I have a question: What are the differences between demineralized water, deoxygenated water, deionized water, and softened water?

Deionized water is water with a very high level of purity; it contains little or no minerals, and is generally obtained through reverse osmosis, ion exchange, or a combination of these methods. Softened water refers to water whose hardness (mainly referring to calcium and magnesium ions in the water) has been removed or reduced to a certain extent. During the softening process of water, only the hardness decreases, while the total salt content remains essentially unchanged. Softened water is generally used in industry for boiler feed water to prevent scaling. Deoxygenated water is water from which oxygen has been removed; it is typically used in boiler circulation systems as feed water, and the purpose of deoxygenation is to prevent oxygen-induced corrosion in boilers.

What was said upstairs is right! As a supplementary note, the boiler feed water is deoxygenated demineralized water! :lol

These are various types of pure water. The types of pure water commonly used in chemical laboratories are distilled water and deionized water; their meanings can be understood by using the method of \"taking the name at face value\".

Generally, boiler water needs to be deoxygenated.

It seems that sometimes there isn’t a clear distinction between desalinated water and softened water

Desalination, deoxidation, water softening, and deionized water are commonly used in boiler systems. Deionized water is widely used.

Here, first-stage demineralized water is referred to as softened water, with the following specifications: conductivity (at 25°C) ≤ 10μs/cm, and silica content ≤ 100μg/L； The specifications for secondary deionized water are as follows: conductivity (at 25°C) ≤ 0.2 μs/cm, and silica content ≤ 20 μg/L ； Deoxygenated water is boiler feed water; deoxygenated water is obtained after the desalinated water has passed deoxygenation tests.

