What Is Polyaluminum Chloride (PAC)?
1. Polyaluminum chloride has properties such as adsorption, coagulation, and precipitation; its stability is poor and it is corrosive. If it splashes onto the skin, rinse immediately with water.
2. Polyaluminum chloride has the advantages of good spray-drying stability, wide adaptability to water areas, fast hydrolysis speed, strong adsorption capacity, large and dense flocs, fast precipitation, low effluent turbidity, and good dewatering performance. Using spray-dried products ensures safety and reduces water accidents, making it very safe and reliable for residents' drinking water. Therefore, polyaluminum chloride is also briefly called high-efficiency polyaluminum chloride, high-efficiency polyaluminum chloride, or high-efficiency spray-dried polyaluminum chloride [3]. Polyaluminum chloride is suitable for raw water of various turbidities and has a wide pH application range, but compared with polyacrylamide, its settling effect is far inferior.
3. Appearance and color [6]: the common solid forms of polyaluminum chloride are yellow, pale yellow, or white powder or granules; the depth of color is mainly related to its raw materials and preparation process. Its color change is mainly related to the iron-ion impurities it contains; when the purity is higher, the color is lighter. White polyaluminum chloride is called high-purity iron-free white polyaluminum chloride, or food-grade white polyaluminum chloride; compared with other polyaluminum chlorides, it is the highest-quality product and can be used in many fields such as paper sizing agents, sugar decolorization and clarification agents, tanning, pharmaceuticals, cosmetics, and precision casting, as well as water treatment. The raw materials of yellow polyaluminum chloride are calcium aluminate powder, hydrochloric acid, and bauxite, mainly used in sewage treatment and drinking-water treatment; the raw materials for drinking-water treatment are aluminum hydroxide powder and hydrochloric acid, plus a little calcium aluminate powder, using the plate-frame pressure-filtration process or spray-drying process. The raw materials of brown polyaluminum chloride are calcium aluminate powder, hydrochloric acid, bauxite, and iron powder. Liquid polyaluminum chloride is usually a pale-yellow to colorless transparent liquid, with a concentration generally of 10%–20%; its color and concentration differ depending on the polymerization degree and acidity of aluminum chloride.
1. The basicity of polyaluminum chloride is a relatively important indicator, especially for drinking-water-grade polyaluminum chloride products. The lower the basicity, the higher the price; each purchaser can operate according to the actual situation of the manufacturer. In addition, different raw materials and different processes produce polyaluminum chloride products with different basicity, which requires the manufacturer to adjust. Increasing the basicity of polyaluminum chloride products can greatly improve the economic benefits of production and use. Increasing basicity from 65% to 92% can reduce production raw-material costs by 20% and use costs by 40% [7].
There are many synthesis methods for polyaluminum chloride, which can be divided according to different raw materials into the metallic aluminum method, activated aluminum hydroxide method, aluminum oxide method, aluminum chloride method, and alkali dissolution method [9].
1 Metallic aluminum method. The raw materials for synthesizing polyaluminum chloride by the metallic aluminum method are mainly aluminum scrap, such as aluminum chips, aluminum ash, and aluminum slag. Aluminum ash is added slowly to hydrochloric acid at a certain ratio while stirring, and after aging, polymerization, and settling, liquid polyaluminum chloride is obtained, then diluted, filtered, concentrated, and dried. The process can be divided into three methods: acid method, alkali method, and neutralization method. The acid method mainly uses HCl, and product quality is difficult to control; the alkali method has higher process difficulty, larger equipment investment, and larger alkali usage, with pH control consuming raw materials and higher cost; the most used is the neutralization method, which can generally meet national standards as long as the ratio is well controlled.
9. Prohibit the use of equipment and tools that easily produce sparks.
3 Aluminum oxide method. Raw materials containing aluminum oxide mainly include gibbsite, bauxite, kaolin, and coal gangue. This production process can be divided into two steps: the first step is to obtain crystalline aluminum chloride, and the second step is to obtain polyaluminum chloride through thermal decomposition or neutralization.
