Updated: Mar 2
Flocculants may not be unfamiliar to professionals who have been working with water
treatment, but which flocculants are there, and how to distinguish and use them correctly seems a very confusing topic among many people.
With the help of this article, YASA ET provide a comprehensive review and explanation of inorganic flocculants such as aluminium and iron salts. Let's take a look at it together.
Flocculant is a type of substance that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, and make dispersed particles coagulate and flocculate into aggregates.
According to chemical composition, flocculants can be divided into three categories:
This article mainly introduces our most commonly used inorganic flocculants for wastewater treatment which include aluminium salts, iron salts and their polymers.
The working principle of flocculants
The colloidal particles in the water have the characteristics of tininess, surface hydration and charging, which makes them have strong stability. When the flocculant is added to the water, it will be hydrolysed into a charged colloid and the surrounding ions to form an electric double layer structure.
The method of rapid stirring after pouring is adopted to promote the collision chance and frequency of the colloidal impurity particles in the water and the micelles hydrolysed by the flocculant. The impurity particles in the water first lose their stability under the action of the flocculant, and then agglomerate into larger particles, and finally settle down or float upwards.
The impurity particles in the water interact with the flocculant to lose or reduce the stability through mechanisms such as the compression of the electric double layer and the electric neutralization, and the process of generating micro flocs is called coagulation. The process of agglomerating to form micro-flocs under the agitation of bridging materials and water flow, through adsorption bridging and sediment net catching and other mechanisms to grow into large flocs is called flocculation.
The combination of mixing, coagulation and flocculation is called coagulation. The mixing process is generally completed in the mixing tank, and the coagulation and flocculation are carried out in the reaction tank.
1. Inorganic Flocculants
Traditionally used inorganic flocculants are low-molecular aluminium and iron salts. Aluminium salts mainly include aluminium sulphate (AL2(SO4)3∙18H2O), alum (AL2(SO4)3∙K2SO4∙24H2O), sodium aluminate (NaALO3) ), iron salts mainly include iron trichloride (FeCL3∙6H20), ferrous sulphate (FeSO4∙6H20) and iron sulphate (Fe2(SO4)3∙2H20).
Since the United States first used aluminium sulphate for water treatment and obtained a patent at the end of the 19th century, aluminium sulphate has been widely used with its excellent cohesion and sedimentation properties. Aluminium sulphate is currently the most used flocculant in the world.
Commercially available aluminium sulphate has two forms: solid and liquid. The solid is divided into refined and crude based on the content of insoluble matter. The solid product alum that is commonly used in drinking water purification in our country is a compound of aluminium sulphate and potassium sulphate. Salt, but not much used in industrial water and wastewater treatment.
The suitable pH range of aluminium sulphate is related to the hardness of the raw water. When processing soft water, the suitable pH value is 5~6.6, when processing medium-hard water, the suitable pH value is 6.6-7.2, and when processing high-hard water, the suitable pH value is 7.2-7.8.
The applicable water temperature range for aluminium sulphate is 20°C to 40°C, and the coagulation effect is poor when it is lower than 10°C. Aluminium sulphate is less corrosive and easy to use, but the hydrolysis reaction is slow, and a consistent amount of alkali needs to be consumed.
Ferric chloride is another commonly used inorganic low-molecular coagulant. The product has solid dark-brown crystals as well as higher-concentration liquids. It has the advantages of being easily soluble in water, large and heavy alum blooms, good precipitation performance, and a wide range of adaptation to temperature, water quality and pH.
The applicable pH range of ferric trichloride is 9-11, the flocs formed are dense, easy to precipitate, and the effect is still very good at low temperature or high turbidity. Solid ferric chloride has strong water absorption, high anti-corrosion requirements for dissolution and dosing equipment, and pungent odour.
The mechanism of iron trichloride is to use ferric iron ions produced by the gradual hydrolysis of various hydroxyl iron ions to achieve the flocculation of impurity particles in the water, and the formation of hydroxyl iron ions requires the use of a large number of hydroxyl groups in the water, so a large amount of alkali is consumed. When the alkalinity of the raw water is not enough, it is necessary to supplement an alkali source such as lime.
Ferrous sulphate is commonly known as green vitriol. It forms flocs quickly and stably, and has a short settling time. It is suitable for high alkalinity and high turbidity conditions, but the colour is not easy to remove, and it is also highly corrosive.
This chemical is normally used for very specific applications and not so common as aluminium and ferric sulphates.
