Regarding boiler scale inhibitors

Scale inhibitors and dispersants – Overview: Scale inhibitors and dispersants are chemical agents that play a significant role in the routine chemical treatment of circulating water. Examples such as hydroxyethylidene diphosphonic acid (HEDP) and aminotrimethylenephosphonic acid (ATMP) work by having certain functional groups in their molecules attract to the active sites on the surface of the crystal nuclei being formed by scale-forming metal salts through electrostatic forces; this prevents the growth of those crystals, keeping many of them in a microcrystalline state, which in turn increases the solubility of the scale-forming metal salts in water ； At the same time, due to the adsorption of scale-inhibiting molecules on the crystal surface, even as the crystals grow, they can only do so in a deformed manner, which causes the crystals to become distorted. The adhesion between the distorted crystal and the metal surface is reduced, so it does not tend to deposit on the metal surface ； Since the functional groups adsorbed on the crystal surface are only some of those in the scale-inhibiting molecule, the functional groups that do not participate in adsorption endow the crystals with ionic properties; the increased repulsive force due to charges causes the crystals to remain in a dispersed state. The simultaneous presence of these three effects means that the amount of scale inhibitor required to prevent the same amount of scaling metal salts from forming scale on metal surfaces in water is much lower than the amount of chelating agent needed to chelate the same amount of such salts in water and prevent their precipitation. This phenomenon is known as the “threshold effect,” also referred to as the “low-limit effect” or the “sub-stoichiometry effect.” Therefore, they can disperse insoluble inorganic salts in water, prevent or inhibit the deposition and scaling of such salts on the surface of metal equipment, maintain good heat transfer performance in the metal equipment, and ensure the normal progress of production. Scale inhibitors mainly include various natural dispersants, phosphonic acids, phosphonocarboxylic acids, phosphosulfonic acids, and polymer compounds; currently, the vast majority of scale-inhibiting dispersants in use are polymer compounds. Polyphosphates are considered to be the first scale inhibitors to be used in water treatment. In addition to its scale-inhibiting effect, polyphosphate also has a corrosion-inhibiting effect. However, its drawback is that it is prone to hydrolyzing into orthophosphates; if not controlled properly, this can turn what was originally a relatively mild issue of calcium carbonate scaling into a very serious problem of calcium phosphate scaling. The introduction of organophosphorus (salt) scale inhibitors in the 1960s marked a breakthrough in scale prevention technology for calcium carbonate in water. It also enables the use of ultra-low chromium treatment formulas for circulating cooling water. This ultra-low chromium formula is used in a pH range of 7.5–8.5. The use of a high pH value reduces the required amount of chromate for corrosion inhibition from 15–25 mg/L in traditional chromium-phosphorus-zinc formulations to 5–10 mg/L, **thereby reducing the adverse impact on the environment. This is possible because the use of organic phosphonic acid (salt) scale inhibitors allows the scale-forming salt calcium carbonate in water to deposit controllably in the cathodic region on the metal surface. This controlled deposition of calcium carbonate acts as a cathodic corrosion inhibitor. The use of organic phosphonic acid (salt) scale inhibitors has also led to the widespread adoption of the classical phosphate-based alkaline treatment formula for circulating cooling water, which consists of polyphosphates, organic phosphonic acids (salts), and polyacrylic acids (salts). The further improvement of phosphorus-based alkaline treatment formulations can be attributed to the advent of calcium phosphate scale-inhibiting copolymer scale inhibitors. Materials such as acrylic acid/methyl acrylate binary copolymers, acrylic acid/hydroxypropyl acrylate/methyl acrylate terpolymer, and acrylic acid/acrylamide/methylpropanesulfonic acid/phosphorous acid terpolymer have been developed to prevent calcium phosphate scaling. While today there are effective scale