Kaolinite, the principal mineral in china clay, has the commonly accepted composition of 2H2O.Al2O3.2SiO2. Kaolin clays are generally found as sedimentary deposits formed by the weathering of feldspathic rocks. The kaolinite is almost invariably associated with impurities such as iron oxides, rutile, silica, feldspar, mica, sulfides and organic matter. For most applications, these impurities have to be removed from the kaolin clay to produce a useful end product.
Processed kaolin clays can be divided into two broad categories:
- Dry-processed clays of low to medium purity, for use in relatively low-cost applications such as ceramics and other structural materials.
- Wet-processed kaolin of high purity and brightness, used mainly as filler and coatings in high-grade paper, and also in paints and plastics.
Low-grade clays are produced by employing relatively low-cost dry processing methods, including air flotation, sizing, and some magnetic separation and froth flotation. On the other hand, high-grade clays are generally produced by employing advanced, state-of the-art technologies, including mostly wet processes, from advanced high-gradient magnetic separation to froth flotation techniques.
Flotation concentration of high-grade kaolin clays is conducted by employing reverse flotation of heavy and colored mineral impurities away from kaolin clay. The majority of U.S. producers, mostly located in the middleGeorgia area, use this reverse flotation process.
In recent years, ever-increasing demand for high-grade, performance products with stricter product specifications, has resulted in several technologically advanced process and equipment developments in the kaolin clay industry. Flotation is normally used along with other innovative processes such as magnetic separation, selective flocculation, etc., to produce high-grade clay products.
Reverse flotation of colored impurities from kaolin clay is a highly competitive and technologically advanced process application. Since fatty acids and their derivatives have, until recently, been the only collector type available for flotation, the industry innovators looked for other ways to improve the overall process. As a result, numerous, highly successful and competitive process applications were developed, based on improved modifiers and equipment during blunging, conditioning, and flotation stages.
However, with the recent introduction of hydroxamic acid collectors by Cytec, further significant improvements have been realized. Hydroxamic acid-based collectors not only simplify the overall process by eliminating activators and cumbersome reagent schemes, but also make it possible to process some types of kaolin clays that are not treatable with standard fatty acids. Some of these developments have been reported by Yoon, Yordan, Basilio and Ravishankar et al.
A typical process for hydroxamic acid flotation includes high solids (50% or higher) and high-intensity blunging to disperse clay minerals from impurities, followed by conditioning with hydroxamic acid collectors and flotation, preferably with the use of columns. Dosages for hydroxamate collectors (AERO 6493, AERO 6494 promoters and REAGENT S-9849 promoter) vary between 0.5 to 1.0 Kg/tonne of flotation feed, depending on the amount of impurity minerals and kaolin clay type. In addition, some of the following advantages can be realized:
- Energy savings
- Reduction in mechanical (conditioning) energy by 50%
- Decrease in equipment wear
- Yield improvements
- Selective removal of titanium impurities
- Elimination of the metal ion activators improving yield
- Water management
- High solids flotation – reduces down stream processing, water recycling issues
- Process advantages
- Simplifying the process by eliminating the number of chemicals added, control and management
- Enabling process-ability of "hard to beneficiate" raw materials
- Cutting back the number of cleaning steps
Cytec's current hydroxamate product line includes AERO 6493, AERO 6494 promoters and REAGENT S-9849 promoter. These collectors are designed to possess different frothing properties to respond effectively to various kaolin clays and flotation concentration methods.
AERO 6493 promoter is also used in the novel selective flocculation process developed recently. This process is especially applicable for the fine kaolin clays. The hydroxamate collector, used in the blunging-conditioning step, adsorbs selectively on the colored impurities which then form large aggregates. These aggregates are selectively flocculated with high molecular weight flocculants, specifically Hydroxamated PAMs.