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How to make colorless and transparent glass products?

admin 2022-08-15 10:20:0203 Comments
1. The coloring principle of impurity ions in colorless transparent glass
2. Decolorization of glass products
3. Process control
4. Case analysis of common white material and high white material

1. The coloring principle of impurity ions in colorless transparent glass
Glass products are made of quartz sand, soda ash and other raw materials mixed by a mixer, transported to the kiln head silo, fed into the kiln by a feeder, melted and clarified by the fuel in the kiln, and finally shaped by various molding machines. However, the various raw materials we use will contain coloring elements such as iron, chromium, and titanium; during the melting process, the molten glass is in contact with the refractory material, and the refractory material also contains certain coloring elements; the fuel used for melting glass will also contain sulfur, Impurity coloring elements such as vanadium and iron; contact and adhesion with equipment and molds during molding will also bring in iron and alloy pollutants, etc. These factors will affect the whiteness, brightness and transparency of colorless glass products,Glass product     .
Common ionic coloring and decolorization in colorless transparent glass

Several common coloring ions and commonly used decolorizing ions in glass
1. Coloring of iron Iron in glass mainly exists in the form of Fe2+ and Fe3+ ions. Fe2+ can make the glass light blue, Fe3+ can make the glass light yellow-green, and the coloring ability of Fe2+ is about 10 times that of Fe3+. The equilibrium state between them is directly affected by the furnace atmosphere and the redox index in the recipe.

2. Chromium coloring Chromium's coloring ability is second only to cobalt. It exists in two valence states of Cr3+ and Cr6+ in glass. In common glass, Cr3+ dominates, Cr3+ makes the glass green, and Cr6+ makes the glass yellow.

3. Coloring of vanadium Vanadium can exist in glass in three valence states of V3+, V4+ and V5+, V3+ makes the glass yellow-green, V4+ is colorless in the visible light region (380-760nm), and V5+ is only in high content. Time to make the glass yellow-brown.

4. Coloring of titanium Titanium exists in two valence states in glass, generally in the valence state of Ti4+ ion. Ti4+ ion is colorless, but Ti4+ ion will strongly absorb ultraviolet rays, causing the glass to produce brownish yellow. If the titanium content in the glass is high, a yellow color is produced. Using this principle, cerium is introduced into the glass at the same time, and the combination of "cerium and titanium" can be colored, which can be colored into a beautiful golden yellow.

5. Coloring of sulfur Sulfur has four valence states in glass: -2, 0, +2, and +4. In a reducing atmosphere, sulfur exists in the valence of S-2 and reacts with iron in the glass to form brown ferrous sulfide FeS. It exists in the valence of S+2 under weak reducing conditions, and reacts with the sodium ion Na+ in the glass to form a Na2Sx orange-yellow compound. Under oxidizing conditions, it exists in the valence of S+4 and reacts with sodium ions Na+ and oxygen ions O-2 in the glass to generate colorless sodium sulfite Na2SO3 and SO2↑ gas. From the perspective of the coloring mechanism of sulfur, in order to overcome the negative impact of S in the fuel on the glass, it is necessary to maintain an oxidizing and weakly oxidizing atmosphere in the kiln or increase the oxidative properties of the formula and control the sulfur content in the fuel (heavy oil and coal). The sulfur content in the fuel should be controlled at <0.5%, and the total sulfur content in industrial natural gas should be controlled at <350㎎/Nm³. If the reducing atmosphere in the furnace is too strong and the time is too long, it is very easy to turn the glass into a yellow hue. In addition, we also need to control the introduction of organic matter in the purchased cullet. During the combustion process of organic matter in the kiln, it will greatly increase the reducing atmosphere in the kiln and increase the probability and degree of yellow lines and yellow hues in the product.

6. Coloring of cobalt Among all the coloring ions, cobalt has the strongest coloring ability, and the content of 0.01% can make the glass become dark blue. Cobalt generally exists in the form of Co2+ in glass, which is stable in coloring and is less affected by melting conditions and basic formula ingredients.

7. Coloring of cerium Cerium often exists in glass in two states of Ce3+ and Ce4+, and Ce3+ ions are close to colorless. Ce4+ absorbs ultraviolet rays and enters the visible light region, making the glass light yellow, but it is relatively weak. Only when the content is sufficient (up to 0.5%), the glass can visually appear yellow or light yellowish brown. Because cerium oxide also has the functions of strong oxidation and high temperature clarification, it is often used as a chemical decolorizer and clarifier for high-white and above-grade glass, and is also commonly used as a colorant in combination with "cerium titanium". In chemical decolorization, it is superior to the general oxidant. After adding cerium oxide, the content of Fe2+ in the glass is significantly reduced, and the product decolorization effect is better. I have done an experiment to adjust the formula of crystal white material. The experimental results are as follows: Group A formula: 100 kg of quartz sand (0.01-0.02% iron content), 32 kg of soda ash, 12.5 kg of aluminum hydroxide, 13 kg of calcite, and 5.5 kg of barium carbonate , 1.5 kg of sodium fluorosilicate, 3 kg of sodium nitrate, 0.24 kg of cerium oxide, 1.2 kg of compound clarifying agent, 18 g of erbium oxide, and 5 g of neodymium oxide.

