What Is Zinc Chloride (ZnCl2)?
Zinc chloride (ZnCl2), an inorganic compound, appears as a white, hygroscopic crystalline solid at room temperature. It dissolves readily in water and thus finds broad applications across various industries. In essence, comprising one zinc atom and two chlorine atoms, zinc chloride’s chemical formula is ZnCl2.
Structural and Properties Of Zinc Chloride
Chemical Structure and Composition
- Zinc chloride (ZnCl2) consists of zinc cations (Zn²⁺) and chloride anions (Cl⁻).
- It can exist as an anhydrous salt or form hydrates with varying water content, such as ZnCl₂·H₂O (monohydrate), ZnCl₂·1.5H₂O, ZnCl₂·2.5H₂O, ZnCl₂·3H₂O, and ZnCl₂·4H₂O.
- Additionally, zinc chloride can also form basic zinc chloride compounds like Zn₅(OH)₈Cl₂·H₂O and Zn₅(OH)₈Cl₂·4[(H₂O)x(NH₃)₁-x].
- Furthermore, it can form complexes with amino acids like zinc-lysine-chloride (ZLC) complex [Zn(C₆H₁₄N₂O₂)₂Cl]⁺Cl⁻.
Physical Properties
- Anhydrous zinc chloride is a white, hygroscopic solid with a high melting point of 283°C.
- Moreover, it is highly soluble in water and forms acidic solutions due to hydrolysis.
- The solubility and hydration behavior depend on temperature and zinc chloride concentration.
- Basic zinc chloride particulates can be synthesized with controlled particle sizes (D₁₀ > 100 μm, D₉₅ > 450 μm) and a spherical shape for better handling.
Preparation of Zinc Chloride
Raw Materials and Synthesis
- Zinc chloride solution is prepared with a zinc concentration of 60-150 g/L.
- Next, an ammonia solution with pH ≥ 10 is prepared, with ammonium chloride added as an inducer to provide chloride ions.
- Consequently, the ammonia-nitrogen concentration is 140-170 g/L and the chloride ion concentration is 80-160 g/L.
- Simultaneously, the zinc-containing raw material complexes and ammonia-leaches in the mixed ammonia/ammonium chloride solution, producing a zinc-ammonia complex solution with 60-150 g/L zinc.
- At the same time, the zinc chloride and zinc-ammonia complex solutions are added in parallel flow to a reaction vessel containing water or mother liquor of basic zinc chloride, heated to 70-90°C.
- By maintaining stirring, the reaction at 1200-1500 rpm for 2-4 hours at pH 6.0-7.0 yields a basic zinc chloride precipitate.
Reaction Conditions and Product Properties
- The reaction temperature is typically 60-90°C, and 20-40 minutes of additional reaction time is required after feeding is complete.
- The precipitated basic zinc chloride (Zn5(OH)8Cl2·H2O) is filtered, washed, and dried at 80-105°C for 4-8 hours to obtain the product.
- As a result, the product exhibits a nearly spherical particle shape, with D10 > 100 μm and D95 > 450 μm for good flowability and reduced dust formation.
- Importantly, this ammonia precipitation method avoids impurities, allows easy wastewater treatment, and is suitable for industrialization.
Alternative Synthesis Routes
- Basic zinc chloride can also be prepared from zinc chloride and calcium hydroxide, zinc oxide, and hydrochloric acid, or basic zinc carbonate and hydrochloric acid.
- Zinc-containing wastes or byproducts like zinc calcine can serve as raw materials through acid-leaching and purification steps.
- Electrochemical synthesis from molten zinc chloride is possible, potentially integrated with zinc production from silicon compounds.
The ammonia precipitation method using zinc chloride and zinc-ammonia complex solutions allows good control over product quality, particle properties, and process parameters for industrial-scale basic zinc chloride production suitable as a feed additive.
Applications of Zinc Chloride
Zinc Chloride as a Versatile Lewis Acid Catalyst
Zinc chloride is a mild Lewis acid that finds extensive use in promoting various organic reactions like cycloadditions, substitutions, additions, couplings, and reductions. It facilitates transmetalation reactions for the preparation of organozinc reagents. It catalyzes Diels-Alder reactions, Friedel-Crafts acylations, and reductive aminations.
Zinc Chloride in Metallurgy and Surface Treatments
In hot-dip galvanizing processes, zinc chloride, often as zinc ammonium chloride (ZnCl₂·NH₄Cl), acts as a flux, removing oxides and promoting the adhesion of zinc coatings to iron and steel surfaces. Zinc chloride solutions are also employed to extract and recover anhydrous zinc chloride, which is used in zinc production via fused salt electrolysis.
Synthesis of Zinc Compounds
Zinc chloride is a precursor for synthesizing various zinc-based compounds like basic zinc chloride (Zn5(OH)8Cl2·H2O), zinc oxide, and zinc hydroxide chloride. These compounds find applications in pharmaceuticals, catalysts, ceramics, and rubber industries. The synthesis routes involve precipitation, calcination, and thermal decomposition of zinc chloride solutions with alkalis or other reagents.
