A Comprehensive Guide to Zinc Sulfate in Two Forms: Monohydrate vs. Heptahydrate – From Molecular Structure to Practical Applications
- Fernando Chen
- 21 minutes ago
- 11 min read
I. Introduction
In orchards, zinc deficiency in citrus trees can lead to chlorotic spots on leaves, slow growth of new shoots, fruit deformities, and even yield reduction. In livestock farms, a lack of zinc in feed may result in stunted growth and weakened immunity in animals. As an essential trace element, zinc is closely tied to both agricultural production and daily life.
Among the many zinc supplementation methods, zinc sulfate is the most commonly used source of zinc due to its high water solubility and excellent absorption efficiency. It is widely applied in agriculture, animal feed, and industrial production. In crop cultivation, it effectively prevents and corrects zinc deficiency symptoms; in animal nutrition, it serves as a vital trace element; while in industry, it is used in electroplating, pharmaceuticals, textiles, and more.
On the market, zinc sulfate is mainly available in two common forms: Zinc Sulfate Monohydrate (ZnSO₄·H₂O) and Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O). Although both belong to zinc sulfate, the difference in the number of water molecules in their crystalline structures leads to significant differences in zinc content, stability, applications, and cost.
Understanding the differences between these two is like distinguishing “different fruits from the same tree.” It has practical importance for selecting the right product, improving agricultural efficiency, and ensuring feed quality. In the following sections, we will provide a systematic comparison from three aspects—physicochemical properties, production processes, and application fields—and finally introduce the product forms and advantages we can offer.
II. Comparison of Physicochemical Properties
1. Basic Chemical Information
Zinc Sulfate Monohydrate (ZnSO₄·H₂O)
Molecular weight: 179.47
CAS No.: 7446-19-7
Appearance: White powder or granules
Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O)
Molecular weight: 287.54
CAS No.: 7446-20-0
Appearance: Colorless or white crystals
2. Crystal Water and Zinc Content
Zinc sulfate monohydrate contains only one water molecule, resulting in a higher zinc content.
Zinc sulfate heptahydrate contains seven water molecules, with a higher degree of hydration and lower zinc content.
This difference means that, under the same weight, monohydrate provides more available zinc, while heptahydrate requires a larger dosage to achieve the same zinc supplementation effect.
3. Physical and Chemical Characteristics
Solubility: Both are easily soluble in water, suitable for agricultural and feed applications.
Stability: Monohydrate is more stable and less prone to moisture absorption and caking; heptahydrate tends to absorb moisture and deliquesce in humid environments.
Storage and Transportation: Monohydrate is more suitable for long-distance transportation and long-term storage, while heptahydrate requires strict moisture-proof measures.
Comparison Table of Physicochemical Properties
Item | Zinc Sulfate Monohydrate (ZnSO₄·H₂O) | Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O) |
Molecular weight | 179.47 | 287.55 |
CAS No. | 7446-19-7 | 7446-20-0 |
Appearance | White powder or granules | White crystals |
Number of water molecules | 1 | 7 |
Solubility | Easily soluble in water | Easily soluble in water |
Storage & transportation | Convenient storage and transport in granular form | Requires strict moisture protection |
👉 Summary:
Zinc sulfate monohydrate has advantages in zinc content and stability, making it more suitable for scenarios that demand higher quality and efficiency. Zinc sulfate heptahydrate, on the other hand, retains an important role in traditional agriculture due to its cost and process advantages.
III. Mainstream Production Processes
Currently, the industrial production of zinc sulfate monohydrate and heptahydrate is primarily based on the “sulfuric acid–zinc oxide synthesis method.” However, due to differences in crystal water requirements, the subsequent concentration, crystallization, and drying processes vary significantly. The detailed process is as follows:
Step 1: Common Basic Process – Preparation and Purification of Crude Zinc Sulfate Solution
Raw material reaction
Industrial-grade zinc oxide (Zn content ≥ 40%) is used as the raw material. It is reacted with diluted sulfuric acid (30–40% concentration) in a reactor at 60–80 °C.
The neutralization reaction is as follows:
ZnO + H₂SO₄ → ZnSO₄ + H₂O
Stirring continues for 30–60 minutes to ensure complete dissolution of zinc oxide and to avoid residual unreacted impurities.
Two-step purification and impurity removal
First step: Add zinc powder. Utilizing the reducing property of zinc, heavy metal ions such as copper, cadmium, and lead are displaced from the solution (e.g., Cu²⁺ + Zn → Zn²⁺ + Cu↓). After stirring, filter out the precipitated metals.
Second step: Add potassium permanganate to the filtrate. This oxidizes Fe²⁺ to Fe³⁺. The pH is adjusted to 3.5–4.0 so that Fe³⁺ forms ferric hydroxide precipitates. After another filtration, a clarified and impurity-free zinc sulfate solution is obtained.