8. Coloring of selenium Se is often used for the decolorization of glass products with high white material and below. Se has four valence states: -2, 0, +4 and +6 valences. Elemental selenium makes the glass pink. It is yellow-green with Fe3+ ions removed from the glass. Se is sensitive to oxidative and reducing atmospheres, and the color of the product is unstable due to improper control. Selenium generates colorless NaSe under reducing atmosphere and colorless SeO2 under oxidizing atmosphere, so selenium is suitable for use in neutral and weakly oxidizing atmospheres. Se and Er are commonly used in high-white materials to decolorize, so as to alleviate and reduce the negative effects caused by the instability of selenium.

9. Coloring of manganese Manganese exists in glass in two forms, Mn2+ and Mn3+. Mn2+ makes the glass pink, but from the electron arrangement 1S22S22P63S23P63d5 and orbital of Mn2+, the 3d orbital is half full, so the coloring ability is very weak. . The tinting ability of Mn3+ is much greater than that of Mn2+, which can make the glass become purple-red. Manganese needs to be in an oxidizing atmosphere to generate a certain amount of Mn3+, and the amount of Mn3+ is only about 0.1% of the total manganese. The purple-red colored by Mn3+ ions and the yellow-green in the glass are complementary colors, so manganese is also used for decolorization of glass products, but manganese will form brown with iron in the glass, reducing the transmittance of the glass, and it is also susceptible to the atmosphere. Influence, there is a phenomenon of decolorization instability. Products used in high white materials are not decolorized much, and they are often only used in combination. When good decolorization cannot be achieved, it is used to adjust the color of the product, so that the product has a little blue light to increase the glass. Brightness (human vision is more sensitive to yellow-green tones, and products with a slight blue tint look brighter than yellow-green tints).

10. Coloring of Erbium Oxide and Neodymium Oxide Erbium and neodymium belong to rare earth elements, Er3+ and Nd3+ ions are generally unchanged in price. Their spectral characteristics and coloring are very stable, and are not affected by the melting process and the composition of the base glass. Er3+ binds the glass. To pink, it can take off the green in the glass, Nd3+ makes the glass blue-purple, can take off the yellow-green in the glass. Erbium oxide and neodymium oxide can also increase the gloss and refractive index of the product, but because the decolorization is much weaker than that of selenium powder (the decolorization ability of selenium powder is about ten times that of erbium), the relative decolorization cost is more expensive than selenium powder. Therefore, erbium oxide and neodymium oxide are mainly used for decolorization of high-grade transparent colorless glass frits above crystal white.

Influencing factors of ionic coloration and decolorization

The valence state, coloring and decolorization of ions in glass are affected by the following factors
1. Basic composition of glass In quartz glass and high boron glass, ions are easy to form low valence and high coordination. For example, Co is mostly coordinated by [Co6] in high boron glass, which can make the glass blue-violet, while in ordinary soda lime silica glass, it is mostly coordinated by [Co4], which can make the glass blue. We use cobalt With this characteristic, the coloring and decolorizing of cobalt can be flexibly controlled, and the ideal effect will be obtained.
The alkali content in the glass affects the valence state of the coloring ions.

2. Redox potential state The reducing atmosphere is conducive to the generation of low-valence ions, and the oxidizing atmosphere is conducive to the generation of high-valence ions. The redox state includes the redox index Redox of the glass batch and the redox properties of the combustion atmosphere in the furnace. The coloring ions of iron, chromium, manganese and selenium are Fe3+, Cr6+, Mn3+, Se4+ (colorless) high valence in the oxidation potential state, and Fe2+, Cr3+, Mn2+, Se2+ (colorless) low valence respectively in the reducing atmosphere. Ordinary flat glass, emerald green beer bottle, brown glass and black glass with manganese-chromium as the main colorant are produced by controlling the redox potential.