2. Factors influencing the effectiveness of flocculants
The pH value of the water has a great influence on the use effect of the inorganic flocculant. The pH value is related to the type of flocculant selected, the dosage and the coagulation sedimentation effect. The H+ and OH- in the water participate in the hydrolysis reaction of the flocculant. Therefore, the pH value strongly affects the hydrolysis rate of the flocculant, the existence form and performance of the hydrolysate.
Take the aluminium salt that achieves coagulation by generating Al(OH)3 charged colloids as an example. When the pH value is less than 4, Al3+ cannot be hydrolysed into Al(OH)3 in a large amount, mainly in the form of Al3+ ions, and the coagulation effect is extremely Difference. When the pH value is between 6.5 and 7.5, Al3+ is hydrolysed and polymerized into Al(OH)3 neutral colloid with a high degree of polymerization, and the coagulation effect is better. After pH value ﹥8, Al3+ is hydrolysed into AlO2-, and the coagulation effect becomes very poor.
The alkalinity of water has a buffering effect on the pH value. When the alkalinity is not enough, lime and other agents should be added to supplement it. When the pH of the water is high, you need to add acid to adjust the pH to neutral. In contrast, polymer flocculants are less affected by pH.
Water temperature affects the rate of hydrolysis of the flocculant and the rate and structure of alum formation. The hydrolysis of coagulation is mostly an endothermic reaction. When the water temperature is low, the hydrolysis rate is slow and incomplete.
At low temperature, the viscosity of water is large, the Brownian motion is weakened, the number of collisions between the flocculant colloidal particles and the impurity particles in the water is reduced, and the shear force of the water increases, which hinders the mutual adhesion of the coagulated flocs; therefore, despite the increase With the dosage of flocculant, the formation of flocs is still very slow, and the structure is loose and the particles are small, making it difficult to remove.
Low temperature has little effect on polymer flocculants. However, it should be noted that when using organic polymer flocculants, the water temperature should not be too high. High temperature will easily cause the organic polymer flocculants to age or even decompose to produce insoluble substances, thereby reducing the coagulation effect.
The uneven particle size of impurities in the water is beneficial to coagulation, and small and uniform particles will lead to poor coagulation effects. Too low concentration of impurity particles is often detrimental to coagulation. At this time, refluxing sediment or adding coagulant can improve the coagulation effect. When the impurity particles in the water contain a large amount of organic matter, the coagulation effect will be worse, and it is necessary to increase the dosage or add oxidants and other agents that play a role in coagulation. Calcium and magnesium ions, sulfides, and phosphides in water are generally beneficial to coagulation, while certain anions and surface active substances have an adverse effect on coagulation.
Types of flocculants
The choice of flocculant mainly depends on the nature and concentration of colloids and suspended solids in water. If the pollutants in the water are mainly in a colloidal state, inorganic flocculants should be the first choice to make them destabilize and agglomerate. If the flocs are small, it is necessary to add a polymer flocculant or use a coagulant aid such as activated silica gel.
In many cases, the combined use of inorganic flocculants and polymer flocculants can significantly improve the coagulation effect and expand the scope of application. For macromolecules, the greater the amount of charge on the chain molecules, the higher the charge density, the more fully extended the chain, the greater the range of adsorption and bridging, and the better the coagulation effect.
When coagulation is used to treat any wastewater, there is an optimal flocculant and an optimal dosage, which are usually determined by experiments. Excessive dosage may cause the stability of the colloid. Generally, the dosage range of ordinary iron salt and aluminium salt is 10-100mg/L, the dosage of polymer salt is 1/2～1/3 of the dosage of ordinary salt, and the dosage range of organic polymer flocculant is 1～5mg/L. .
Dosing sequence of flocculant
When using multiple flocculants, it is necessary to determine the best dosing order through experiments. Generally speaking, when the inorganic flocculant is used together with the organic flocculant, the inorganic flocculant should be added first, and then the organic flocculant should be added.
When the particle size of the treated impurities is above 50μm, the organic flocculant is often added to absorb the bridge first, and then the inorganic flocculant is added to compress the electric double layer to destabilize the colloid.
In the mixing stage, the flocculant and water are required to mix quickly and evenly. In the reaction stage, it is necessary to create enough collision opportunities and good adsorption conditions to allow sufficient growth opportunities for the flocs, and to prevent the small flocs that have been formed from being Broken, so the stirring intensity should be gradually reduced, and the reaction time should be long enough.
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