Step 2: Production of Zinc Sulfate Heptahydrate (Low-temperature retention of 7 water molecules)
Concentrate the clarified solution at 50–60 °C until supersaturation is reached.
Cool down to 20–25 °C and allow crystallization to occur, precipitating heptahydrate crystals.
Separate crystals by centrifugation and dry them at a low temperature (40–50 °C) to prevent loss of crystal water.
Final product: ZnSO₄·7H₂O, a white or colorless crystalline material.
Step 3: Production of Zinc Sulfate Monohydrate (High-temperature retention of only 1 water molecule)
This requires “high-temperature dehydration” to remove excess crystal water. In industrial practice, two pathways are commonly adopted depending on production scale:
Pathway 1: High-temperature conversion from heptahydrate
The wet crystals (or finished heptahydrate product) are fed into a rotary dryer.
Temperature is controlled at 100–150 °C, which precisely removes 6 molecules of crystal water, leaving only 1 bound to zinc sulfate.
Drying continues for 2–3 hours. Circulating hot air carries away evaporated water, while moisture content is monitored in real time.
After cooling to room temperature, the product can be milled into powder or granulated by extrusion, yielding ZnSO₄·H₂O with zinc content of approximately 35–36%.
Pathway 2: Direct drying of purified solution (preferred for large-scale production)
The purification step is followed directly by spray drying, without heptahydrate crystallization.
The solution is fed into a spray dryer with inlet air temperature at 180–220 °C and outlet air temperature at 80–90 °C.
The atomized droplets instantly contact hot air, evaporating water and forming zinc sulfate monohydrate powder (with only 1 water molecule).
If granular products are needed, the powder is mixed with a small amount of approved binder, extruded into granules, and cured at 80–100 °C to ensure hardness and flowability.
In summary: Both processes share identical raw material preparation and purification steps. By controlling the key parameter of temperature, manufacturers can differentiate the number of crystal water molecules, thereby meeting varying demands for zinc sulfate forms and zinc content across different applications.
IV. Comparison of Applications
4.1 Agricultural Applications (Fertilizers / Soil and Foliar Nutrition)
Role and Targets
Zinc (Zn) is an essential micronutrient: it participates in chlorophyll synthesis, hormone metabolism, and enzyme activation. Common zinc deficiency symptoms include chlorosis of young leaves, smaller and narrower leaves, shortened internodes, weak root systems, etc. Typical crops include rice, maize, wheat, citrus, grapes, cotton, peanuts, and more.
Zinc sulfate provides a water-soluble zinc source: it can be applied via basal fertilization (soil mixing/row application), foliar spraying, drip irrigation/fertigation, blending into compound fertilizers, or as part of water-soluble fertilizer formulations.
Monohydrate vs. Heptahydrate: Application Forms and Efficiency
Zinc Sulfate Monohydrate (ZnSO₄·H₂O)
Suitable scenarios: water-soluble fertilizers, foliar sprays, drip irrigation; high-content formulations; NPK+TE blending (granular form).
Advantages: higher zinc content, lower dosage needed for the same zinc input; better storage and transport stability with reduced caking risk; more suitable for precision fertilization and industrialized formula control.
Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O)
Suitable scenarios: traditional soil basal application, cost-sensitive field crops; sometimes used in local markets for self-mixed fertilizers or simple water-soluble applications.
Trade-offs: lower zinc content, requiring higher dosage for the same zinc supply; prone to moisture absorption and caking in humid environments, imposing stricter requirements on storage and handling.
4.2 Feed Applications (Trace Mineral Additive)
Role and Requirements
Functions: supplies essential zinc for animals, supporting skin and coat health, bone development, reproduction, immune function, and enzyme systems.
Quality considerations: solubility and bioavailability, heavy metal limits, impurity control, batch consistency, particle size (compatibility with premix processes).
Monohydrate vs. Heptahydrate: Selection Logic
Zinc Sulfate Monohydrate
Advantages:
Strong stability: low water content (only one water molecule), less prone to moisture absorption and caking; can be stably mixed with other feed components (vitamins, mineral premixes) for long-term storage without uneven distribution.
Good palatability: powder or granular form has no irritating odor, minimal impact on feed taste, reducing feed refusal by livestock.
High bioavailability: high water solubility ensures zinc ions are quickly absorbed in the animal’s intestine; absorption efficiency is comparable to heptahydrate, but stronger stability allows retention of high absorption rates even after long-term storage.
Zinc Sulfate Heptahydrate
Features:
Lower cost: simpler production process (no high-temperature dehydration), typically 10–15% cheaper than monohydrate.
Lower zinc content: about 21–22% in pure product, 20–21% in industrial grade; higher dosage required for equivalent supplementation.
