Zinc Sulfate: A Comprehensive Analysis of Properties, Applications and Safety Protection

Zinc sulfate (chemical formula: ZnSO₄) is an important inorganic compound. It is a colorless, hygroscopic crystal at room temperature, easily soluble in water, with an weakly acidic aqueous solution. It decomposes at 680℃, with a density of about 3.8g/cm³ and a solubility in water of 22g/100ml at 20℃. The logarithm of its octanol/water partition coefficient is -0.07. As a core category of zinc-containing compounds, zinc sulfate has been widely used in medicine, food, agriculture, industry and many other fields due to its unique physical and chemical properties. At the same time, because of its certain hazards, standardized operation and protection are particularly important. This article will comprehensively analyze the core value and usage points of zinc sulfate from four dimensions: basic properties, production processes, application scenarios and safety protection.

I. Basic Physical and Chemical Properties of Zinc Sulfate

Zinc sulfate exists in various forms, among which the most common is zinc sulfate heptahydrate (ZnSO₄·7H₂O), also known as white vitriol. In addition, there are variants such as zinc sulfate monohydrate and anhydrous zinc sulfate. Different variants have slightly different physical and chemical properties, but their core chemical properties are consistent. Its core characteristics can be summarized as follows:

• Physical Properties: The pure product is colorless and transparent crystals or white powder, which is easy to absorb moisture. When exposed to air, it will gradually lose crystal water and become anhydrous zinc sulfate; it has no special odor, tastes astringent, and its melting point varies with the content of crystal water. Zinc sulfate heptahydrate will lose crystal water at 56-79℃, while the melting point of zinc sulfate monohydrate is between 231-276℃ (under nitrogen atmosphere).

• Chemical Properties: The aqueous solution is weakly acidic, which can react with alkalis to form zinc hydroxide precipitates and with barium salts to form barium sulfate precipitates; it has certain oxidizing properties, but it is non-flammable and does not pose an explosion hazard; it is not easy to deteriorate when exposed to air for a long time and has strong stability, but it should be avoided to contact with oxidants and other substances.

• Purity Standard: Different application fields have different requirements for the purity of zinc sulfate. The main content of industrial-grade zinc sulfate is usually stably between 98.0%-99.5%, and the total content of heavy metal impurities (such as Pb, Cd, As) is less than 10ppm, which meets the first-class product standard of HG/T 2523-2022 "Industrial Zinc Sulfate"; pharmaceutical-grade and food-grade zinc sulfate have higher purity requirements, and the impurity content must be strictly controlled within ppm level.

II. Main Production Processes of Zinc Sulfate

The production processes of zinc sulfate are mainly divided into two categories: ore leaching method and waste zinc recovery method. In addition, there are auxiliary processes such as electrolytic zinc by-product method. Different processes are suitable for different raw material scenarios, each with its own advantages and disadvantages, as follows:

(I) Ore Leaching Method (Mainstream Process)

This process uses zinc-containing ores such as smithsonite and sphalerite as raw materials, and is the main production method of industrial-grade zinc sulfate. The specific steps are divided into four steps:

1. Ore Roasting: Smithsonite (ZnCO₃) is calcined at high temperature to convert into zinc oxide (ZnO), and sphalerite (ZnS) turns dark yellow after calcination, which is convenient for subsequent acid leaching to extract zinc elements.

2. Acid Leaching for Zinc Extraction: The roasted ore powder is reacted with dilute sulfuric acid, the pH value is controlled at about 5.2, and impurities such as silica and iron hydroxide are removed by filtration to obtain the initial zinc sulfate solution. The reaction formula is ZnO + H₂SO₄ → ZnSO₄ + H₂O, the reaction temperature is controlled at 60-90℃, and the leaching rate can reach more than 98.5%.

3. Deep Purification: Zinc powder replacement method is used to remove residual heavy metal impurities such as copper and cadmium in the solution, and potassium permanganate oxidation method is used to remove impurities such as iron and manganese to ensure that the solution purity meets the standard.

4. Concentration and Crystallization: The purified zinc sulfate solution is evaporated until a crystal film appears, cooled and crystallized, centrifugally dehydrated, and dried at low temperature to obtain the finished zinc sulfate crystal. Zinc sulfate heptahydrate or anhydrous zinc sulfate can be produced according to requirements.

(II) Waste Zinc Recovery Method (Green and Environmentally Friendly Process)

This process uses zinc-containing waste materials such as waste battery zinc sheets, zinc slag and electroplating sludge as raw materials, which is in line with the "double carbon" strategy and the orientation of resource recycling. In recent years, its application proportion has been continuously increasing. The specific steps are as follows:

1. Raw Material Pretreatment: Waste zinc sheets need to be cleaned to remove oil and dried, and zinc slag, electroplating sludge, etc. need to be crushed to a particle size ≤2mm to increase the reaction contact area and improve the leaching efficiency.

2. Acidolysis Reaction: The pretreated waste zinc raw materials are reacted with 20%-30% dilute sulfuric acid, the pH value is controlled at 4.8-5.0, and the temperature is 80-90℃ to generate zinc sulfate solution. The reaction formula is Zn + H₂SO₄ → ZnSO₄ + H₂↑.

3. Impurity Removal and Purification: Oxidants (such as NaClO₃) are added to oxidize Fe²⁺ in the solution to Fe³⁺, and the pH value is adjusted to 3.5-5.0 to generate iron hydroxide precipitation; then zinc powder is added to replace heavy metals such as copper and cadmium, and the filter residue is removed by filtration to obtain pure zinc sulfate solution.

4. Evaporation and Crystallization: The filtrate is evaporated to 49-52°Be (saturated state), cooled to 12-18℃ to precipitate zinc sulfate heptahydrate crystals, and the finished product is obtained after centrifugal dehydration.

