Technical Specifications for the Polyaluminum Chloride Dryer

Poly aluminum chloride (PAC) is an inorganic polymeric coagulant. It is an inorganic polymeric water treatment chemical with a high molecular weight and high charge, produced through the bridging action of hydroxide ions and the polymerization of polyvalent anions. Its molecular formula is as follows: [Al₂(OH)_nCl_(6-n)]_m (where n = 1–5 and m ≤ 10). Basicity: B = n/6 × 100%. Its flocculation mechanism is as follows:

a. Strong electrostatic neutralization of colloidal substances in water.

b. Excellent bridging and adsorption of suspended solids in water by hydrolysis products.

c. Selective adsorption of soluble substances.

The primary form of polyaluminum chloride in water is Al₁₃O₄(OH)₂⁴⁷⁺.

Currently, the domestic industry generally employs a one-step acid-leaching process, in which bauxite is first dissolved and reacted, followed by adjustment of the basicity using calcium aluminate.

There are three main challenges in PAC production in China:

1. Product purity:

It is generally believed that the higher the alumina content, the better the purity and the more effective the product. Most enterprises produce a single, low-quality polyaluminum chloride product using bauxite, calcium aluminate, and by-product hydrochloric acid. Production scales are small, and the technology is rudimentary; the alumina content varies from 9 to 13, with inconsistent quality levels.

II. The issue of insolubles:

The mineral raw materials used generally have complex compositions. The finer the grinding, the higher the aluminum oxide leaching rate; however, this also makes it difficult for insolubles to settle. Currently, solid-liquid separation in China is generally achieved through the following three methods: 1. Natural sedimentation, which typically takes a very long time and requires a large site to construct sedimentation tanks; 2. Filtration using plate-and-frame filter presses, which involves high capital investment and operating costs; some facilities also use horizontal screw centrifuges, which also entail significant investment; 3. Adding flocculants such as polyacrylamide—when the dosage is carefully controlled, this method yields good results.

III. Base Number Issues:

Generally, the higher the base number, the better the product’s flocculation performance. Generally, aluminum shavings, sodium aluminate, calcium carbonate, aluminum carbonate, sodium hydroxide gel, lime, and other substances can be added to low-basicity products to increase their basicity. If the goal is to avoid introducing heavy metals and other impurities, the methods of adding aluminum shavings and sodium aluminate are generally preferred. However, these methods are more costly than calcium aluminate and aluminum dross; currently, many domestic enterprises use calcium aluminate to adjust basicity.

Poly aluminum chloride is a highly efficient water purifier capable of eliminating bacteria, odors, and discoloration. Its primary industrial application is as a water treatment agent. Due to its outstanding characteristics and broad applicability—it requires over 30% less dosage than traditional water purifiers and offers cost savings of over 40%—it has become a universally recognized superior water purifier both domestically and internationally. In addition, polyaluminum chloride can also be used for the purification of drinking water and for special water quality treatment in water supply systems.

Because polyaluminum chloride is highly corrosive, hygroscopic, and abrasive, and the product has low added value, the requirements for drying equipment—in terms of material selection, configuration optimization, energy conservation, and consumption reduction—are very high. The two commonly used drying methods for polyaluminum chloride are drum dryers and spray dryers. The traditional drying method involves drum dryers, which use saturated steam as a heat source. However, because polyaluminum chloride solutions are highly corrosive at higher operating temperatures, this method results in a short equipment service life, limited drum diameter, low single-unit output, high energy consumption, poor product quality, and a harsh operating environment that poses significant health risks to operators. Poly aluminum chloride powder produced using spray drying exhibits good dispersibility and dissolves rapidly in water.