Weaker stability: with 7 water molecules, it readily absorbs moisture and cakes, requiring immediate use after mixing and unsuitable for long-term storage.
Formulation and Processing Considerations
Dosage control: must be added according to zinc requirements by animal species and growth stage to avoid both excess (which interferes with copper and iron absorption, causing anemia) and deficiency. In general, total zinc content in feed must comply with Feed Additive Safety Standards.
Mixing uniformity: whether monohydrate or heptahydrate, both should first be premixed with a small carrier (e.g., cornmeal, bran) before being thoroughly mixed into bulk feed, avoiding uneven distribution due to small dosages.
Compatibility: avoid direct mixing with strongly alkaline components (e.g., sodium bicarbonate), as zinc ions may form insoluble zinc hydroxide precipitates in alkaline conditions, reducing absorption. If feed contains high calcium or phosphorus, zinc sulfate dosage may need to be increased due to competitive absorption.
4.3 Industrial Applications (Chemicals and Material Processing)
Typical Applications of Zinc Sulfate Monohydrate
Used in man-made fibers, pesticides, dyes, electroplating, and as raw material for other zinc salts and lithopone.
Rubber/leather/paper industries: acts as a functional salt or process aid in certain formulations and artificial fiber production.
Typical Applications of Zinc Sulfate Heptahydrate
Leather tanning: functions as an auxiliary tanning agent together with chrome tanning, improving leather softness and aging resistance.
Paper industry: used as a sizing agent modifier, helping form a more uniform fiber structure and enhancing paper strength.
Monohydrate vs. Heptahydrate: Industrial Choice
Zinc Sulfate Monohydrate: preferred in processes requiring high purity, higher content, and consistent batch quality (facilitating formula calculations and process control).
Zinc Sulfate Heptahydrate: used in cost-sensitive bulk processes where precise zinc content and moisture are less critical.
4.4 Summary Table of Application Differences
Dimension / Application Stage | Zinc Sulfate Monohydrate (ZnSO₄·H₂O) | Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O) |
Zinc supply efficiency | High (Zn ≈ 35–36%); customizable Zn content in granular blends | Moderate (Zn ≈ 21–22%) |
Typical agricultural uses | Powder: water-soluble fertilizers / foliar spray / drip irrigation; NPK+TE formulations; Granular: blending | Traditional soil basal application; self-mixed fertilizers; cost-sensitive field crops |
Feed applications | High-standard premix/compound feeds; precise dosing | Cost-sensitive scenarios; higher dosage required |
Industrial uses | Precision processes with strict purity/stability requirements | Conventional bulk processes with less sensitivity to zinc content |
Storage & stability | Good: resistant to deliquescence, low caking risk | Moisture-sensitive, prone to caking; strict moisture control required |
V. Market and Application Choices
5.1 Market Trends and Structure
Agricultural Market
With the development of precision agriculture and fertigation, demand for high-content, high-purity, and easily soluble zinc sources has increased significantly. Zinc sulfate monohydrate is gaining a growing share in water-soluble fertilizers, foliar fertilizers, and drip irrigation.
Zinc sulfate heptahydrate still maintains a large presence in traditional agriculture, especially in cost-sensitive markets (such as zinc supplementation for field crops in some developing countries).
Feed Market
Under stricter regulations and the trend toward large-scale animal farming, the industry increasingly prefers zinc sulfate monohydrate to reduce additive volume and enhance stability.
Zinc sulfate heptahydrate, due to higher dosage requirements and strong hygroscopicity, is being gradually replaced by some companies, though it still maintains a presence in the low-end feed market due to its cost advantage.
Industrial Market
The applications of zinc sulfate monohydrate are expanding into electroplating, catalysts, pharmaceuticals, and other fine chemical industries.
Zinc sulfate heptahydrate remains mainly in paper, leather, and textile industries, where zinc content requirements are not strict, and it continues to hold market share with its lower cost.
5.2 Application Scenarios Compared
Zinc Sulfate Monohydrate
Precision agriculture (foliar spraying, drip irrigation, water-soluble fertilizers, blended fertilizers).
High-standard premixes and compound feeds.
Industrial applications requiring high purity.
Zinc Sulfate Heptahydrate
Cost-sensitive field agriculture (basal application, broadcasting).
Certain low-end feed markets.
Traditional industrial processes with low requirements for purity and stability.
5.3 Selection Logic (Customer Perspective)
To help customers make quick decisions during procurement or use, the following three dimensions can be considered:
Target Zinc Supply Level
If higher zinc supplementation efficiency is required → choose zinc sulfate monohydrate.
If higher application volume is acceptable due to stronger cost pressure → choose zinc sulfate heptahydrate.
Application Method and End-Use Requirements
Water-soluble fertilizers / foliar fertilizers / drip irrigation → monohydrate (powder form).
Blended fertilizers / mechanized field application → monohydrate (granular form).
