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How much do you know about the coagulation principle of polyaluminum chloride?

Author:小编 Date:2021-01-29 17:37:56 Hits:

Polyaluminium chloride (PAC) water purification technology is widely used. Let's take a look at which industries it is used in:

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

2. Industrial water supply and industrial circulating water purification.

3. Purification of polluted water in cities.

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

5. Recovery of pulverized coal from industrial coal washing wastewater and recovery of kaolin in ceramic manufacturing industry.

6. Medical refining, glycerin refining, sugar refining.

7. Cement fast setting, casting molding.

8. Tanning leather, cloth wrinkle - proof.

9. Cosmetic raw materials.

10. Catalyst carrier.

11. Paper sizing.

Coagulation principle of polyaluminium chloride


1. Compression double electric layer

The structure of the micelle double electric layer determines that the concentration of counterions at the surface of the colloidal particles is high, and the concentration of counterions is lower with the greater distance from the surface of the colloidal particles, and the concentration of ions in the solution is equal.

When electrolyte is added to the solution, the concentration of ions in the solution increases, the thickness of the diffusion layer decreases.

When the two colloidal particles are close to each other, their mutual repulsive force decreases due to the decrease of the thickness of the diffusion layer and the decrease of the ξ potential, that is, the repulsive force between the colloidal particles with a high ion concentration in the solution is less than that of the colloidal particles with a low ion concentration.

The suction between the colloidal particles is not affected by the composition of the water phase, but because the diffusion layer is thinner, they collide at a smaller distance, so there is more suction between them.

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

This mechanism can better explain the sedimentation phenomenon in the harbor, because when fresh water enters the seawater, the salt increases, the ion concentration increases, and the stability of fresh water carrying 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 concentration of coagulation, more excess counterions will not enter the diffusion layer, and it is impossible for the colloidal particles to change sign and make the colloidal particles re-stable.

This mechanism is to explain the effect of electrolyte on the instability of colloidal particles by electrostatic phenomenon alone, but it does not consider the effect of other properties in the process of instability (such as adsorption), so it cannot explain some other complex instability phenomena.

For example, the amount of trivalent aluminum salt and ferric salt used as coagulants is too much, and the coagulation effect decreases, or even re-stabilizes; Another example is that the polymer or high molecular organic matter with the same electric number as the colloidal belt may have good coagulation effect. The isoelectric state should have good coagulation effect, but the ξ potential is greater than zero in the production practice, the coagulation effect is very good... And so on.

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

How much do you know about the coagulation principle of polyaluminum chloride?(图1)


2, adsorption electric neutralization

Adsorption electric neutralization refers to the particle surface of different ion, different number colloidal particles or chain molecules with different number charge parts have strong adsorption, because of this adsorption neutralized part of its charge, reduce the electrostatic repulsion, so easy to close to other particles and adsorption each other.

Electrostatic attraction is often the main aspect of these effects, but in many cases, other effects are more than electrostatic attraction.

, for example, with Na + and dodecyl ammonium ion (C12H25NH3 +) caused by removing the negatively charged silver iodide solution turbidity, found with organic amine is one of the ion to take off the stability capacity is much bigger than the Na +, Na + additive will not result in excessive micelle stability again, ions and organic amine otherwise, more than a certain cast rear can make colloidal particles to stability occur, reverse ion colloidal particle adsorption way too much, make originally the negative into a positive.

When aluminum salt and iron salt are added in high amount, the phenomenon of re-stability and charge change sign also occur. The above phenomenon is well explained by the mechanism of adsorption neutralization.


3, adsorption bridging effect

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

High polymer flocculant with linear structure, they have with parts play a role of colloidal particle surface functional groups, when high polymer contact with colloidal particles, groups with special reaction and colloidal particles surface adsorption, while the rest of the polymer molecules spread in solution, can have vacancy of colloidal particles with another surface adsorption, it ACTS as a bridge to connect the polymer.

If there are fewer colloidal particles, the extended part of the polymer does not adhere to the second colloidal particle, then sooner or later the extended part will be adsorbed on other parts of the original colloidal particle, the polymer can not play a bridging role, and the colloidal particle is in a stable state.

When the amount of polymer flocculant is too large, the surface of colloidal particles will be saturated and stabilized again. If the bridged flocculated colloidal particles are stirred violently for a long time, the bridging polymer may break off from the surface of another colloidal particle and roll back to the surface of the original one, resulting in a re-stable state.

The adsorption of polymer on colloidal surface comes from various physical and chemical actions, such as van der Waals attraction, electrostatic attraction, hydrogen bond, coordination bond, etc., which depends on the characteristics of the chemical structure of polymer and colloidal surface. This mechanism can explain the good flocculation effect of non-ionic or ionic polymer flocculants with the same electric number.


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 flocculants, and when quantities are added large enough to precipitate metal hydroxides (such as Al(OH)3, Fe(OH)3, Mg(OH)2 or metal carbonates (such as CaCO3), colloids in water can be netted by these precipitates during formation.

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

The four coagulation mechanisms introduced above are often not isolated phenomena in water treatment, but may exist at the same time. They are only given priority to a certain phenomenon under certain circumstances. At present, it seems that they can be used to explain the coagulation phenomenon of water.

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