3. Melting temperature During the melting process of glass, there is a dynamic balance between the high and low valence states of ions, as follows:
Cr6+ (yellow green)+3e=Cr3+ (green)
Mn3+ (purple)+e=Mn2+ (colorless)
The higher the furnace temperature and the longer the melting time, the more favorable the coloring ions are to change from high valence state to low valence state.
4. The concentration of coloring ions The more the content of coloring ions is, the more the valence state of the coloring ions in the glass changes to a higher valence state. If the iron content in the glass increases, the ratio of Fe3+/Fe2+ in the glass increases, and the glass deepens from yellow-green to green and develops towards dark green.

2. Decolorization of glass products

In theory, if no impurity coloring elements are introduced into the glass, there will be no selective absorption in the visible light region of 380-760 nm, and the glass will be colorless and transparent. However, in the actual production process, some impurities will inevitably be introduced, which will cause the glass to selectively absorb light under the light after coloring, and the glass will produce certain colors. If the glass absorbs the light energy of each wavelength in the visible spectrum uniformly, after the light passes through the glass, the intensity of the spectrum is only slightly weakened but the composition does not change, and the transmitted light is also approximately white light. A certain amount and a certain type of decolorizing agent are added to the glass, so that the color produced by it is complementary to the color formed by the coloring ions in the glass, so that the light is absorbed evenly, and it becomes a colorless and transparent glass.
Decolorization of glass is divided into chemical decolorization and physical decolorization. The chemical decolorizer can release oxygen when decomposed at high temperature, so that the Fe2+ ions with strong coloring ability are oxidized to Fe3+ ions with weak coloring ability. Chemical decolorization only weakens the coloring ability of coloring ions to a certain extent, but cannot completely eliminate the color of these coloring ions, so many transparent glasses need further physical decolorization. Practice has proved that using chemical decolorization plus physical decolorization, the decolorization effect is better. Physical decolorization is a complementary color method. A certain type and amount of colorants are added to the glass batch to make the color produced by the impurity Fe2+, Cr3+, Fe3+, Cr6+, V3+, Ti4+ in the glass. It absorbs and transmits uniformly in the visible light region, so as to obtain transparent colorless glass.

1. The commonly used chemical decolorants for chemical decolorization are sodium nitrate, potassium nitrate, arsenic, antimony trioxide, cerium oxide, halogen compounds, and the commonly used halogen compounds are sodium fluorosilicate, sodium chloride, and fluorite powder. It decomposes to release oxygen, but it can react with impurity iron ions to generate volatile gas FeF3, FeCl3 or colorless sodium ferric fluoride Na3FeF6, so as to achieve decolorization effect.
2. Physical decolorization In physical decolorization, selenium and cobalt are commonly used for decolorization. Crystal white materials are generally decolorized with erbium oxide and cobalt. The most commonly used crystal materials are erbium oxide and neodymium oxide for decolorization. Because manganese and iron will form ferromanganese brown coloring groups, and manganese ions are easily affected by the atmosphere of the kiln, and the decolorization is unstable, so manganese is used less for decolorization. In some areas, manganese is used for decolorization. on high white material. The raw materials of manganese are manganese powder and potassium permanganate. The purity of manganese powder is not high, which will increase the introduction of impurities and reduce the transparency of glass products. If manganese is used for decolorization, potassium permanganate is better than manganese powder, but potassium permanganate is a dangerous chemical under national control and requires certain procedures to purchase.
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Physical decolorization pay attention to the following:

(1) The combination of erbium oxide and neodymium oxide is not suitable for use in formulas with a large amount of neodymium oxide. Because neodymium oxide has a two-color effect, it appears purple-red under full-color light and blue-violet under monochromatic light. Our daily debugging formulas are mainly based on the color of the products observed under full-color light during the day, while the product trading malls and product showcases are generally indoors and need to be illuminated. For products that are decolorized with erbium oxide and neodymium oxide, if the amount of neodymium oxide is too large, the light observed at this time will change, and the color of the product will change, which will have a certain impact on the product.

(2) Glauber's salt will inhibit the excessive amount of selenium decolorization. Glauber's salt will not only react with the ferric oxide Fe2O3 in the glass to form ferrous sulfide FeS and Fe2S3, and form a brown coloring group - sodium ferric sulfide with sodium sulfide, which makes the glass With brown tint, it reduces the transmittance of glass products, affects the color of glass products, and also reacts with selenium to form colorless selenate, increasing the amount of selenium. Na2SO4+Se+O2=Na2SeO4+SO2↑.

(3) When using selenium and cobalt for decolorization, the redox atmosphere of the kiln and the redox index of the batch should be well controlled. Selenium is suitable for decolorization in a neutral and weakly oxidizing atmosphere.

(4) Control the moisture of the ingredients, water will accelerate the volatilization of selenium.

 

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