Aluminium sulphate used in water treatment

Aluminum sulfate is one of the common aluminum salts, and exists in anhydrous and various hydrate forms (16, 18, 27, etc.). The more stable ones are anhydrous aluminum sulfate and octahydrate aluminum sulfate. It is easy to hydrolyze, and the aqueous solution is acidic.

There are iron-free aluminum sulfate and industrial aluminum sulfate. The industrial product is yellow-green due to the iron content, and tastes sour and astringent. It is stable in the air. It loses crystal water when heated to 250℃, and begins to decompose into aluminum trioxide, sulfur trioxide and water vapor when heated to above 700℃. It is easily soluble in water, and the aqueous solution reacts acidically.

Al2(SO4)3+2H2O→Al2(SO4)2(OH)2+H2SO4, when the hydrate is heated, it expands violently and becomes spongy. When heated to red heat, it decomposes into sulfur trioxide and aluminum oxide.

Flocculent or sponge-like Al(OH)3 has a strong adsorption capacity and can adsorb pigments and fiber fabrics well. Therefore, it is used as a mordant in the printing and dyeing industry; it is also used to purify drinking water; in addition, in the papermaking industry, it is added to the pulp together with rosin to make the fibers bonded.

Product Category

1. According to its purity and manufacturing process, it is divided into refined aluminum sulfate and crude aluminum sulfate.

2. According to its use, it is divided into industrial aluminum sulfate specifically used for water treatment and food-grade aluminum sulfate used in the production of food packaging paper.

3. According to its physical state, it is divided into solid aluminum sulfate and liquid aluminum sulfate.

Product properties

White flakes, blocks, particles, powder, liquid

Coagulation mechanism

The coagulation mechanism of aluminum sulfate mainly includes three types: electrical neutralization, destabilization and adsorption aggregation:

1. Electrical neutralization:

Aluminum sulfate dissolves in water to generate charged ions, which interact with suspended colloidal particles. Due to the existence of isoelectric potential, they attract and collide with each other to form particles with a specific gravity greater than 1, and then agglomerate.

‌2. Destabilization‌:

In addition to the electrical neutralization effect, Al2(SO4)3 will also generate attached hydroxides after dissolution. This hydroxide colloid can adsorb and condense suspended solids in water. ‌

3. Adsorption and aggregation‌:

For suspended particles that are not colloids, the dissolved products of Al2(SO4)3 not only generate charged ions, but also generate attached hydroxides. These hydroxide colloids can adsorb and condense suspended solids in water, aggregate them together, and adsorb and precipitate other suspended small particles. ‌

In addition, aluminum sulfate, as the main accelerating component of alkali-free accelerator, has a good accelerating effect on cement hydration. With the increase of aluminum sulfate dosage, the setting time first shortens and then tends to be flat, indicating that aluminum sulfate has a good accelerating effect on cement. When the dosage of aluminum sulfate is 57%, the accelerating effect is the best. ‌

How to use aluminum sulfate in water treatment?

The use of aluminum sulfate mainly depends on the specific scenario of its application, such as water purification or sewage treatment. Here are some general usage steps and precautions:

1. Dissolution and preparation

Add an appropriate amount of aluminum sulfate to clean water. Note that the aluminum sulfate should be gradually added to the water first and stirred thoroughly until it is completely dissolved.

The dissolution ratio of aluminum sulfate is usually 1:3, that is, 1 part of aluminum sulfate is dissolved in 3 parts of water. There is also a saying that 0.5-1.0 grams of aluminum sulfate is added per liter of water, but the specific amount needs to be determined according to the water quality and treatment requirements.

2. Adjust the pH value

Before using aluminum sulfate, it may be necessary to test and adjust the pH value of the water body. Generally speaking, a neutral or weakly alkaline environment (pH between 6.5-8.5) has a better effect on aluminum sulfate. In sewage treatment, aluminum sulfate has a better flocculation effect when the pH value is between 6.0-7.0.

3. Add aluminum sulfate

Add the prepared aluminum sulfate solution evenly to the water to be treated. There are two ways of adding: static adding and dynamic adding. Static adding is to pour the aluminum sulfate solution evenly into the water body and stir it fully, while dynamic adding is to continuously add it into the water body through equipment such as pumps.

The dosage should be determined according to the water quality and treatment requirements, usually between 10-80 mg/L, and there is also a saying that the dosage per ton of water is 10-100 grams. Too high or too low dosage will affect the treatment effect.

4. Mixing and precipitation

In the process of mixing aluminum sulfate with water, the aluminum ions in aluminum sulfate will react chemically with suspended matter, colloids and organic matter in the water to form floccules. Over time, the floccules gradually increase and settle to the bottom of the water body.

Stirring can promote full contact and mixing between aluminum sulfate and impurities in the water body, but the stirring time needs to be determined according to the size of the water treatment system and the water quality, generally 10-30 minutes.

5. Subsequent treatment

After the precipitation is completed, the supernatant can be gently extracted from the top to avoid muddying the sediment again.

If the water quality needs to be further improved, the water can be filtered using a filter or filter to remove smaller suspended matter and particles.

What are the uses for aluminum sulfate?

1. Used as a paper sizing agent in the papermaking industry to enhance the water resistance and anti-seepage properties of paper;

2. After dissolving in water, it can make the fine particles and natural colloid particles in the water condense into large flocs, thereby removing them from the water, so it is used as a coagulant for water supply and wastewater;

3. Used as a turbid water purifier, also used as a precipitant, color fixative, filler, etc. In cosmetics, it is used as a raw material for antiperspirant cosmetics (astringent);

4. In the fire protection industry, it is used with baking soda and foaming agent to form a foam fire extinguishing agent;

5. Analytical reagents, mordants, tanning agents, oil decolorizers, and wood preservatives;

6. Stabilizers for albumin pasteurization (including liquid or frozen whole eggs, egg whites or egg yolks);

7. It can be used as a raw material for the manufacture of artificial gemstones and high-grade ammonium alum, other aluminates;

8. In the fuel industry, it is used as a precipitant in the production of chrome yellow and lake dyes, and it also acts as a color fixer and filler.

9. It is used as an effective cross-linking agent for animal glue and can increase the viscosity of animal glue. It is also used as a curing agent for urea-formaldehyde adhesives, and the 20% aqueous solution cures quickly.

Case application

1. Water treatment: 

In a large water plant, industrial aluminum sulfate is used as a coagulant. By adding an appropriate amount of aluminum sulfate to the raw water, suspended matter, organic matter and heavy metal ions in the water can be effectively removed, and the clarity and safety of the water quality can be improved. Specific data shows that after adding aluminum sulfate, the turbidity in the water decreased by more than 80%, reaching the national drinking water standard.

2. Papermaking industry: 

In the papermaking industry, aluminum sulfate is used as a filler and coating to improve the whiteness, glossiness and smoothness of paper, making the final product more beautiful and durable. According to statistics, the sales volume of paper added with aluminum sulfate has increased by 20% in the market.