4 Aluminum chloride method. Using aluminum chloride powder as raw material to process polyaluminum chloride. This method is the most widely used. Crystalline aluminum chloride can be subjected to boiling thermal decomposition at 170°C, then aged and polymerized with water, and then solidified and dried.
5 Alkali dissolution method. First react aluminum ash with sodium hydroxide to obtain sodium aluminate solution, then adjust the pH with hydrochloric acid to prepare polyaluminum chloride solution. The product prepared by this method has good color and appearance and fewer insoluble matters, but has high sodium chloride content, high raw-material consumption, low alumina content in the solution, and large industrial production cost.
The most widely used field of polyaluminum chloride is water treatment, especially the purification of drinking water, sewage, and industrial wastewater. Because of its excellent flocculation performance and low use cost, polyaluminum chloride is very widely used in the treatment of urban water supply, industrial wastewater, domestic sewage, and river water.
Drinking-water treatment: as a flocculant, polyaluminum chloride can effectively remove suspended solids, colloidal particles, heavy-metal ions, and other harmful substances from water. Its advantage is significant flocculation effect, achieving good treatment effect at low dosage.
Sewage treatment: polyaluminum chloride is used to remove harmful substances such as oil stains, dyes, petroleum substances, and other organic pollutants from industrial and domestic wastewater. Because of its high removal capacity, polyaluminum chloride is often used together with other water-treatment agents (such as coagulant aids and flocculation auxiliaries) as a flocculant.
River-water treatment: under eutrophication and water-quality deterioration, polyaluminum chloride can be used to treat algal blooms and harmful substances in river water and improve water quality.
In many industrial production processes, wastewater often contains large amounts of pollutants such as suspended solids, oil stains, and heavy-metal ions, and traditional chemical treatment methods often fail to meet environmental-protection requirements. As a strong flocculant, polyaluminum chloride is widely used in industrial wastewater treatment.
Pulp and paper industry: in papermaking wastewater treatment, polyaluminum chloride can effectively remove pollutants such as suspended solids, pulp fibers, and surfactants from water, reducing COD (chemical oxygen demand) and BOD (biochemical oxygen demand).
Chemical industry: in chemical production, wastewater often contains pollutants such as acids, alkalis, salts, and metal ions; polyaluminum chloride can effectively remove these pollutants through flocculation, reducing water-body pollution.
Food industry: in wastewater treatment of food-processing industries such as sugar refineries and breweries, polyaluminum chloride can remove suspended solids, color, and grease.
In the paper-production process, polyaluminum chloride can be used not only as a flocculant for wastewater treatment, but also for pulp purification and wastewater recovery during production.
4. Food and beverage industry
The application of polyaluminum chloride in the food and beverage industry is mainly reflected in clarification and removal of insoluble substances, especially in the production of fruit juice, beverages, and other liquid foods.
Juice clarification: in the production of fruit juice, polyaluminum chloride can be used to remove suspended particles, turbidity, and some dissolved substances from the juice, making the juice more transparent.
Building on this, industry experts have also carried out extensive research and improvements.
Soil improvement: polyaluminum chloride can be used for acid-regulation of soil in agriculture. By adjusting the soil's pH, it improves the crop growth environment and increases crop yields.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
1. Externally packed in plastic woven bags with an inner plastic film liner, each bag net 25 kg; can also be modified according to user requirements; liquid polyaluminum chloride is also sold.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
2 Aluminum hydroxide method. Aluminum hydroxide powder has relatively high purity; the synthesized polyaluminum chloride has low toxic-substance content such as heavy metals, and generally uses the heating-and-pressurizing acid-dissolution production process. This process is relatively simple, the raw-material purity is high, and the generated polyaluminum chloride has little effect on water quality, but the basicity of the produced polyaluminum chloride is low, so generally the aluminum hydroxide heating-pressurizing acid-dissolution plus calcium aluminate ore powder neutralization two-step process is used.
1. Liquid polyaluminum chloride is the undried form; it has the advantages of no need for dilution, convenient loading and unloading, and relatively cheap price, but the disadvantage is that transport requires tank trucks, increasing unit transport cost (each ton of solid is equivalent to 2–3 tons of liquid), making it more suitable for users within 100 km.
