The coagulation principle of polyaluminum chloride

To understand the coagulation principle of polyaluminum chloride, we must first have a basic understanding and understanding of the structural characteristics and coagulation mechanism of polyaluminum chloride.

The following editor will introduce it to you to deepen your knowledge and understanding of it.

Polyaluminum chloride is composed of a series of inorganic polymer compounds with different polymerization degrees and has the best morphological distribution.

The main component is AI4 (OH) 24 (H2O) 24 (H2O) 127+, which is a highly charged polymer ring chain with a Keggin structure. It has a high degree of electrical neutralization and bridging effects on colloids and particles in water, and can remove micro Toxic and heavy metal ions, stable properties.

In the water, it neutralizes the negative charge of the colloidal particles instantaneously, destabilizing the colloid, and the colloidal particles quickly coagulate, and further bridge the formation of flocs for rapid precipitation. 

1. Polyaluminum chloride is referred to as PAC, also known as polyaluminum chloride, composite polyaluminum chloride, and basic aluminum chloride.

2. Molecular formula: [AL2(OH)nCL6-n?XH2O]m, where: 1≤n≤5, m≤10.

3. Molecular weight: about 2000.

4. Inspection method: according to the international GB15892-2003 standard.

5. Polyaluminum chloride is non-toxic, harmless, tasteless, easily soluble in water, liquid products are yellow or light yellow or colorless liquid; solid products are white or light yellow powder resinous solid, solid products are easy to absorb moisture in the air.

The water purification technology of polyaluminum chloride has been widely used, let's take a look at which industries have applications:

1. Purification of urban water supply rivers, reservoirs and groundwater.

2. Industrial water supply and industrial circulating water purification.

3. Urban pollution water purification.

4. Industrial printing and dyeing, papermaking, sugar making, leather, brewing, meat processing, coal washing, metallurgy, ore washing, pharmaceuticals and other water purification, as well as fluorine-containing, oil-containing, and heavy metal-containing wastewater purification.

5. Recovery of pulverized coal in industrial coal washing wastewater, recovery of kaolin in ceramic manufacturing.

6. Medical refining, glycerin refining, sugar liquid refining.

7. Cement is quick-setting and casting molding.

8. Anti-wrinkle for tanned leather and cloth.

9. Cosmetic raw materials.

10. Catalyst carrier.

11. Paper sizing.

>>> 1. Compress double electric layer

The structure of the micellar electric double layer determines the maximum concentration of counter ions on the surface of the colloidal particles. As the distance from the surface of the colloidal particles increases, the concentration of counter ions decreases, which is finally equal to the ion concentration in the solution.

When the electrolyte is added to the solution to increase the ion concentration in the solution, the thickness of the diffusion layer is reduced.

When two colloidal particles are close to each other, because the thickness of the diffusion layer decreases and the zeta potential decreases, their mutual repulsion force is reduced, that is, the repulsive force between the colloids with high ion concentration in the solution is smaller than that with low ion concentration.

The suction force between the colloidal particles is not affected by the composition of the water phase, but due to the thinning of the diffusion layer, the distance between them when they collide is reduced, so that the mutual suction force is greater.

It can be seen that the resultant force of repulsion and attraction has changed from repulsive force to suction force (the repulsive potential energy disappears), and the colloidal particles can quickly condense.

This mechanism can better explain the sedimentation phenomenon in the harbor. When the fresh water enters the seawater, the salt increases, the ion concentration increases, and the stability of the fresh water entrained colloidal particles decreases, so clay and other colloidal particles are easy to deposit in the harbor.

According to this mechanism, when the added electrolyte in the solution exceeds the critical aggregation concentration for agglomeration, there will not be more excess counter ions into the diffusion layer, and it is impossible for the colloidal particles to change signs to stabilize the colloidal particles again.

This mechanism is based on the simple electrostatic phenomenon to illustrate the effect of electrolyte on the destabilization of colloidal particles, but it does not consider the effects of other properties in the destabilization process (such as adsorption), so it cannot explain other complex destabilization phenomena.

