it was a slow process. Also, because of the need to supply air to the bacteria, an appreciable amount of coal lost its calorific value.

Two more technologies — gasification and liquefaction — can help solve the problem. In coal gasification, pyrite forms sulphur-containing gases which can be scrubbed to yield a clean gaseous fuel. In coal liquefaction using hydrogen under pressure, coal is transformed into a liquid fuel while pyrite forms hydrogen sulphide, which can be separated and recovered in form of elemental sulphur.

The current problems posed by pyrite

Tailings from beneficiation processes represent a large disposal problem because the aqueous oxidation on exposure to weathering conditions of the present pyrite generates sulphuric acid:

FeS2 + H2O + 7⁄2 O2 Æ FeSO4 + H2SO4

The formation of acid is accelerated by the presence of microorganisms such as thiobacillus ferrooxidans. The acid generated will solubilize other minerals thus releasing metal ions in solution. As a result, terrains measuring hundreds of hectares must be prepared to stockpile the tailings, either dry or under water. Precautions must be taken to avoid the breakage of the dams, leaks, seepage to under­ground water, etc. Thus, large costs have to be incurred to monitor, maintain and re-vegetate the ponds. A research group known as “Mine Drainage” was founded in 1975 at the Canada Centre for Mineral Technology in Ottawa to research solutions to this problem.

In most cases, pyrite is associated with arsenopyrite (FeAsS). In the roasting process, volatile arsenic oxide is formed and collected in the dust-collecting chambers. This creates significant disposal-related problems. When pyrite is treated by a hydrometallurgical process, arsenic is precipitated as ferric arsenate for disposal.

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