2. Solid polyaluminum chloride is the dried form of liquid polyaluminum chloride; it has the advantage of convenient transport without needing tank trucks, but the disadvantage is that it needs dilution before use, increasing work intensity.
The structure of the electrical double layer of the micelle determines that the counter-ion concentration is greatest at the micelle surface and decreases with distance outward from the micelle surface, eventually equalizing with the ion concentration in solution. When electrolyte is added to the solution, increasing the ion concentration in the solution, the thickness of the diffuse layer decreases.
When two micelles approach each other, because the diffuse-layer thickness decreases and the xi potential decreases, the repulsive force between them decreases—that is, the inter-micelle repulsion at high solution ion concentration is smaller than at low ion concentration. The attraction between micelles is not affected by the aqueous-phase composition, but because the diffusion layer thins, the distance at collision decreases, so the mutual attraction increases. It can be seen that the resultant of repulsion and attraction changes from being repulsion-dominated to attraction-dominated (the repulsion potential energy disappears), and the micelles coagulate rapidly. This mechanism better explains the deposition phenomenon at harbors: when fresh water enters seawater, the salt content increases and the ion concentration increases, reducing the stability of clay and other colloidal particles carried by the fresh water, so clay and other colloidal particles easily deposit at harbors.
According to this mechanism, when the externally added electrolyte in solution far exceeds the critical coagulation concentration for coagulation, no more excess counter-ions enter the diffuse layer, and it is impossible for the micelle to change sign and re-stabilize. Such a mechanism explains the effect of electrolyte on micelle destabilization by purely electrostatic phenomena, but it does not consider other properties' effects in the destabilization process (such as adsorption), so it cannot explain some other complex destabilization phenomena—for example, when the dosing of trivalent aluminum and iron salts as coagulants is too high, the coagulation effect instead decreases or even re-stabilizes; another example is that polymer or macromolecular organics with the same charge sign as the micelle may have good coagulation effect: the isoelectric state should have the best coagulation effect, but in production practice the coagulation effect is often least when the xi potential is greater than zero, etc.
In fact, adding coagulant to an aqueous solution to destabilize micelles involves the interactions among micelles and coagulant, micelles and aqueous solution, and coagulant and aqueous solution—a comprehensive phenomenon.
Adsorption charge neutralization refers to the strong adsorption of the micelle surface to counter-ions, oppositely charged micelles, or the oppositely charged parts of chain macromolecules. Because this adsorption neutralizes part of its charge and reduces electrostatic repulsion, it easily approaches and adsorbs other particles. At this time, electrostatic attraction is often the main aspect of these effects, but in many cases other effects exceed electrostatic attraction.
For example, when using Na and dodecylammonium ions (C12H25NH) to remove the turbidity caused by negatively charged silver iodide solution, it was found that the destabilization ability of monovalent organic amine ions is much greater than that of Na; excessive Na dosing will not cause micelle re-stabilization, while organic amine ions, when exceeding a certain dosage, can cause re-stabilization of micelles, indicating that the micelle adsorbs too many counter-ions, changing its original negative charge into a positive charge. Aluminum and iron salts at high dosage also experience re-stabilization and charge reversal. The above phenomena are well explained by the adsorption charge-neutralization mechanism.
The adsorption bridging mechanism mainly refers to the adsorption and bridging of macromolecular substances with micelles. It can also be understood as two large like-charged micelles connected by an unlike-charged micelle in between. Polymer flocculants have a linear structure and chemical groups that can act on certain parts of the micelle surface; when the polymer contacts the micelle, the groups can react specially with the micelle surface and mutually adsorb, while the rest of the polymer molecule extends in solution and can adsorb onto another micelle surface with vacancies, so the polymer plays a bridging role. If there are few micelles, the extended part of the polymer cannot reach a second micelle, and this extended part will sooner or later be adsorbed back onto the original micelle at another site, so the polymer cannot play a bridging role and the micelle remains stable. When the dosage of polymer flocculant is too large, the micelle surface becomes saturated and re-stabilization occurs. Bridged flocs, if subjected to intense prolonged stirring, may have the bridging polymer detach from another micelle surface and curl back to the original micelle surface, causing re-stabilization.
