Principle and quantitative method of polyaluminum chloride water purification

 Product introduction

Poly aluminum Chloride code name PAC. Also commonly known as water purifier or coagulant, it is a water-soluble inorganic polymer between AlCl3 and Al(OH)3. The general chemical formula is [Al2(OH)nCl6-n]m Where m represents the degree of polymerization and n represents the neutrality of the PAC product. The color is yellow or light yellow, dark brown or dark gray resinous solid.This product has strong bridging adsorption properties. During the hydrolysis process, physical and chemical processes such as aggregation, adsorption and precipitation occur. The fundamental difference between polyaluminum chloride and traditional inorganic coagulants is that traditional inorganic coagulants are low-molecular crystalline salts, while the structure of polyaluminum chloride is composed of polycarboxylic complexes with various shapes. It has fast flocculation and sedimentation speed and is suitable for use. It has a wide PH value range, is non-corrosive to pipeline equipment, and has an obvious water purification effect. It can effectively remove SS, COD, BOD and heavy metal ions such as arsenic and mercury in water. This product is widely used in drinking water, industrial water and sewage treatment fields. 

 Synthesis method

There are many methods for synthesizing polyaluminium chloride. According to different raw materials, it can be divided into metal aluminum method, activated aluminum hydroxide method, aluminum trioxide method, aluminum chloride method, alkali solution method, etc.

1. Metal aluminum method. The raw materials used to synthesize polyaluminum chloride using the metal aluminum method are mainly scraps from aluminum processing, such as aluminum chips, aluminum ash and aluminum slag. Hydrochloric acid is slowly added to aluminum ash in a certain proportion under stirring for reaction, and liquid polyaluminum chloride is obtained through aging, polymerization, and sedimentation. It is then diluted, filtered, concentrated, and dried. In terms of technology, it can be divided into three types: acid method, alkali method and neutralization method. The acid method mainly uses HCl, and the product quality is not easy to control; the alkali method has a higher production process, higher investment in equipment and a large amount of alkali, and the pH value control requires raw materials and the cost is higher; the neutralization method is more commonly used. As long as the ratio is controlled well, it can generally reach national standards.

   
   

2. Aluminum hydroxide method. The purity of aluminum hydroxide powder is relatively high, and the synthesized polyaluminum chloride has low content of heavy metals and other toxic substances. It generally adopts the production process of heating and pressurizing acid dissolution. This process is relatively simple, but the polyaluminum chloride produced has a low base degree, so it generally uses aluminum hydroxide, heated and pressurized acid dissolution, and calcium aluminate ore powder neutralization.

3. Aluminum trioxide method. The raw materials containing aluminum oxide mainly include gibbsite, bauxite, kaolin, coal gangue, etc. The 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 pyrolysis or neutralization.

4. Aluminum chloride method. Aluminum chloride powder is used as raw material to process polyaluminum chloride. This method is more commonly used. It can be obtained by boiling pyrolysis of crystalline aluminum chloride at 170°C, adding water for aging and polymerization, and then solidifying and drying. 

5. Alkali solution method. First, aluminum ash and sodium hydroxide are reacted to obtain a sodium aluminate solution, and then the pH value is adjusted with hydrochloric acid to prepare a polyaluminum chloride solution. The products produced by this method have better color appearance and less insoluble matter, but have high sodium chloride content, high raw material consumption, low solution alumina content, and high industrial production costs.