　　Desalinated water, also known as purified water, is water from which highly soluble electrolytes that can be easily removed have been eliminated or reduced to a certain level. The residual salt content in the desalinated water should be between 1 and 5 milligrams per liter. The main methods for producing desalinated water are as follows: ① Distillation, which involves heating salty water to cause it to evaporate, and then condensing the vapor to obtain desalinated water ； ②In the ion exchange method, salt-containing water is passed through an exchange column filled with zeolite or ion exchangers (see ion exchange); ions such as calcium and magnesium remain in the exchange column, while the filtered water is desalinated water ； ③In electrodialysis, thanks to the selective permeability of ion exchange membranes to ions, under the influence of an external electric field, the cations and anions in the water between the two ion exchange membranes move respectively toward the cathode and anode through those membranes. Thus, the area between the membranes becomes a fresh water zone, while the area outside the membranes is a concentrated water zone. Water drawn from freshwater areas is desalinated water. 　　Distillation is commonly used in laboratories to clean containers or prepare solutions, and it is suitable for situations where small quantities are involved and high purity is required. Ion exchange and electrodialysis methods are commonly used in the chemical industry, for example in boiler water, to reduce scaling and corrosion; they are suitable for applications where large volumes are handled and the purity requirements are not very high. 　　Deoxygenated water, also known as deoxidized water. Water contains oxygen, which is determined by the solubility of oxygen in water. At 1 atmosphere pressure and 0 degrees, the dissolved oxygen in water is 14 milligrams per liter; at 90 degrees, it is 1.6 milligrams per liter ； At 0.4 atmospheres and 70 degrees, the dissolved oxygen in water is 0.4 milligrams per liter at 1 atmosphere. 　　Deoxygenated water is mainly used for boiler feed water. Corrosion caused by dissolved oxygen in boiler feedwater is known as oxygen corrosion, also referred to as oxygen depolarization corrosion. The dissolved oxygen in boiler feedwater acts as a depolarizer for the cathodic process in corrosion reactions, causing oxygen corrosion in steel equipment. Since the final product of corrosion is iron oxide, this in turn leads to a deterioration in the quality of the boiler feed water. Therefore, boilers have strict requirements regarding the oxygen content in the boiler feed water. The higher the boiler pressure, the lower the specified allowable oxygen content in the boiler feedwater. When the steam pressure at the boiler outlet is less than or equal to 1.6 MPa, the oxygen content in the feed water is required to be ≤ 0.1 ppm ； For low-pressure boilers with an outlet steam pressure of 1.6–2.5 MPa, the dissolved oxygen content in the feed water is required to be ≤0.05 ppm ； For high-pressure steam boilers: at pressures of 3.8–5.8 MPa, the oxygen content in the feedwater should be ≤0.015 ppm; at pressures greater than 5.9 MPa, it should be ≤0.007 ppm. Deoxygenation methods include thermal deoxygenation, vacuum deoxygenation, and chemical deoxygenation. For its mechanism and methods, reference can be made to professional materials on boiler water treatment. Its chemical properties remain unchanged, with no change in pH. 　　 　　Soft water refers to water that contains little or no soluble calcium and magnesium compounds. Soft water does not easily form scum with soap, unlike hard water. Natural soft water generally refers to water from rivers, streams, and lakes (freshwater lakes). Softened hard water refers to water that has had its calcium and magnesium salts reduced to levels of 1.0–50 milligrams per liter. Although boiling can turn temporarily hard water into soft water, it is extremely uneconomical to use this method for treating large amounts of water on an industrial scale. 　　Methods for softening water include: 1) Lime-soda method. First, the hardness of the water is determined, after which a fixed amount of calcium hydroxide and sodium carbonate is added; the calcium and magnesium ions in the hard water then precipitate out. 　　Ca(HCO3)2 + Ca(OH)2 = 2CaCO3↓ + 2H2O 　　2) Phosphate softening method. For boiler water, sodium phosphite (NaPO3) can be added as a water softener; it forms complexes with calcium and magnesium ions, preventing these ions from precipitating out when the water is boiled, and thus avoiding the formation of scale. This method is not suitable for softening drinking water. 　　Mg(HCO3)2 + 2Ca(OH)2 = Mg(OH)2↓ + 2CaCO3↓ + 2H2O 　　3) Ion exchange method. Although both zeolites and ion exchangers are insoluble in water, the sodium and hydrogen ions contained in them can undergo exchange reactions with the calcium and magnesium ions in hard water, causing these calcium and magnesium ions to be adsorbed by the zeolites, artificial zeolites, and ion exchangers and thus removed. Zeolites and ion exchangers that become ineffective after long-term use can be reused through regeneration; therefore, this method is an economical and advanced softening technique. 　　CaSO4 + Na2CO3 = CaCO3↓ + Na2SO4 　　4) Chemical addition method: Adding specialized scale inhibitors to water can change the way calcium and magnesium ions combine with carbonate ions, thereby preventing the formation and deposition of scale. There are many scale inhibitors available for industrial use today. The advantage of this method is that it requires less initial investment and has wide adaptability ； However, the operating costs are high when the water volume is large and soft; due to the presence of chemical substances, its use is heavily restricted, and it generally cannot be used for drinking, food processing, industrial production, and other purposes. It is also rarely used in civilian applications. 　　5) Electromagnetic method: A certain electric or magnetic field is applied to water in order to alter the properties of ions, thereby changing the rate of deposition of calcium carbonate (magnesium carbonate) as well as its physical properties during deposition, and thus preventing the formation of hard water scale. Its features are: low equipment investment, easy installation, and low operating costs ； However, its effectiveness lacks stability, and there are no uniform standards for measurement. Moreover, since its main function is merely to affect the physical properties of scale within a certain range, the duration for which the treated water can be used and the distance over which it can be utilized are limited. It is mainly used for the treatment of circulating cooling water in commercial applications such as central air conditioning, and cannot be applied to the treatment of water used in industrial production or as boiler make-up water (also because the mechanism behind this type of equipment has not been fully theoretically proven). 　　6) Membrane separation method: Both nanofiltration membranes (NF) and reverse osmosis membranes (RO) can remove calcium and magnesium ions from water, thereby significantly reducing its hardness. The advantage of this method is that it yields significant and stable results, and the treated water can be used in a wide range of applications ； However, it requires a high inlet water pressure, and both the equipment investment and operating costs are high. It is generally used less often, and is mostly applied in specialized softening treatments. 　Deionized water: Water is passed through an anion and cation exchange resin bed; through this exchange process, the anions and cations in the water are removed, resulting in deionized water, which is commonly used for purposes requiring water in chemical laboratories. This post was last edited by Simonia on 2008-10-22 at 14:30]

Deoxygenated water is generally used in boilers; the purpose of deoxygenation is to reduce the corrosion caused by boiler feedwater on the boiler itself, as well as to minimize steam-water shock

In boiler feedwater treatment, water with a conductivity of less than 3 uS/cm (at 25°C) is referred to as distilled water. Water with a conductivity of less than 5 uS/cm (at 25°C) and a SiO2 content of less than 100 ug/L is called primary deionized water; water with a conductivity of less than 0.2 uS/cm (at 25°C) and a SiO2 content of less than 20 ug/L is called secondary deionized water. Water with a conductivity of less than 0.2 uS/cm (at 25°C), concentrations of Cu, Fe, and Na of less than 3 ug/L, and a SiO2 content of less than 3 ug/L is known as high-purity water or ultra-pure water.