1. The role of chromium ore powder in glass coloring
Main components
The main component of chromium ore powder is chromium oxide (mainly Cr₂O₃), which may contain a small amount of impurities such as Fe₂O₃ and Al₂O₃.
The coloring effect of Cr³+: In the glass melt, Cr³+ ions absorb light of a specific wavelength (blue-violet light) and reflect yellow-green light, making the glass appear green (typical applications such as the dark green of beer bottles).
Color regulation
Concentration effect: The amount of Cr₂O₃ added is usually 0.1%-2% of the total mass of the glass. Excessive addition will cause the color to be too dark (dark green) or turbidity.
Redox state:
Oxidation conditions (oxygen-rich smelting): Cr³+ is stable and appears green.
Reduction conditions (oxygen-deficient smelting): Some Cr³+ is reduced to Cr²+, which may appear blue-green or uneven in color.
2. Application steps of chromite powder as glass colorant
1. Raw material pretreatment
Purification and grinding:
Remove impurities (such as iron and silicon) in chromite powder to avoid affecting the color development effect.
Grind to 100-200 mesh to ensure uniform dispersion in the glass batch.
2. Formula design
Basic glass composition: mainly soda-lime glass (SiO₂-Na₂O-CaO) or borosilicate glass.
Amount of chromite powder added:
Light green: 0.1%-0.5% Cr₂O₃.
Dark green: 0.5%-2% Cr₂O₃.
Auxiliary colorant:
Match with Fe₂O₃: adjust the color tone (such as olive green).
Add CoO: enhance the blue-green tone.
3. Melting process
Melting temperature: 1400-1600℃ (depending on the type of glass).
Redox control:
Introduce air or add sodium nitrate (NaNO₃) to maintain an oxidizing atmosphere to ensure the stability of Cr³+.
Avoid the mixing of reducing substances (such as carbon) to prevent Cr³+ from being reduced.
Homogenization treatment: fully stir the melt to avoid color difference or streaks.
4. Molding and annealing
Molding: Molding into the required products through blowing, calendering or float process.
Annealing: Slowly cool down (500-600℃ range) to eliminate internal stress and prevent glass cracking.
III. Advantages and limitations of chromium ore powder application
Advantages
Color stability: Cr³+ is resistant to high temperature and aging in glass, and the color lasts.
Low cost: Natural chromium ore powder is more economical than synthetic chromium oxide (Cr₂O₃).
Environmental compatibility: Cr³+ in chromium ore powder has low toxicity and meets the requirements of environmentally friendly glass production (Note: the generation of Cr⁶+ needs to be avoided).
Disadvantages
Impurity sensitivity: iron impurities may cause yellowish or turbid color.
Strict process control: the smelting redox state needs to be precisely controlled, otherwise color difference is easy to occur.
Health risks: improper operation (such as inhalation of dust) may cause harm to workers’ health.
IV. Key points
Impurity control:
Use high-purity chromium ore powder (Cr₂O₃ content> 90%), or remove iron impurities through magnetic separation and pickling.
Safety protection:
Wear a dust mask during operation to avoid inhalation of chromium dust.
Waste slag needs to be treated as hazardous waste (if it contains Cr⁶+).
Alternatives:
If a purer green is required, synthetic chromium oxide (Cr₂O₃) can be used instead, but the cost is higher.
For scenes with extremely high environmental protection requirements (such as food containers), it is necessary to ensure that there is no risk of Cr³+ dissolution.
5. Application Examples
Beer bottle glass: add 0.5%-1% chromium ore powder, adjust to dark green with Fe₂O₃, block ultraviolet rays to protect the flavor of beer.
Art glass: mix with cobalt blue (CoO) to make blue-green gradient glass.
Filter glass: use the ultraviolet absorption property of Cr³+ for optical instruments or lamps.
Summary
The core of chromium ore powder as a glass colorant is to achieve green color through the selective absorption of Cr³+ ions. Its application needs to focus on raw material purity, smelting redox control and process stability. Reasonable use can take into account both cost and performance, but impurities and operational safety must be strictly managed to avoid environmental and health risks.
