Soda ash big science popularization

I. Definition and properties of soda ash

(A) Definition of soda ash

Soda ash, also known as soda, lye ash, lye noodles or washing soda, is composed of sodium carbonate, with molecular formula Na2CO3 and molecular weight 105.99.

 Soda ash is an important basic chemical raw material and one of the two bases in the “three acids and two bases”, widely used in building materials, petrochemicals, metallurgy, food, textiles, national defense, medicine and many other areas of the national economy, and occupies a very important position in the national economy.

(B) the physical properties of sodium carbonate

Sodium carbonate at room temperature for the white odorless powder or particles, absorbent, exposed in the air gradually absorb water, will form lumps. There are three kinds of sodium carbonate hydrates containing water of crystallization, namely sodium carbonate monohydrate (Na2CO3-H2O), sodium carbonate heptahydrate (Na2CO3-7H2O) and sodium carbonate decahydrate (Na2CO3-10H2O). Sodium carbonate is soluble in water and glycerol, slightly soluble in anhydrous ethanol, insoluble in propanol. 20 ℃ per 100 grams of water can dissolve 20 grams of sodium carbonate, 35.4 ℃ when the maximum solubility of 49.7 grams of sodium carbonate per 100 grams of water can be dissolved.

(iii) Chemical properties of sodium carbonate

Sodium carbonate is a sodium salt. Hydrolysis of sodium carbonate in aqueous solution, ionized carbonate ions and hydrogen ions in water combined into bicarbonate ions, resulting in a reduction of hydrogen ions in solution, leaving the ionized hydroxide ions, so its aqueous solution is alkaline (pH = 11.6). Sodium carbonate is somewhat corrosive and stable, but it can also decompose at high temperatures to form sodium oxide and carbon dioxide. When exposed to air for a long time, sodium carbonate can absorb water and carbon dioxide in the air to generate sodium bicarbonate. Sodium carbonate can react with acid, salt and alkali. Complex decomposition reaction can occur with acid, such as in the case of sufficient hydrochloric acid, the generation of sodium chloride and carbonic acid, unstable carbonic acid immediately decomposed into carbon dioxide and water, sodium carbonate and other types of acid can also occur similar reactions. Similar reactions can also occur with other kinds of acids. Complex decomposition reactions can occur with salts, for example, some calcium salts and barium salts can produce precipitates and new sodium salts with sodium carbonate. With calcium hydroxide, barium hydroxide and other bases can occur complex decomposition reaction, the formation of precipitate and sodium hydroxide, the industry will use this reaction to prepare caustic soda (sodium hydroxide), commonly known as causticization method.

Second, the classification and use of soda ash

(A) Classification of soda ash

According to the different density, soda ash is mainly divided into light soda ash (hereinafter referred to as light soda ash) and heavy soda ash (hereinafter referred to as heavy soda ash), whose chemical composition is sodium carbonate, but the physical form is different: the density of light soda ash is 500-600kg/ m3, which is in the form of white crystalline powder; the density of heavy soda ash is 1000-1200kg/ m3, which is in the form of white fine particles. Compared with light alkali, heavy alkali has solid, large particles, high density, low moisture absorption, not easy to caking, not easy to fly, good fluidity and so on. In addition, there are also ultra-light soda ash and ultra-heavy soda ash, with densities of about 370kg/ m3 and 1550-2553kg/ m3 respectively.

1, according to the different uses, soda ash can be divided into industrial soda ash and edible soda ash. Industrial soda ash implements the national standard GB/T 210-2004 “Industrial Sodium Carbonate and its Test Methods” (hereinafter referred to as the national standard) issued by the People's Republic of China, including GB/T210.1-2004 “Industrial Sodium Carbonate and its Test Methods Part 1 Industrial Sodium Carbonate” and GB/T210.2-2004 “Industrial Sodium Carbonate and its Test Methods Part 2 Industrial Sodium Carbonate Test Methods”. Edible soda ash implements the standard GB1886-1992 “Sodium Carbonate for Food Additives”, which increases the content limit of arsenic and heavy metals on the basis of meeting the low saline standard of industrial soda ash.

2、According to the different uses, soda ash can be divided into industrial soda ash and edible soda ash. Industrial soda ash implements the national standard GB/T 210-2004 “Industrial Sodium Carbonate and its Test Methods” (hereinafter referred to as the national standard) issued by the People's Republic of China, including GB/T210.1-2004 “Industrial Sodium Carbonate and its Test Methods Part 1 Industrial Sodium Carbonate” and GB/T210.2-2004 “Industrial Sodium Carbonate and its Test Methods Part 2 Industrial Sodium Carbonate Test Methods”. Edible soda ash implements the standard GB1886-1992 “Sodium Carbonate for Food Additives”, which increases the content limit of arsenic and heavy metals on the basis of meeting the low saline standard of industrial soda ash.