inhibitors available for calcium carbonate and calcium phosphate scales, there are not yet such effective inhibitors for magnesium calcium silicate scales, iron hydroxide scales, zinc hydroxide scales, calcium sulfate scales, and barium sulfate scales, although many chemicals have shown scale-inhibiting effects on these types of scales. Current research on scale inhibition and dispersants focuses on the development of various copolymers. Moreover, efforts are underway to develop a versatile water treatment agent. Furthermore, the development of environmentally friendly scale inhibitors in recent years has contributed to protecting the environment and water resources from pollution. Natural polymers and natural scale inhibitors are a type of water treatment scale inhibitor that have been used since early times. They are generally made by further processing natural products. Recently, the effectiveness of such scale-inhibiting products is inferior to that of newer organic products, but since they are generally non-toxic, they are still used in some water treatment systems. 1.1 Sodium lignosulfonate – Also known as sodium lignin sulfonate, lignosodium, sulfite lignin, or MN-type water-reducing agent. Molecular formula: Typical softwood sulfonate, C9H8.5O2.5(OCH3)0.85(SO3H)0.4. Molecular weight: 5000–100,000. 1) Physical and chemical properties: It is divided into soft pulp type and hard pulp type depending on the raw materials used in its production. Cork lignin is currently the one that is widely used. Lignosulfonates, also known as sulfite lignin, have varying molecular weights and structures; they are heterogeneous anionic polyelectrolytes with polydispersity. The solid product is a brown, free-flowing powder that is soluble in water and prone to absorbing moisture. It is soluble in water of any hardness and is not affected by changes in pH, but it is insoluble in ethanol, acetone, and other common organic solvents. It has good chemical stability in aqueous solutions. It has good diffusion properties and heat stability. The aqueous solution is brown to black in color and exhibits colloidal properties; its viscosity increases as the concentration rises. Lignosulfonates have little effect on reducing the interfacial surface tension between liquids; they cannot lower the surface tension of water nor form micelles. Their dispersing action relies primarily on the adsorption-desorption of the matrix and the generation of charges. 2) Technical specifications are shown in the table below. Reference specifications for sodium lignosulfonate in China: Parameter Name, Parameter Value; Parameter Name, Parameter Value. Appearance: Yellow-brown or brown solid; Water-insoluble matter/%: ≤0.4; Moisture/%: 7; Total reducing substances/%: ≤3; Total calcium and magnesium content/%: ≤0.6; pH value: 9.0–9.5; Sodium sulfate/%: ≤3. 3) Toxicity and safety: Sodium lignosulfonate is non-toxic, with an LD50 value of >5 g/kg. The U.S. Food and Drug Administration has approved the use of this product in the manufacturing and processing of various foods and food packaging. 4) Applications: Sodium lignosulfonate contains phenolic hydroxyl groups and carboxyl groups in its structural units, which enable it to form insoluble protein complexes; it is used to control suspended solids and iron scale, and possesses dispersion properties. It is a dispersant and binder, as well as a corrosion inhibitor. In circulating water treatment agents, this product is utilized for its chelating effect with zinc, thereby storing zinc in the water and continuously supplying a certain amount of zinc ions to suppress corrosion in water systems. It is combined with polyol phosphates and zinc to form composite agents. It acts as a dispersant and scale inhibitor in boiler water, exhibiting good thermal stability and maintaining excellent dispersing properties even at 250°C. It is also effective in dispersing hydrated oxides and organic dirt in water. Sodium lignosulfonate has dispersing, bonding, chelating, and emulsifying properties. Its stabilizing effect allows it to be widely used in various industries. However, its biggest drawback is its unstable composition, with performance often fluctuating; it tends to decompose under high temperatures and pressures, which is detrimental to modern water treatment. However, due to its advantages such as easy availability, low cost, and non-polluting nature, it is still used in small quantities in composite formulations for circulating cooling water treatment. This ingredient is present in the components of water treatment chemicals such as N-7350 and N-4324 produced by the American company Narco. 