Views: 302 Author: Edie Publish Time: 2023-06-01 Origin: LinkedIn
It is generally accepted that a hardness of less than 500ppm has no effect on textile dyeing, and 200ppm is already very good quality water.
The key is the content of metal ions, especially iron ions, which can lead to changes in colour and light, so it is advisable to measure the conductivity of the water.
Many printing and dyeing factories will discharge wastewater to meet the standard for bleaching and dyeing production, thus reducing water consumption, but the reuse of wastewater for reprinting and dyeing, the finished product is easy to produce colour spots and flowers, the main reason is the high metal ions lead to high conductivity of the water, so the reuse of bleaching and dyeing wastewater needs to test the conductivity of the wastewater. The metal ions that cause high conductivity cannot be effectively removed by conventional wastewater treatment methods and must be removed by reverse osmosis membrane treatment.
Excessive water hardness can cause water debris to form on the boiler walls, and calcium and magnesium ions in the water can cause soap to precipitate. Uneven dyeing, poor fabric feel and yellowing of the fabric will reduce the activity of the enzymes used for desizing and reduce the solubility of the sizing material, which is not compatible with the chemical agents used in the finishing formula.
Uneven dyeing (poor dyeing) is a problem if the dyeing water is not softened or if the thick fabric often contains metal ions. Therefore it is necessary to add a chelating and dispersing agent during the dyeing process to prevent the effect of metal ions. Usually we can treat the water with EDTA or sodium tripolyphosphate, or buy special metal ion chelating agents.
However, EDTA (ethylenediamine-diacetic acid), DTPA, NTA, phosphate. Chelating and dispersing agents such as gluconic acid may chelate out the metal ions in the dyestuff, resulting in a reduction in sunlight fastness and fading. It is therefore important to use the appropriate chelating agent according to the characteristics of the dye itself.
Example of a Factory Dyeing Problem
The water used for dyeing cotton in the factory is groundwater, which has not been softened and filtered. The dyed cylinder yarn can obviously be seen as a layer of mud and dirt, and now a small amount of bleaching powder has been added to the water to improve it a little, but the cylinder yarn is dyed with an astringent feel at the bottom and a light and dark colour. About 130ppm.
Causes and Treatment Options
The water quality is too hard, groundwater yarn dyeing, water quality is too hard and too many impurities, may lead to colour yarn tube inner layer yarn layer dirt, the best solution is to first pump the groundwater to the ground to settle, soft water treatment, then filter the water with a filter, in addition can also be added to the dyeing process chelating agent.
Cationic soft working fluid in river water is not stable, which will flocculate with microorganisms in the river water, producing yellow flocculent, stained on the fabric, after drying, shaping，thus forming yellow spots on the fabric, so that's why there are always problems in May to June when the temperature is more suitable for easy algae reproduction is particularly prone to quality problems, while in the autumn and winter season there are few such situations.
The quality of the water has a great impact on the stability of the soft working solution and the whiteness of the fabric after soft finishing, so dyeing factories shall use treated water when producing.
This is because the microorganisms are very small and the activated carbon cannot be completely adsorbed and filtered, so adding cationic surfactant to the sedimentation tank before water treatment makes it flocculate with the microorganisms in the water, and then filtering by activated carbon can improve the effect of water treatment; in addition, the activated carbon must be cleaned or replaced regularly.
If the above methods are not feasible or still cannot eliminate water quality problems, it is recommended that the factory use non-ionic or anionic softener flakes or use non-ionic softener flakes or silicone oil of hand finishing when the algae bloom is high.
For reactive dyeing, when the pH value is 7~9, the colour difference of the product is normal, which means that the pH range has little effect on the colour difference of the product, and the presence of a small amount of alkali will not change or destroy the colour developing group of the reactive dye molecule.
When the pH value is 8.5, the soaping colour fastness and wet rubbing fastness of the fabric is grade 3, which cannot meet the customer's requirements; when the pH value increases further, the colour fastness becomes worse, which shows that the increase of alkalinity in the water will affect the colour fastness of the dyed products.
The feed water quality of wastewater reuse plants generally requires pH <9 to ensure that the microbial growth of the treatment system is not endangered by alkalinity and can operate normally, while the quality standard for reuse water requires pH to fall 8.5, preferably close to 7.
The salts in the reuse system are mainly sodium sulphate, which has two sources.
(1) Addition as a dyeing promoter in the dyeing process;
(2) Caustic soda and soda ash added in the boiling and colour fixing processes, which are neutralised by sulphuric acid and converted into sodium sulphate.
The limit salinity in the reuse system is 924 mg/L, which does not affect the fabric dyeing quality.
Pretreatment iron ions cause the hydrogen peroxide to gather and decompose faster, resulting in holes in the fabric.
Iron ions can lead to darkening of the dyed fabric surface and can easily cause colour spots. As for the removal method, it depends on the content, which is usually below 15ppm, but if it is slightly higher, chelating agents can be added.
Chloride ions alone have no effect on reactive dyeing, as some people use salt as a dyeing agent, but it does have an effect on the cylinder, depending on whether the steel you use for the cylinder is chlorine resistant.
If, as in the case of tap water, it contains active chlorine for bleaching and sterilisation and exceeds a certain standard, it will have an effect on the dye.
Edit by Edie
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