The adsorption of polymer on the micelle surface originates from various physicochemical actions, such as van der Waals forces, electrostatic attraction, hydrogen bonding, and coordination bonds, depending on the chemical-structure characteristics of both the polymer and the micelle surface. This mechanism explains the phenomenon that non-ionic or same-charge ionic polymer flocculants can achieve good flocculation effects.
When metal salts (such as aluminum sulfate or ferric chloride) or metal oxides and hydroxides (such as lime) are used as coagulants, when the dosage is large enough to rapidly precipitate metal hydroxides [such as Al(OH)3, Fe(OH)3, Mg(OH)2] or metal carbonates (such as CaCO3), the micelles in the water can be entrapped by these precipitates during formation. When the precipitate is positively charged (Al(OH)3 and Fe(OH)3 in neutral and acidic pH ranges), the settling speed can be accelerated by the presence of anions in the solution, such as sulfate ions. In addition, the micelles in the water can themselves serve as cores for the formation of these metal-oxide hydroxide precipitates, so the optimal dosage of coagulant is inversely proportional to the concentration of the substance to be removed—that is, the more micelles, the less metal coagulant needed.
In addition, this technology has also been widely applied and practiced in related fields.
Dissolve the solid product at a 1:3 ratio with water into liquid, then dilute with 10–30 times clear water to the required concentration before use. The optimal pH for dosing is 3.5–5.0; selecting the optimal pH for dosing can maximize the coagulation benefit. The dosage can be determined according to the different turbidity of the raw water. For raw-water turbidity of 100–500 mg/L, the dosage is 10–20 kg per 1,000 tons. When raw-water turbidity is high, the dosage can be appropriately increased; when turbidity is low, the dosage can be appropriately reduced.
For rural use, the agent can be put into a water vat, stirred evenly, and left to stand; the supernatant can be used, adding about 1 g of this agent per 50 kg. If this agent is used together with the high-molecular flocculant produced by the company, the effect is better. For dosing, the company's anionic or cationic polyacrylamide can be dissolved together with polyaluminum chloride into a composite flocculant for use, or polyaluminum chloride can be added first to the treated water body to form flocs, then the company's anionic polyacrylamide added for adsorption bridging into large flocs.
In addition, from the perspective of industrial development, market demand is also driving technological progress.
II. In domestic and production sewage, first add about 30 g of polyaluminum chloride product per ton of sewage. Then add the diluted polyacrylamide product. (If the effect is not obvious, please reduce or increase the product dosage as appropriate.)
IV. When raw-water turbidity is 100–500 mg/L, the dosage is 5–10 mg, i.e., 5–10 kg per 1,000 tons of water. It is best to conduct a small test according to water-quality characteristics before use to select the optimal value, then apply.
2. Flocculation stage: the process of flocs growing and coarsening, requiring appropriate turbulent intensity and sufficient retention time (10–15 min), until in the later stage a large number of flocs are observed gathering and slowly sinking, forming a clear surface layer [14]. In the beaker test, first stir at 150 rpm for about 6 minutes, then at 60 rpm for about 4 minutes until suspended.
3. Settling stage [14]: this is the floc-settling process carried out in the settling tank, requiring slow water flow; to improve efficiency, inclined-tube (plate) settling tanks are generally used (air flotation for floc separation is best), where large coarse flocs are blocked by the inclined-tube (plate) walls and deposit at the bottom of the tank, the upper water is clear, and the remaining small, low-density flocs slowly descend while continuing to collide and grow, until in the later stage the residual turbidity is basically unchanged. The beaker test should use slow stirring at 20–30 rpm for 5 minutes, then static settling for 10 minutes, to measure residual turbidity.
5. Applied to environmental protection and industrial wastewater treatment; the usage method is roughly the same as that of a water-treatment plant. For raw water with high color, high COD, and high BOD, the effect is excellent with the aid of auxiliary agents.
In addition, many factors must be considered in practical engineering applications.
8. This product must be dissolved before use; the dissolving equipment and dosing facilities should be made of corrosion-resistant materials.