Water purification principle

Compressed double electric layer: The structure of the micellar double electric layer determines that the concentration of counter ions is greater on the surface of the colloidal particles. As the distance outward from the surface of the colloidal particles increases, the concentration of counter ions becomes lower, which is equal to the ion concentration in the solution. . When an electrolyte is added to the solution to increase the ion concentration in the solution, the thickness of the diffusion layer decreases. When two colloidal particles approach each other, since the thickness of the diffusion layer decreases and the potential decreases, their mutual repulsive force decreases. That is, the repulsive force between colloidal particles with a high ion concentration in the solution is smaller than that with a low ion concentration.The suction between colloidal particles is not affected by the composition of the water phase, but due to diffusion thinning, the distance between them when they collide is reduced, so the suction between them becomes greater. It can be seen that the combined force of repulsion and attraction changes from repulsion to suction (repulsion potential energy disappears), and the colloidal particles can quickly condense. This mechanism can better explain the sedimentation phenomenon in harbors. When fresh water enters seawater, salts increase, the ion concentration increases, and the stability of colloidal particles carried by fresh water decreases. Therefore, clay and other colloidal particles are easy to deposit in harbors. According to this mechanism, when the external electrolyte in the solution exceeds the critical condensation concentration for condensation by a large amount, no more excess counterions will enter the diffusion layer, and it is impossible for the colloidal particles to change their sign and re-stabilize the colloidal particles.This mechanism uses simple electrostatic phenomena to explain the effect of electrolytes on the destabilization of colloidal particles, but it does not consider the effects of other properties (such as adsorption) in the destabilization process, so it cannot explain other complex destabilization phenomena, such as trivalent If the amount of aluminum salt and iron salt used as coagulant is too much, the coagulation effect will decrease or even stabilize again.For example, polymers or high-molecular organic matter with the same electric number as the colloidal particles may have good coagulation effect: the isoelectric state should It has better coagulation effect, but in production practice, when the potential is greater than zero, the coagulation effect is often less. In fact, the phenomenon of destabilizing colloidal particles by adding coagulant in an aqueous solution involves the interaction between colloidal particles and coagulant, colloidal particles and aqueous solution, and coagulant and aqueous solution. It is a comprehensive phenomenon. Adsorption and electricity neutralization: Adsorption and electricity neutralization means that the particle surface has a strong adsorption effect on the parts with different signs of ions, colloid particles or chain ion molecules that have different charges. This adsorption neutralizes part of it. The electric charge reduces the electrostatic repulsion, so it is easy to get close to other particles and adsorb each other. At this time, electrostatic attraction is often the main aspect of these effects, but in many cases, other effects exceed electrostatic attraction.For example, Na and dodecyl ammonium ions (C12H25NH) were used to remove the turbidity caused by the negatively charged silver iodide solution. It was found that the destabilizing ability of the same monovalent organic amine ion was much greater than that of Na. Excessive addition of Na Adding water will not cause the colloidal particles to restabilize, but this is not the case for organic amine ions. When the addition exceeds a certain amount, the colloidal particles can restabilize, indicating that the colloidal particles have absorbed too many counter ions, causing the original negative charge to change into a negative charge. positive charge. When the dosage of aluminum salt and iron salt is high, re-stabilization phenomenon will also occur and the charge will change. The above phenomenon is very suitable to be explained by the mechanism of adsorption and electroneutralization. Adsorption and bridging: The mechanism of adsorption and bridging mainly refers to the adsorption and bridging of polymer substances and colloidal particles.It can also be understood that two large colloid particles of the same size are connected together because there is a colloid particle of different sizes in the middle. Polymer flocculants have a linear structure, and they have chemical groups that can interact with certain parts of the colloidal particle surface. When the polymer comes into contact with the colloidal particles, the groups can produce special reactions with the colloidal particle surface and adsorb each other. The rest of the polymer molecule stretches in the solution and can adsorb to another colloid with vacancies on its surface, so that the polymer acts as a bridge connection. If there are few colloidal particles and the stretched part of the polymer cannot adhere to the second colloidal particle, then sooner or later this extended part will be adsorbed to other parts by the original colloidal particles, and the polymer will not be able to play a bridging role, and the colloidal particles will In a stable state again. When the dosage of polymer flocculant is too large, the surface of the colloidal particles will be saturated and cause re-stabilization. If the colloidal particles that have been bridged and flocculated are subjected to vigorous and long-term stirring, the bridging polymer may detach from the surface of another colloidal particle and roll back to the original surface of the colloidal particle, resulting in a restabilized state. The adsorption of polymers on the surface of colloidal particles comes from various physical and chemical effects, such as van der Waals attraction, electrostatic attraction, hydrogen bonds, coordination bonds, etc., and depends on the characteristics of the chemical structures of the polymer and the surface of the colloidal particles. This mechanism can explain the phenomenon that non-ionic or ionic polymer flocculants with the same electric signal can achieve good flocculation effects.

Instructions

The solid product is dissolved in a certain proportion and directly added to the sewage through a diaphragm pump or other dosing equipment. The dissolved concentration is determined by the controllability of the on-site dosing equipment and the amount of medicine used. In short, dissolution is to facilitate the dosing of medicines. When mixed with raw water, there is no specified dissolution concentration, but it is recommended that the alumina concentration be no less than 1.5% when used for dissolution. Can bring out the greatest benefit of coagulation.

1.Method summary Use natural raw water (surface water source water such as rivers, lakes, reservoirs, etc.), use a coagulation sedimentation test mixer to conduct a coagulation sedimentation test, and judge the coagulation performance based on the test results.

2.Instruments and equipment

(1)Coagulation and sedimentation test mixer.

(2)Scattered light turbidity meter.

3.Coagulation sedimentation test

(1)Preparation of polyaluminum chloride diluent: Weigh the polyaluminum chloride sample, put it into a 100ml volumetric flask, add water to dilute to the mark, shake well, so that the Al2O3 content of the diluent is 1.0 mg/ml ~ 10 mg/ml. This solution should be prepared on the day of use.

(2)Set the test program as follows: mixing 500r/min～600r/min 30s～60s flocculation 20r/min～200r/min 10min～30min sedimentation

(3)Pour raw water into six beakers of the same model and add to the 1000ml mark. Pipette the required polyaluminum chloride diluent and put it into the dosing test tube in sequence.

(4)Start the coagulation sedimentation test mixer and conduct the test according to the procedure. When the sedimentation time is up, take a clear water sample and measure the remaining turbidity and other water quality indicators.

(5)During the test, the formation time, shape, size and settlement status of the flocs were simultaneously observed and recorded.

4.To evaluate the coagulation and sedimentation effect, draw a curve or table based on the coagulant dosage, residual turbidity of clarified water and other water quality indicators as well as observation conditions. Brief description of the method: Dissolve 1g PAC into a 100ml volumetric flask to prepare a 1% PAC solution. Weigh the raw water and place it in a 1000ml beaker for the flocculation and stirring experiment (without a stirrer, you can stir manually with a glass rod). Add 1ml each time The dosage of this PAC solution corresponds to adding 10kg PAC (solid) per 1,000 cubic meters of raw water, and the on-site dosage is determined based on the use of PAC in the small test.