5) Usage: Generally, water treatment manufacturers directly incorporate it into composite water treatment agents according to the requirements of the formula. 1.2 Tannins, also known as tannic acid, gallic acid, galloyl tannic acid, or tannins in general, have the molecular formula C76H52O46 and a molecular weight of 1701.22. 1) Physical and chemical properties: Tannins are actually a general term referring to complex mixtures composed of gallic acid, resorcinol, hydroxycinnamaldehyde, and other phenolic derivatives; they often occur together with sugars. Based on their chemical structure, they can be classified as: 1) hydrolyzable tannins, which are tannins containing ester bonds and glycosidic bonds. It is easily hydrolyzed under the action of acids and enzymes. 2) Condensed tannins, in which all aromatic nuclei are connected by carbon bonds. Under the action of strong acids and strong oxidizing agents, intermolecular condensation can occur. Tannins appear as a pale yellow amorphous powder, or as sparsely arranged, shiny flaky or spongy solids; they turn black when exposed to air. Odorless, with a strong astringent taste and an acidic character. Flammable. Its solubility in 100 g of water is 300 g; it is also soluble in ethanol and propanol, but insoluble in benzene, chloroform, carbon disulfide, carbon tetrachloride, ether, and petroleum ether. It begins to melt at 210°C and decomposes mostly into pyrogallic acid and carbon dioxide. Flash point 198.9°C. Autoignition temperature: 526.7°C. In aqueous solution, it can be precipitated using strong acids or salts (NaCl, Na2SO4, KCl). In alkaline solutions, it is easily oxidized by air, causing the solution to turn dark blue. Tannins act as reducing agents, and can react with albumin, starch, gelatin, and most alkaloids to produce insoluble precipitates. Tannins oxidize easily when exposed to air and sunlight, causing their color to darken and leading to clumping due to moisture absorption; therefore, they should be stored in a sealed container away from light. 2) Technical specifications are shown in the table below. According to GB5308-85 (Industrial tannic acid): Parameter Name, Grade 1, Grade 2, Grade 3; Appearance: Pale yellow to light brown amorphous powder; Tannic acid content (on a dry basis)/%, ≥81.0, ≥78.0, ≥75.0; Loss on drying/%, ≤9.0, ≤9.0, ≤9.0; Water-insoluble substances/%, ≤0.6, ≤0.8, ≤1.0; Total color value, ≤2.0, ≤3.0, ≤4.0. 3) Toxicity and safety: It has low toxicity; the oral LD50 in mice is 6000 mg/kg. 4) Applications: It is mainly used in industries such as pharmaceuticals, inks, printing and dyeing, leather, rubber, and metallurgy, as well as in water treatment. Tannins play roles in water treatment such as flocculation, deoxygenation, corrosion inhibition, scale prevention, and sterilization. It is a dispersant in circulating cooling water treatment. Due to its large number of hydroxyl groups and carboxyl groups formed after partial hydrolysis, it has the ability to form complexes with various metal ions such as Fe3+, Ca2+, and Mg2+, thereby serving a dispersing function. Furthermore, tannins can prevent dissolved oxygen from causing cathodic depolarization, or they can react with iron ions or iron oxide on the metal surface to form an impermeable protective film with a network structure, thereby suppressing the corrosion of iron. Furthermore, taking advantage of tannins’ tendency to absorb oxygen in acidic media, boiler oxygen corrosion can be effectively prevented. It can be used in conjunction with zinc salts in circulating water treatment. Furthermore, tannins have a certain bactericidal effect on sulfate-reducing bacteria in water. 5) Usage method: a. When used as a scale-inhibiting and dispersing agent in a circulating cooling water system, the appropriate pH range is 6–8, with a concentration of around 50 mg/L. b. When used as a deoxidizer in boilers, a pH value of around 11 is recommended. At a pH of 11, tannins possess good deoxygenation capabilities, similar to hydrazine or sodium sulfite. It can inhibit the corrosion of carbon steel, with a corrosion inhibition rate as high as 97%.