For example, if the amount of trivalent aluminum salt and iron salt used as coagulant is too much, the coagulation effect will decrease or even stabilize again. Another example is that the polymer or polymer organic matter with the same charge as the colloidal particles may have a good coagulation effect: etc. The electrical state should have the best coagulation effect, but the coagulation effect is often the best when the ξ potential is greater than zero in production practice...etc.

In fact, adding a coagulant to the aqueous solution to destabilize the colloidal particles involves the interaction of the colloidal particles and the coagulant, the colloidal particles and the aqueous solution, and the coagulant and the aqueous solution, which is a comprehensive phenomenon.

>>>> 2. Adsorption and neutralization

Adsorption charge neutralization refers to the strong adsorption of the particle surface to the opposite-sign ions, the different-sized colloidal particles or the part of the chain-like molecule with the opposite-sign charge. Because this adsorption neutralizes some of its charges and reduces the static electricity Repulsive force, so it is easy to get close to other particles and adsorb each other.

At this time, electrostatic gravity is often the main aspect of these effects, but in many cases, other effects exceed electrostatic gravity.

For example, using Na+ and dodecylammonium ion (C12H25NH3+) to remove the turbidity caused by negatively charged silver iodide solution, it is found that the same monovalent organic amine ion has a much greater destabilization ability than Na+. Addition will not cause the colloidal particles to be stable again, while organic amine ions do not. When exceeding a certain amount, the colloidal particles can be stabilized, which means that the rubber particles have absorbed too much counter ions, and the original negative charge will become positive charge.

When the dosage of aluminum salt and iron salt is high, the stabilization phenomenon and the charge change will also occur. The above phenomenon is explained by the mechanism of adsorption and neutralization.

>>>> 3. Adsorption and bridging

The mechanism of adsorption bridging mainly refers to the adsorption and bridging of polymer substances and colloidal particles. It can also be understood that two large rubber particles of the same size are connected together due to a different size rubber particle.

Macromolecular flocculants have a linear structure. They have chemical groups that can interact with certain parts of the surface of the colloidal particles. When the high polymer is in contact with the colloidal particles, the groups can react with the surface of the colloidal particles to adsorb each other. The rest of the polymer molecule stretches out in the solution and can be adsorbed to another colloidal particle with vacancies on the surface, so that the polymer acts as a bridge.

If there are few rubber particles, the stretched part of the polymer mentioned above cannot adhere to the second rubber particle. Sooner or later, the stretched part will be adsorbed on other parts by the original rubber particles. This polymer will not function as a bridge, and the rubber particles It is in a stable state again.

When the dosage of high polymer flocculant is too large, the surface of the colloidal particles will be saturated and stable again. If the colloidal particles that have been bridged and flocculated are subjected to vigorous and prolonged agitation, the bridged polymer may separate from the surface of another colloidal particle and rewind to the original surface of the colloidal particle, resulting in a stable state.

The adsorption of polymer on the surface of colloidal particles comes from various physical and chemical effects, such as van der Waals attraction, electrostatic attraction, hydrogen bond, coordination bond, etc., depending on the characteristics of the chemical structure of the polymer and the surface of the colloidal particle. This mechanism can explain the phenomenon that non-ionic or ionic polymer flocculants with the same charge can obtain good flocculation effects.

>>>> 4. Sediment net catching mechanism

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 quickly precipitate metal hydroxides (such as Al(OH)3, In the case of Fe(OH)3, Mg(OH)2 or metal carbonate (such as CaCO3), the colloidal particles in the water can be caught by these precipitates when they are formed.

When the precipitate is positively charged (Al(OH)3 and Fe(OH)3 are in the neutral and acidic pH range), the precipitation rate can be accelerated due to the presence of anions in the solution, such as silver sulfate ions. In addition, the colloidal particles in water can serve as the core of these metal oxide precipitates, so the optimal dosage of coagulant is inversely proportional to the concentration of the substance to be removed, that is, the more colloidal particles, the less the dosage of metal coagulant.

The four mechanisms of coagulation introduced above are often not isolated phenomena in water treatment, but may often exist at the same time, but under certain circumstances, they are mainly based on certain phenomena. At present, they can be used to explain water.的coagulation phenomenon.