3. According to the difference of chloride content, soda ash can be divided into common alkali, low-salt alkali, ultra-low-salt alkali and special low-salt alkali. According to the national standard, the mass fraction of sodium chloride of common soda ash is ≤1.20%; the mass fraction of sodium chloride of low-salt alkali is ≤0.90%; the mass fraction of sodium chloride of ultra-low-salt alkali is ≤0.70%; the mass fraction of sodium chloride of special low-salt alkali is ≤0.30%.

(II) Uses of soda ash

Soda ash is one of the important chemical raw materials, the use mainly includes the following fields:

First, the field of building materials, soda ash as one of the main raw materials of flat glass, providing sodium ions for the reaction, but also the main component of clarifier.

Secondly, in the field of chemical production, soda ash is widely used in the manufacture of sodium silicate (commonly known as effervescent soda, water glass), sodium bicarbonate (commonly known as baking soda), sodium fluoride, dichromate and other products.

Thirdly, in the field of printing, dyeing and metallurgy, soda ash is used as a water softener in the textile production process, or as a co-solvent in smelting, a flotation agent in ore dressing, and a desulfurizer in steel making.

Fourthly, in the field of food processing, soda ash is used as a pasta additive to play the role of neutralizer, bulking agent, buffering agent, dough improver, to increase the texture and pliability of pasta, and can also be used as the main auxiliary additives in the production of monosodium glutamate (MSG) and soy sauce.

In addition, soda ash is also widely used in environmental protection, desulfurization, pharmaceutical products, tanning, papermaking, etc. High-end soda ash can also be used in the manufacture of CRT glass shells and optical glass.

Third, the production process of soda ash

Before there is no industrial production method, people from the natural alkali lake or from the alkali plant ash manufacturing soda ash, used in detergents and glass, soap, leather manufacturing and so on. With the development of industry, the original way of soda ash extraction can no longer meet the demand for alkali, large-scale industrial production of alkali production process came into being. At present, the alkali production process is mainly divided into synthetic alkali method and natural alkali method, the world soda ash production capacity of synthetic alkali method accounted for about two-thirds, natural alkali method accounted for about one-third.

(i) Synthetic alkali method

1. History of development

Phase I: Lubranpha

In 1788, the French chemist Lublanc researched the alkali production method using salt as raw material, i.e. the Lublanc Process. The Leblanc Process was the first large-scale industrial alkali production method in history, which led to the production of a series of chemical products such as sulfuric acid, hydrochloric acid, bleaching powder, manganese nitrate and sulfur. However, the purity of the finished soda ash produced was low, the production costs were high, and it was difficult to sell the highly corrosive hydrochloric acid recovered.

Phase II: Ammonia-base method

In the mid to late 19th century, the Solvay brothers in Belgium developed the Solvay Process, which is also known as the Ammonia Alkali Process because ammonia is used as a medium in the production process. Because ammonia is used as a medium in the production process, it is also called ammonia-alkali method. The raw materials of ammonia-alkali method are cheap and easy to obtain, the production is continuous, the purity of the products is high, which is suitable for large-scale production.

Stage III: the Union Base Method

In the 1930s, China's “modern chemical pioneer” alkali expert Dr. Hou Deping, combined with the actual production situation in China, developed Hou's Process, also known as the joint alkali production method or combined alkali method after a lot of experiments. This method builds ammonia plant and soda ash plant together and produces ammonium chloride as a by-product while producing soda ash. The Hou's Process organically combines the two production systems of ammonia synthesis and alkali production, which pushes the world alkali production technology level to a new height, and becomes the mainstream process of synthetic alkali production together with the ammonia-alkali process.

2, ammonia alkali production process

The production process of the ammonia-alkali method is shown in the figure, and the main reaction process is as follows: ammonia is passed into saturated brine to produce ammonia brine, then carbon dioxide is absorbed through the ammonia brine to obtain sodium bicarbonate (commonly known as baking soda), and finally, the sodium bicarbonate is calcined, that is, the light alkali is obtained.

The ammonia used in ammonia-alkali production comes from an ammonia plant, and the carbon dioxide comes from the calcination of limestone. The main component of limestone is calcium carbonate, which when calcined produces carbon dioxide and calcium oxide (quicklime). The carbon dioxide is used in the main reaction process to make soda ash, while the calcium oxide reacts with water to produce calcium hydroxide (slaked lime), which is added to the mother liquor after precipitating sodium bicarbonate to produce ammonia gas and a waste liquid and slag, mainly composed of calcium chloride, which is recycled to the main reaction process.

The limitations of the ammonia-alkali method are mainly manifested in the low utilization rate of raw salt and the pollution of the environment by waste liquid and waste residue. The utilization rate of sodium chloride in ammonia alkali method can only reach 75% at the highest, and the rest is discharged with waste liquid and waste residue. According to statistics, one ton of soda ash produced by ammonia alkali method should be discharged about ten tons of waste liquid. The waste liquid and waste residue contain a large amount of calcium chloride, which is difficult to be utilized and highly polluting. Therefore, the soda ash enterprises adopting the ammonia alkali production method are generally laid out along the coast or along the river, and the site selection of the plant is restricted.