Bulk field basal fertilization → heptahydrate (crystalline form).
Storage and Logistics Conditions
Limited storage conditions, long transportation cycles → monohydrate is safer.
Local procurement, short-term use → heptahydrate is feasible, but requires strict moisture control.
5.4 Application Choice Comparison Table
Dimension | Zinc Sulfate Monohydrate (ZnSO₄·H₂O) | Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O) |
Market positioning | Efficient, precise, modernized use | Traditional, bulk, cost-sensitive use |
Agricultural uses | Water-soluble fertilizers, foliar spray, drip irrigation, blended fertilizers | Field crop basal fertilization, low-cost fertilization |
Feed uses | High-standard premixes, compound feeds | Cost-sensitive feed additives |
Industrial uses | Electroplating, catalysts, pharmaceuticals, fine chemicals | Paper, leather, textiles, traditional industries |
Storage adaptability | Stable, low risk of deliquescence | Prone to moisture absorption, requires strict moisture control |
Cost structure | More complex process, higher cost but greater efficiency | Simpler process, lower cost but higher usage volume |
Market trend | Share increasing, driven by precision agriculture | Stable or slightly declining share, concentrated in traditional uses |
👉 Summary:
With the advancement of agricultural modernization, feed industry standardization, and industrial refinement, the demand for zinc sulfate monohydrate is steadily growing, making it the mainstream trend. Zinc sulfate heptahydrate, on the other hand, relies more on its cost advantage to maintain market share in price-sensitive segments. Customers should evaluate their target application, cost budget, and storage/logistics conditions to make a rational choice.
VI. Product Highlights from Kelewell
Within our zinc sulfate product line, Kelewell offers both Zinc Sulfate Monohydrate and Zinc Sulfate Heptahydrate, tailored in different specifications and forms to meet the diverse needs of agricultural, feed, and industrial customers.
6.1 Zinc Sulfate Monohydrate (ZnSO₄·H₂O)
"Powder form emphasizes rapid solubility and precision fertilization, while granular form ensures physical stability and compatibility with mechanized application—together addressing the dual demands of modern agriculture for efficiency and practicality."
Powder Form
Features: Rapid dissolution, suitable for water-soluble fertilizers, foliar fertilizers, and drip irrigation systems.
Application Advantages: Provides crops with zinc quickly; ideal for foliar spraying and fertigation applications.
Technical Parameters:
Zinc content: ≥ 35.0%
Particle size ≤ 250 μm: ≥ 95%
Packaging: 25 kg bags
Granular Form (Blending Grade)
Features: Uniform granules with high hardness; suitable for blending into compound fertilizers and mechanized field application.
Application Advantages: Excellent flowability, even distribution, low dust, and convenience for precision fertilization in field crops.
Technical Parameters (customizable):
Zinc content: 21% / 25% / 31% / 33%
Particle size: 1–2 mm / 2–4 mm / 4–6 mm
Packaging: 25 kg bags / 500 kg / 1000 kg jumbo bags
6.2 Zinc Sulfate Heptahydrate (ZnSO₄·7H₂O)
Product Form: Crystals or granules
Typical Applications: Zinc supplementation for field crops, feed additive, leather processing, and papermaking in traditional industrial applications
Application Advantages: More competitive pricing, suitable for cost-sensitive markets
Technical Parameters (to be supplemented):
Zinc content: ≥ 21%
Appearance: White crystals
Packaging: 25 kg bags
6.3 Our Comprehensive Advantages
Complete specifications: Powder and granular forms available to meet multi-scenario needs in water-soluble fertilizers, blended fertilizers, feed, and industrial applications.
Stable quality: Strict control of impurities and heavy metals.
Customization: Flexible zinc content, particle size, and packaging options according to customer requirements.
Global supply capacity: Reliable and long-term production and logistics system ensuring sufficient supply and efficient delivery.
Regulatory support: Comprehensive product documentation (COA, SDS, TDS) to help customers enter local markets smoothly.
👉 If you are interested in this product, please contact us at: info@kelewell.de
Conclusion
In summary, zinc sulfate monohydrate and heptahydrate each have distinct characteristics in physicochemical properties, production processes, and application fields:
Monohydrate offers higher zinc content and better stability, making it more suitable for precision agriculture, regulated feed applications, and fine chemical industries.
Heptahydrate has simpler processing and lower costs, maintaining relevance in traditional agriculture and certain industrial sectors.
Kelewell provides both powder and granular zinc sulfate monohydrate, along with conventional zinc sulfate heptahydrate, covering a wide range of applications from water-soluble fertilizers and foliar sprays to field fertilization, animal nutrition, and industrial uses.
For customers, this means not only greater flexibility in procurement but also a clearer understanding of product suitability—helping them make more informed and scientific decisions in real-world applications.
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