(III) Electrolytic Zinc By-product Method (Low-cost Process)

This process relies on the hydrometallurgical zinc smelting process of large zinc smelters, and uses the zinc-containing solution (Zn²⁺ concentration about 80-120g/L) generated in the purification process to directly concentrate and crystallize after adjusting the composition, saving the raw material procurement and primary leaching links. The comprehensive cost per ton of products is much lower than other processes, and the energy consumption is also lower. The comprehensive energy consumption per ton of products is only 0.32 tons of standard coal, which is 29% lower than the ore leaching method. However, this process is highly dependent on the main process capacity, and the product purity is restricted by the impurity control level of the main process. Extraction and refining technology is required to meet the needs of high-end fields.

III. Multi-field Application Scenarios of Zinc Sulfate

The application of zinc sulfate covers four core fields: medicine, food, agriculture and industry. The application scenarios and action mechanisms in different fields are different, which fully reflects its versatility and important value.

(I) Medical Field: Auxiliary Treatment and Protection

Zinc sulfate has antiseptic and astringent effects in the medical field, and is divided into oral and external dosage forms. The specific applications are as follows:

• External Application: Eye drops can be used for relieving itching and anti-inflammation of chronic conjunctivitis and trachoma, and alleviating eye discomfort; local wet compress with solution can be used for auxiliary treatment of eczema and dermatitis, reducing redness, swelling and itching symptoms; it can also be used for local care of canker sores to promote wound healing.

• Oral Application: As a zinc supplement, it is used to prevent and treat zinc deficiency-induced anorexia, anemia, growth retardation, nutritional dwarfism and acrodermatitis enteropathica; it can also be used for auxiliary treatment of pica, rheumatoid arthritis, intermittent claudication, hepatolenticular degeneration (for those who cannot use penicillamine), acne and chronic ulcers; in addition, taking advantage of its adverse reactions such as nausea, vomiting and diarrhea when taken orally, it can be used for emesis in case of pesticide and drug poisoning.

• Precautions: Oral zinc sulfate should be taken after meals to reduce gastrointestinal irritation; it is contraindicated for those allergic to zinc sulfate, and should be used with caution for those with allergic constitution; eye drops should be used as soon as possible after opening to avoid deterioration of the medicinal solution.

(II) Food Field: Nutritional Fortifier

Zinc is one of the indispensable trace elements for human development, sexual maturity, intellectual development and body immunity, especially crucial for children's growth and development. About 40% of children in China are in a critical state of zinc deficiency, and zinc sulfate is one of the six mineral nutritional fortifiers approved for use in China.

Its applications in the food field are mainly divided into two categories: one is directly used as a nutritional fortifier, added to infant formula food, dairy products, cereal products, beverages and other products to supplement the zinc needed by the human body; the other is used to produce zinc-rich yeast, adding zinc sulfate to the yeast culture system to form a biological zinc supplement, which is more easily absorbed and utilized by the human body and widely used in the health product field.

(III) Agricultural Field: Trace Element Fertilizer and Animal Feed Additive

Zinc sulfate is a commonly used trace element fertilizer in the agricultural field, and also an important preventive and therapeutic agent for animal zinc deficiency. The application scenarios are divided into plant planting and animal breeding:

• Plant Planting: It is used to prevent diseases of fruit trees and nurseries, supplement the zinc needed for crop growth, and can be used as base fertilizer, foliar fertilizer and seed dressing fertilizer. The specific usage is: take 0.04~0.06 kg of zinc fertilizer, mix with 1 kg of water, dress 10 kg of seeds, pile and stuff for 2~3 hours before sowing; apply zinc fertilizer to the rhizosphere layer before sowing, 0.75~1 kg per mu; if the leaf color is light during the seedling stage, zinc fertilizer can be sprayed, 0.1 kg per mu, which can effectively improve crop yield and quality, and prevent crops from small leaf disease, chlorosis and other diseases caused by zinc deficiency.

• Animal Breeding: It is used to treat zinc deficiency in animals, mostly mixed with feed for oral administration. When animals are deficient in zinc, they will have symptoms such as incomplete keratinization of the skin, loss of appetite, slow growth, decreased plasma alkaline phosphatase activity, growth stagnation and abnormal epithelial cell metabolism. It will also lead to reduced sperm production and motility, dermatitis, lack of feathers, low immune function, easy infection of diseases, affecting growth and development and feed utilization rate. Supplementing zinc sulfate can effectively alleviate the above symptoms.

(IV) Industrial Field: Raw Materials and Auxiliaries for Multiple Industries

Zinc sulfate is widely used in the industrial field, involving chemical industry, chemical fiber, papermaking, leather, electroplating and many other industries, as follows:

• Chemical Industry: It is used to manufacture lithopone and various zinc salts, and is an important raw material for chemical synthesis; it can also be used as a background resist agent for phthalocyanine brilliant blue and an anti-alkali agent for vanadium blue dyeing to improve dyeing effect and stability.

• Chemical Fiber Industry: It is used as an acid bath treatment agent and coagulant for chemical fiber yarn, rayon, artificial wool, viscose chemical fiber, etc., to help chemical fiber forming and improve product quality.

• Papermaking and Leather Industry: In the papermaking industry, it is used as a bleaching agent to improve the whiteness of paper; in the leather industry, it is used as a preservative to extend the service life of leather and prevent mildew.

• Electroplating Industry: It is used to prepare zinc plating solution, imitation gold plating solution and cyanide-free copper-zinc-tin ternary alloy imitation gold plating solution, improve the gloss and adhesion of the electroplating layer, and is widely used in the electroplating processing of hardware, electronic and other products.