The effect of calcium carbonate “alkalinity” (PH) on rubber and plastic products

As an important filler, calcium carbonate is widely used in the processing and production of rubber products, but the rubber produced by many manufacturers has problems such as yellowing and brittleness to varying degrees. Today, the editor mainly talks about the influence of the alkalinity of calcium carbonate on the brittleness and yellowing of rubber products, because alkalinity is often a factor that many manufacturers tend to ignore.

Industrial Precipitated Calcium Carbonate Alkalinity is actually the free base in our production process for some reason. Free base refers to the substance in the production of calcium carbonate, which is not converted into calcium carbonate but exists in the form of calcium hydroxide. If the alkalinity is too high, it will react with other plasticizers of rubber, causing rubber products to become brittle, yellow, and white. It can be seen that free alkali is an important technical index in calcium carbonate products and must be strictly controlled in production.

The main reason for the high alkalinity of calcium carbonate is the formation of basic calcium carbonate.

1 lime overcooked

In the process of lime calcination, due to the different size of lumps and poor control, it is easy to cause the phenomenon of over-burning of lime. Over-burned lime consumes a lot of water for digestion, and the water temperature is low and the digestion is incomplete, resulting in lime particles. Calcium carbonate generated during carbonization takes fine particles as nuclei and is deposited on it to form calcium carbonate-coated calcium oxide particles. We know that calcium oxide crystals are cubic and calcium carbonate crystals are rhombic hexahedrons. The intercrystalline angles of these two crystals are different, and different expansion coefficients occur after heating up, causing the crystal grains to be broken and calcium oxide to be released. Alkaline.

2 high free base

Basic calcium carbonate, especially in cold weather, due to the low temperature and high solubility of calcium hydroxide, in the carbonization process, calcium hydroxide solids and water-soluble calcium hydroxide ions exist in the lime milk, and then carbon dioxide reacts with carbon dioxide. Occurs in alkaline solutions, thus producing basic calcium carbonate. This basic calcium carbonate changes with the temperature of the carbonization solution and the amount of carbon dioxide introduced, and is transformed into calcium carbonate with three different structures (calcite type, spirit stone type, aragonite type). When carbonization reaches the end point, PH=8~10, it is alkaline and cannot destroy basic calcium carbonate. These basic calcium carbonates have not had time to change, and enter the next process. When entering the rotary dryer, the temperature rises and basic calcium carbonate decomposes into calcium hydroxide and calcium carbonate, making the alkalinity high.

High alkalinity, cold weather is more prominent than hot weather, the key is that hot weather is high temperature, water temperature is also high, lime digestion is better, and the temperature of the carbonization tower is also relatively high, and calcium hydroxide is low in solubility, and it is not easy to produce plate-like basic carbonic acid Calcium, easily converted into calcium carbonate. It can be seen that the alkalinity of hot weather is lower than that of cold weather.
Therefore, when processing rubber products, we should not only pay attention to the particle size, whiteness, moisture, sedimentation volume, mineral elements, etc. of calcium carbonate, but also pay attention to the alkalinity of calcium carbonate!

How to choose a good calcium carbonate?

After understanding the characteristics of calcium carbonate itself and the effect of calcium carbonate on the properties of filled rubber, it is relatively simple to put forward the basic requirements for calcium carbonate for rubber.

1. The content of calcium carbonate should be high, the compounds of elements such as silicon and iron should be as low as possible, and the content of harmful heavy metal elements should be strictly required.

2. The whiteness should be as high as possible. Whether heavy calcium or light calcium, its whiteness mainly depends on the resource. For rubber materials, the degree of whiteness does not affect the mechanical properties and processing properties of the material, but high whiteness gives people a good feeling, and the same performance with high whiteness has a more competitive advantage.

3. The same mesh and reinforcing performance, the lower the oil absorption value, the better. The maximum amount of butylene phthalate (DBP) that 100g of powder material can absorb is called the oil absorption value of the material.

For some rubber products, such as soft polyvinyl chloride, artificial leather, cable materials, etc., a plasticizer is required. The higher the oil absorption value of calcium carbonate, the easier it is to adsorb the plasticizer into the filler, making it lose the plasticized rubber. Therefore, in order to achieve a certain softness, the amount of plasticizer needs to be increased, resulting in an increase in cost. By treating the surface of calcium carbonate and coating the surface of calcium carbonate particles, its oil absorption value can be reduced. For example, the oil absorption value of light calcium carbonate treated with coupling agent can be reduced from 92.91g/100g to 49.33g/100g.

4. The fineness should be appropriate, not the finer the better, and the particle size distribution should also be determined according to the needs.

5. Activation and non-activation should be determined according to the needs of downstream users.

Common production methods of light calcium carbonate:

Light calcium carbonate, also known as precipitated calcium carbonate, referred to as light calcium, molecular formula CaCO, molecular weight 100.09. Light calcium is calcined limestone and other raw materials to form lime (the main component is calcium oxide) and carbon dioxide, and then add water to digest lime to form lime milk (the main component is calcium hydroxide), and then carbon dioxide is introduced to carbonize the lime milk to form calcium carbonate precipitation. It is obtained by dehydration, drying and crushing. Or first use sodium carbonate and calcium chloride to carry out a metathesis reaction to generate calcium carbonate precipitate, and then dehydrate, dry and pulverize it. Because the sedimentation volume of light calcium carbonate (2.4-2.8mL/g) is larger than that of light calcium carbonate (1.1-1.4mL/g), it is called light calcium carbonate.

light calcium properties

White powder, tasteless, odorless, specific gravity about 2.71. It decomposes at 825~896.6℃, and the melting point is 1339℃. There are two forms of amorphous and crystalline, and the crystalline can be divided into orthorhombic and hexagonal, which are columnar or rhombus. Insoluble in water and alcohol. Soluble in acid, while releasing carbon dioxide, an exothermic reaction. Also soluble in ammonium chloride solution. Stable in air with slight moisture absorption.