3. Combined alkali production process

The co-soda method combines ammonia production with soda ash production, utilizing ammonia and carbon dioxide from the ammonia plant to produce both soda ash and ammonium chloride at the same time.

The reaction process of soda ash produced by the combined alkali method is the same as that of the ammonia alkali method, and the same is to pass ammonia and carbon dioxide into saturated brine successively to produce sodium bicarbonate, and then calcine the sodium bicarbonate to produce soda ash.

Compared with the ammonia-base method, the core innovation of the co-base method is to utilize the change of solubility at different temperatures to cool down the remaining mother liquor of the reaction (the main components are sodium bicarbonate, sodium chloride and ammonium chloride) and add sodium chloride crystals, so that the ammonium chloride can precipitate out in the form of crystals. The remaining saturated brine mother liquor can be recycled as a raw material for the reaction.

In addition, as a result of co-production with an ammonia plant, both ammonia and carbon dioxide in the reaction of the soda ash method can be directly supplied by the ammonia plant. Therefore, there is no need for a series of reactions such as calcination of limestone to generate carbon dioxide in the SOCA process.

Compared with the ammonia-alkali method, the combined alkali method, which is improved and developed on the basis of the ammonia-alkali process, mainly has the following advantages: firstly, it can get soda ash and ammonium chloride as by-products at the same time; secondly, it comprehensively utilizes raw material resources, which not only raises the utilization rate of raw salt to more than 95%, but also saves the inputs of resource raw materials, such as coke and limestone; thirdly, it avoids the inputs of a series of auxiliary equipments, such as mining, mine transportation and lime kiln; fourthly, it avoids the emission of a large amount of waste slag and waste liquid in the ammonia-alkali method. Fourth, it avoids the emission of a large amount of waste slag and waste liquid in the ammonia-alkali method.

However, compared with the ammonia-alkali method, there are also some shortcomings in the combined alkali method, mainly because of the relative complexity of maintenance and the difficulty in controlling the quality of the output soda ash. In addition, the profit of the production enterprises of the soda ash method is affected by the cost of the upstream raw material synthetic ammonia and the downstream ammonium chloride market price at the same time.

(ii) Natural alkali method

1. History of Development

In 1849, pioneers found sodium bicarbonate in Sweetwater River, Wyoming, which was used in detergents and pharmaceuticals. 1905, the United States for the first time to use the natural alkali of Sears Lake (Searles Lake) to try to produce soda ash on a large scale. 1938, the U.S. Intermountain Fuel Supply Company found the world's largest natural alkali mine in the Green River Basin of Wyoming (Green River Basin, Wyoming) when exploring oil and gas resources. In 1938, the U.S. Intermountain Fuel Supply Company in Wyoming, Green River Basin (Green River Basin, Wyoming) exploration for oil and gas resources, discovered the world's largest natural alkali mine, and then the U.S. soda ash industry to its unique natural alkali resources advantage of the rapid development of the soda ash industry. To 1976, the United States through the natural alkali method output of soda ash accounted for 70% of its total domestic production, to 1982 the proportion as high as 94%, and then the United States soda ash is almost entirely produced by natural alkali method.

At present, the world's proven natural alkali mines are only distributed in the United States, China, Turkey, Mexico and southern Africa and a few other countries and regions, of which the United States, Turkey and China are the main natural alkali production countries. The application of natural alkali process is limited by the geographic location of resources, supply and other basic conditions, compared with the other two mainstream production processes, the biggest advantage is the lower cost.

2、Natural alkali production process

According to the different composition of alkali ore, natural alkali production process is mainly divided into evaporation method and carbonization method. For the natural alkali ore with crystal alkali stone (Na2CO3? NaHCO3? 2H2O) as the main component, the general use of evaporation production process, through the decomposition or neutralization of the way to make the crystal alkali stone in the decomposition of sodium bicarbonate, and then evaporation and purification to produce soda ash. For alkali, nitrate, salt symbiosis of vesicular alkali type natural alkali water lake water or the main component of vesicular alkali (Na2CO3? 10H2O) solid ore, generally use the brine carbonization method production process, extraction of natural brine for carbonization, drying and calcination, the production of soda ash.

(iii) Light alkali to heavy alkali process

Whether it is synthetic alkali method or natural alkali method, the soda ash produced is light alkali. Light alkali is reprocessed to produce heavy alkali. At present, the mainstream processing technology has two kinds of water law and extrusion method. The water method is to crystallize the light alkali with water to generate sodium carbonate monohydrate, and then heat to drive out the water of crystallization, the produced heavy alkali retains the crystalline form, and the structure is more dense than light alkali. Extrusion method adopts pure machinery between the two rollers through high-pressure light alkali extruded into thin and hard lye cake, and then crushed lye cake screened out the best particle size, and then too coarse particles re-crushing, too fine particles to return to the extrusion, that is, the production of heavy alkali.