Purified and roasted
Over the course of just five years, Molymet has built a modern plant for processing molybdenum ore concentrate.
The 30-hectare site of its subsidiary Molynor in Mejillones is located in the desert in the north of Chile, right on the Pacific coast, around 70 kilometers from the city of Antofagasta. "To satisfy the global demand for molybdenum, we built this plant in the heart of the largest mining region of Chile and southern Peru," explains John Graell, Executive Chairman of Molymet.
During the initial phase, the new plant had the capacity to produce around 15,000 tonnes of molybdenum per year, and this capacity was quickly doubled. To achieve this, Molymet invested in a second roasting plant, increasing the efficiency of its sulfur dioxide separation plant, as well as in a second module for doubling desalination plant capacity, while also enlarging the related auxiliary facilities.
A cleaning plant for the molybdenum ore concentrate was also built. This removes impurities such as copper, lead and calcium from the molybdenite concentrate prior to roasting.
Tidiness, cleanliness and safety: Guiding principles during construction of the new molybdenum production plant.
From ore to concentrate
To obtain molybdenite, the mined molybdenum ore is first crushed to form material the size of gravel, with a large part of the gangue (worthless rock) being separated out. This material is then ground to form a coarse powder. Flotation separates the less dense molybdenum ore from the denser gangue to give up to 90-percent pure molybdenite.
Cleaning the concentrate
Because the molybdenum ore concentrates processed by Molynor can sometimes contain a relatively high proportion of lead, a leaching process using ferric chloride is ideal for dissolving not only the lead, but also sodium and copper from the ore concentrate.
This is done by slurrying the molybdenite concentrate with ferric chloride, hydrochloric acid and water and heating it for some time up to 120°C in a glass-lined reactor under pressure and constant stirring. When the slurry has cooled and been filtered, the result is the cleaned molybdenite and a ferrous solution containing the dissolved impurities, primarily copper. This ferrous solution is then further processed to produce such things as cement copper for use in producing copper matte and additives for suppressing dust in highway engineering. The majority, however, is conditioned and reused for cleaning the molybdenite.
Because leaching with ferric chloride is an extremely corrosive and dangerous process, Molymet took considerable care over safety at work and protection of the environment when designing the processes, plant, piping and metering.
Roasting the molybdenite
Molybdenite is roasted in multi-level hearth furnaces. Roasting in air at high temperatures between 500 and 650 °C oxidizes the primary component, molybdenum sulfide (MoS2), to form technical molybdenum oxide (MoO3). Multi-level furnaces are made up of steel shell clad with refractory bricks and multi fire-resistant hearths. During the roasting process, the molybdenite passes through the furnace from top to bottom against a strong air current. Rotary rakes move the molybdenite on the hearths floors to promote the chemical reaction.
The effluent gases from the roaster are cooled, dust is electrostatically precipitated and the gases are scrubbed to clean them before final treatment in a sulfuric acid plant to obtain up to 170 tonnes of sulfuric acid per day from the sulfur oxides they contain.
One important by-product of roasting is rhenium oxide, which is present in some of the molybdenite concentrate in concentrations of less than 0.1 percent.
The technical molybdenum oxide contains at least 57 percent molybdenum and is used in the steel industry, primarily for ferromolybdenum. A small proportion is cleaned either thermally or chemically and ultimately reduced to molybdenum metal as pure oxide.