Light calcium use

It can be used as a filler in industries such as rubber, plastics, paper, coatings and inks. Widely used in organic synthesis, metallurgy, glass and asbestos production. It can also be used as a neutralizer for industrial wastewater, an antacid for gastric and duodenal ulcer disease, an antidote for acidosis, an SO remover in SO-containing waste gas, a dairy cattle feed additive and an anti-sticking agent for linoleum. It can also be used as a raw material for tooth powder, toothpaste and other cosmetics.

Production Process

Judging from the production of light calcium carbonate abroad, compared with China, it is more advanced in automation control and environmental protection measures. Foreign light calcium carbonate is mostly used in the production of pulp-like products, so it is widely used in the paper industry. The relevant technical indicators in foreign countries are different from those in China. On the whole, they are more suitable for the needs of light calcium carbonate users than those in my country.

The production process of light calcium carbonate is mainly divided into two types: intermittent bubble carbonization process and continuous spray carbonization process.

1. Production method of traditional light calcium carbonate

Traditional light CaCO production process The traditional production process of light calcium carbonate in the industry uses limestone as raw material, and the limestone is calcined at over 900 ℃ to generate quicklime and release CO2. The production process is: lime calcination-slaked lime digestion-lime milk carbonization-solid-liquid separation-drying-packaging.

The main chemical reactions are as follows:

Limestone is calcined to release CO CaCO→CaO+CO

Lime (calcium oxide) reacts with water to form slaked lime CaO+HO→Ca(OH)

CO is injected into slaked lime and reacted to form light calcium carbonate Ca(OH)+CO→CaCO+HO

2. Steel slag production of light CaCO process

The process of producing light CaCO by steel slag acetic acid method is: acetic acid medium extracts calcium ions from steel slag, silicon removal, CO dissolution and carbonation reaction, pure calcium carbonate precipitation and filtration and separation. The main chemical reactions are as follows:

Extraction of steel slag in acetic acid medium:

Ca+CaSiO+2CHCOOH → Ca++2CHCOO-+SiO+HO

Alkalization with NaOH:

Ca(CHCOO)+2NaOH → 2CHCOONa+Ca(OH)

Carbonation reaction:

Ca(OH)+CO → CaCO+HO

3. Preparation of high-quality light calcium carbonate by leaching phosphogypsum calcium residue with nitric acid

Crude Ca(NO) solution: leaching phosphogypsum calcium residue with an appropriate amount of nitric acid solution, react until no bubbles are generated, filter, remove nitric acid insoluble matter, and obtain crude Ca(NO) solution containing impurities. The main component of impurities is Fe ( NO), Al(NO), Mn(NO) and other soluble nitrates, as well as SO2-4, PO-4 and other anions.

Refined Ca(NO) solution: pass NH into the crude Ca(NO) solution, adjust the pH value of the solution, and adjust the alkali to remove impurities. At this time, the cations such as Al+, Fe+, Mn+ in the solution generate hydroxide and are insoluble , SO-4, PO-4 and other anions generate calcium salts that are insoluble, and these interfering impurities are removed by filtration to obtain a refined Ca(NO) solution. The relevant chemical reaction equations are:

M++2OH-→M(OH)↓(M=Fe, Mn, etc.); Ca++ SO-4→CaSO↓M++3OH-→M(OH)↓(M=Fe, Al, etc.); 3Ca++ 2PO- 4→Ca(PO)↓

Carbonization: Dilute the refined Ca(NO) solution to a certain volume and transfer it to a four-necked flask, control the reaction temperature, and the flow of NH and CO2 gas for carbonization. The chemical reaction equation is as follows:

Ca(NO)+2NH+CO+HO→CaCO↓+2NHNO

Washing: The precipitate is filtered and washed several times until the filtrate is free of nitrate ions. Drying: The washed precipitate is dried in an oven at 120°C for 2 hours to obtain a light calcium carbonate product.

4. Preparation of light calcium carbonate by ammonium chloride method

Quicklime reacts with NHCl to generate CaCl and NH, and excess quicklime reacts with water to generate Ca(OH), and the solution is strongly alkaline. In addition to alkali metal ions, other metal ions such as Mg2+, Fe3+, Mn2+, etc. generate hydroxides and are insoluble. Other anions except halogen ions, such as SO2-4.PO-4, generate calcium salts and are insoluble. They can be mixed with other insoluble impurities. Removed by filtration. The filtrate is passed into CO and carbonized to form CaCO precipitate, and other soluble impurities can be removed by filtration and washing. The reaction formula is as follows:

CaO+2NHCl → CaCl+2NH+HO

CaO+HO → Ca(OH)

M2++OH- → M(OH) (M=Mg, Fe, Mn, etc.)

M3++OH- → M(OH) (M=Fe, Al, etc.)

Ca2++SO2- → 4CaSO

Ca2++PO3- → 4Ca(PO4)

CaCl+2NH+CO+HO → CaCO+2NHCl

The filtered CaCO mother liquor, that is, the NHCl solution, can be recycled.