Although iron is the fourth most abundant element in the Earth's crust, the vast majority is bound in silicate or, more rarely, carbonate minerals. The thermodynamic barriers to separating pure iron from these minerals are formidable and energy intensive, therefore common sources of iron used by human industry exploit comparatively rarer high-grade iron oxide minerals, primarily hematite. Most reserves of such high-grade ore have now been depleted, leading to development of lower-grade iron ore sources, for example, magnetite and taconite. The iron content of these lower-grade ores may be concentrated (upgraded) to a higher iron content through various concentration (beneficiation) processes, for example, to meet the quality requirements of iron and steel industries.
The processing of lower grade ore sources involves the removal of gangue, which are the unwanted minerals (such as silicates and carbonates) that are an intrinsic part of the ore rock itself. In these beneficiation processes, the gangue is separated using techniques like crushing, grinding, milling, gravity or heavy media separation, screening, magnetic separation, and/or froth flotation to improve the concentration of the desired minerals and remove impurities.
One such beneficiation technique is froth flotation. In froth flotation the ore is ground to a size sufficiently small to liberate the desired mineral or minerals from the gangue. The ground ore is combined with water to generate a slurry containing the mineral particles and the gangue particles. The slurry is then aerated, such as in a tank or column called a flotation cell. Froth flotation physically separates the ground particles based on differences in the ability of air bubbles to selectively adhere to specific mineral surfaces in the slurry. The particles with attached air bubbles are carried to the surface of the slurry, forming a froth that may be removed, while the particles that remain completely wetted stay in the solid/liquid phase.
An additional step that may be utilized in combination with the flotation process involves the removal of the ultra-fine particles by desliming. Ultra-fine particles are generally defined as those less than 5 to 10 microns in diameter. The desliming process may be accompanied by or followed by a flocculation step or some other type of settling step, such as the use of a cyclone separating device. This step is typically followed by a flotation step wherein gangue materials are separated from the desired mineral or minerals in the presence of collectors and/or frothers.
The chemistry of the slurry can be modified to control or enhance how certain particles interact with the bubbles or, alternatively, settle to the bottom. For example, “collectors,” typically surfactants, can be added to the slurry to interact with the surface of certain particles, causing an increase in the surface hydrophobicity of the particle and facilitating flotation. “Depressants” can be added to the slurry to selectively interact with the surface of certain particles to reduce the surface hydrophobicity and inhibit the flotation, i.e., facilitate the depression, of that type of particle.
In mineral flotation systems, it is common to depress or hold down the undesirable gangue materials while floating the desirable mineral or minerals. In differential or reverse flotation systems, it is common to depress or hold down the desired mineral or minerals while floating the undesirable gangue. That is, the normal flotation system is reversed, with the silicate being enriched in the flotate and the iron ore in the bottom fraction. Such reverse froth flotation systems are disclosed in U.S. Pat. No. 4,732,667.
Common depressants include materials derived from natural substances such as gums, dextrins and starches. See U.S. Pat. No. 3,292,780 to Frommer et al., and U.S. Pat. No. 3,371,778 to Iwasaki and U.S. Pat. No. 4,339,331.
Synthetic depressants have been developed for use in the separation of gangue from desirable minerals, for example, as described in U.S. Pat. Nos. 4,360,425 and 4,289,613, 2,740,522, 3,929,629, and 4,808,301.
With the use of depressants in reverse or differential flotation systems, some portion of the desired minerals will inadvertently be removed with the gangue. That portion of the valuable mineral or minerals that is inadvertently removed with the gangue is normally permanently lost from the process and can have a significant economic impact. Even a small decrease in the amount of desired mineral or minerals that are inadvertently floated with the gangue can therefore result in significant economic benefits.