Some copper deposits presents a small amount of fluorine associated with copper sulfide ores. Depending on the initial fluorine content and the mineralogical characteristics of the ore, the fluorine may report to the copper concentrate as an impurity. Fluorine levels of more than 200 ppm are subject to penalties in their commercialization.
The presence of fluorine in copper concentrates is usually harmful to sulfuric acid plants attached to copper smelters (Beer, 1992). Fluorine can deteriorate acid proof tiles, fiberglass pipes, packing and catalyser substrate which contains SiO2. It can also damage, to a less extent, cast iron pipes.
During the smelting process of copper concentrates, almost all the contained fluorine volatizes as HF and other compounds. Usually, most of the gases generated by the smelter are collected and sent to a sulfuric acid plant, since these gases contain mainly SO2. The sulfuric acid plant has three basic systems:
- gas washing and purification;
- SO2 catalytic converter;
- acid system.
The sulfur dioxide is reacted in the converter, generating sulfur trioxide by the catalytic action of vanadium pentoxide. Is in this stage that the damage due to fluorine is more serious, since it destroys the catalyser substrate, which is composed mainly by silicon compounds.
To cope with this problem, some plants have a sub-system to remove fluorine from its gas washing section.
The gases are fed to packed towers with silica packaging (rings) that react with HF producing HSiF6 and SiF4. These compounds are dissolved with water that flows counter-currently to the gases.
The custom smelters which buy concentrates for smelting and refining usually use a blending of different concentrates in order to obtain a suitable feed for the plant. The maximum acceptable fluorine content for this concentrate mixture is about 500 ppm. Penalties of about US$ 0,09/t are applied for each 10 ppm of fluorine that exceeds 200 ppm, to the limit of 1000 ppm. Concentrates with more than 1000 ppm are usually difficult to sell.
There are two main alternatives to the problem of fluorine in copper concentrates:
- changes in sulfuric acid plants to treat high fluorine copper concentrates, considering the options of building a new smelter of modifying a third-part smelter;
- Removal of fluorine from copper concentrates, in order to have a “clean” product which doesn’t have market restrictions.
An alternative is often tried by depressing the fluorine bearing minerals in flotation, but is possible only if fluorite is liberated or associated with the gangue. If, however, the fluorite is associated or encapsulated in the copper sulfides, the depressing of fluorine without significant loss of copper is not possible.
A new process was then developed to remove fluorine in concentrates by an hydrometallurgical route. With the fluorine removal process, the fluorine level in a chalcocite/bornite concentrate was reduced from 2215 to 163 ppm.
The fluorine removal process has also the advantage of producing a fraction of the copper as cathode and upgrading the copper concentrate, turning a low-value/high-commercialization-risk product into two high value products.