Pharmaceutical and Medical Applications
Zinc chloride exhibits antimicrobial and tissue-preserving properties, making it useful in pharmaceutical formulations and medical treatments. It is employed as a fixative mixture with plant alkaloids like sanguinarine and chelerythrine for treating skin cancers and melanoma. Zinc chloride hydroxide is explored for sustained zinc ion release in pharmaceutical agents.
Electroplating and Surface Finishing
In electroplating baths, zinc chloride helps deposit zinc coatings onto metal surfaces. Novel electroplating chemistries using zinc chloride improve productivity, enhance deposit properties, and offer economic benefits, particularly in barrel plating operations.
Application Cases
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Zinc Chloride Catalysts | Zinc chloride acts as a mild Lewis acid catalyst, facilitating various organic reactions like cycloadditions, substitutions, additions, couplings, and reductions. It enables transmetalation for organozinc reagent preparation and catalyses Diels-Alder, Friedel-Crafts acylation, and reductive amination reactions. | Organic synthesis, fine chemical manufacturing, pharmaceutical and agrochemical industries. |
| Hot-Dip Galvanizing | Zinc chloride, often as zinc ammonium chloride, serves as a flux to remove oxides and promote zinc coating adhesion on iron and steel surfaces during hot-dip galvanizing processes. | Surface treatment and corrosion protection of iron and steel components in automotive, construction, and manufacturing industries. |
| Zinc Production | Zinc chloride solutions are used to extract and recover anhydrous zinc chloride, which is then employed as a feed material for zinc production via fused salt electrolysis. | Primary zinc production from zinc ores and secondary zinc recovery from waste streams. |
| Zinc Compound Synthesis | Zinc chloride acts as a precursor for synthesizing various zinc compounds like zinc oxide, zinc sulfide, and zinc phosphate, which have applications in ceramics, pigments, and phosphors. | Ceramic, pigment, and phosphor manufacturing industries. |
| Textile Processing | Textile dyeing and printing industries, particularly for natural fiber processing. | Textile dyeing and printing industries, particularly for natural fibre processing. |
Latest Innovations of Zinc Chloride
Zinc Chloride Synthesis and Production
- Novel methods for preparing basic zinc chloride with improved purity, process control, and industrial scalability. Key aspects include parallel flow synthesis, temperature control (60-90 °C), filtration, washing, and drying steps.
- Techniques for recovering anhydrous zinc chloride from aqueous solutions, involving extraction with organic solvents (e.g. tributyl phosphate), stripping with ammonium chloride/hydroxide, and thermal decomposition of zinc ammine complexes.
Electrochemical and Analytical Innovations
- A continuous flow process for producing aqueous zinc chloride electrolyte saturated with chlorine gas, enabling efficient zinc electrowinning.
- Novel zinc chloride electroplating baths offering productivity gains, improved deposit properties, and economic benefits for barrel plating operations.
- Analytical methods like EDTA complexometry for accurate determination of zinc chloride content in industrial samples.
Fundamental Studies
- Investigations into zinc chloride speciation and coordination in aqueous solutions using Raman spectroscopy, identifying species like Zn(H2O)6^(2+), ZnCl^+, ZnCl2, and ZnCl4(H2O)2^(=).
- Electrochemical studies on fused zinc chloride electrolysis, elucidating reactions involving zinc oxide formation and hydrogen evolution.
- Reviews summarizing the properties and applications of zinc chloride as a mild Lewis acid catalyst for organic reactions and as a reducing agent in reductive elimination and coupling reactions.
Technical Challenges of Zinc Chloride
| Synthesis of High-Purity Basic Zinc Chloride | Developing novel methods for synthesising basic zinc chloride with improved purity, process control, and industrial scalability, focusing on aspects such as parallel flow synthesis, temperature control, filtration, washing, and drying steps. |
| Recovery of Anhydrous Zinc Chloride | Techniques for recovering anhydrous zinc chloride from aqueous solutions, involving extraction with organic solvents, stripping with ammonium chloride/hydroxide, and thermal decomposition of zinc ammine complexes. |
| Sustained-Release Zinc Chloride Formulations | Developing zinc chloride hydroxide compositions with sustained zinc ion release for pharmaceutical applications, containing zinc chloride, zinc hydroxide, and/or zinc oxide. |
| Zinc Chloride Fixative for Skin Cancer Treatment | Formulating zinc chloride fixative mixtures for treating skin cancers like melanoma, leveraging the tissue-preserving and hemostatic properties of zinc chloride, and potentially incorporating anticancer plant alkaloids. |
| Particulate Basic Zinc Chloride for Feed Applications | Producing basic zinc chloride particulate matter with large particle size, good flowability, and low dust generation for use in animal feed applications. |
To get detailed scientific explanations of zinc chloride, try Patsnap Eureka.
