introduction to Zhiqiao: the use of hard water in the papermaking workshop will bring different degrees of negative effects to the production process. After the inorganic salts such as carbonate and sulfate in the water reach saturation, it will precipitate out from the water to form a hard solid, which will be adsorbed on the equipment and pipelines to form scale. Influence of scale on papermaking equipmentthe temperature of the short circulation system in the paper production workshop is relatively high, which makes the salts in the water easy to separate out.1) adsorption on the pipeline and equipment (such as pressure sieve and headbox, etc.), the pressure sieve scaling leads to the surface of the sieve drum is rough, the sieve seam is blocked, and the screening efficiency is reduced;2) the scaling of the pipeline will hang up the fibers in the slurry, forming fiber bundles. The fiber bundles will fall off after being impacted. The fiber bundles in front of the pressure sieve will concentrate on the sieve seam on the surface of the sieve drum, and sometimes pass through the sieve seam, will form paper disease;3) the equipment will expand and contract due to the change of temperature. The scale on the surface of the equipment will sometimes fall off into the slurry. A large amount of the scale will block the slag removal equipment, and the fine particles will reach the paper and form a paper disease.
Impact on auxiliary systems
The influence of scale on the auxiliary system is mainly to reduce the equipment efficiency of the heat exchange system, such as the tail gas of the steam condensation system, which usually exchanges its remaining energy with clear water through the heat exchanger, hot water is used for other process needs. Clean water is heated in the heat exchanger, which will produce scale on the inner wall of the exchanger. After a long time, the tubes in the heat exchanger are blocked by the mixture of scale and rust, and cannot be used normally.Scale is a kind of material with extremely poor thermal conductivity, which is only 1% to one tenth of that of steel. Therefore, the appearance of scale greatly reduces the heat exchange efficiency. A large amount of clean water is needed to recover the energy in the tail gas, resulting in an increase in production water consumption.
Influence on vacuum pump
The formation of scale will increase the failure rate of the vacuum pump, and the temperature of the vacuum pump is relatively high during operation. Whether it is a water ring vacuum pump or a Rhodes vacuum pump, the carbonate in the clear water that plays a sealing role during its operation, inorganic salt substances such as sulfate will separate out from the water at high temperature and be adsorbed on the pump body. The gap between the rotor of the water ring vacuum pump and the shell on both sides of the pump body is very small, only about 1mm, scale can fill this gap in a very short time, resulting in increased resistance and power consumption during operation, and in severe cases, the rotor is stuck and cannot operate, thus increasing the downtime of the machine.
Influence on dry and wet parts of Paper machine
Scaling of dewatering elements in paper machine network usually only occurs on paper machines with high temperature (general paper machines will not occur).The paper machine with high temperature will produce straight arch printing due to scale, which shortens the service life of the net; In the drying part, the surface of the dryer and the calender roller will also scale, because the scale is relatively hard, therefore, it is difficult for ordinary non-mobile scraper to completely remove it.In most cases, longitudinal marks will be formed on the surface of drying cylinder and calender roller, resulting in uneven transverse distribution of paper thickness, smoothness and moisture, resulting in paper disease.In addition, scaling on the dryer affects heat transfer efficiency and increases steam consumption. Treatment of scale
According to the different equipment materials of different parts of the paper machine, the thickness of the scale layer and the different components of the scale, different chemical cleaning agents can be used to treat the scale. The scale components of each part of the paper machine are mainly calcium carbonate, therefore, the main object of handling scale is carbonate scale.
Treatment of scale in short circulation system
The structure of the short circulation system is relatively complex, and the cleaning objects are stainless steel pipes and equipment. In addition, there are many instruments in the system, such as slurry pumps, flange welding ports, flow meters, pressure sensors, etc, the part exposed in the pipeline is relatively complex in material and is not all stainless steel.Therefore, the corrosivity of the cleaning agent must be fully considered when cleaning to ensure the safety of the equipment. Therefore, organic cleaning agent aminosulfonic acid should be 选择 as the main cleaning agent.
Cleaning with sulfamic acid
Sulfamic acid does not volatilize, has no special smell and is less toxic. It is a strong acid with weak corrosion to metals and generally does not cause intergranular corrosion of stainless steel. It is suitable for carbon steel, stainless steel, descaling and derusting of brass, red copper and other materials.When the temperature is higher than 130 degrees, the sulfamic acid aqueous solution with higher concentration will quickly decompose and generate a large amount of steam in the closed container, thus causing explosion. Therefore, sulfamic acid is not suitable for cleaning the closed container.
Amino sulfonic acid aqueous solution is almost not decomposed below 60 degrees, but the decomposition speed is faster at 80 degrees, and the cleaning effect will fail. Therefore, when using sulfamic acid as cleaning agent, the temperature of the cleaning solution should be controlled below 60 degrees.
The corrosion effect of sulfamic acid on metals is less than that of sulfuric acid and hydrochloric acid. Sulfamic acid has general solubility in metal oxides and good solubility in carbonate and phosphate scale, adding appropriate corrosion inhibitor can ensure its good corrosion inhibition performance.
Matters needing attention during processing
1) since the composition of the polyester net is complex, it is not clear whether the aqueous solution of sulfamic acid will cause potential harm to it. Therefore, it is recommended to choose before changing the net or to cover the net.
2) before emptying the cleaning solution of sulfamic acid, it is necessary to measure its pH value. If the acidity is too large, it should be neutralized with a certain amount of alkaline substance to prevent the cleaning solution from corroding the sewage pipe and polluting the environment.
3) during the cleaning process, a large amount of ammonia gas is released. Attention should be paid to proper safety protection and on-site ventilation should be maintained.
Treatment of pulp box scale
Although the headbox can also be cleaned during short process cleaning, Minfeng paper company uses an open headbox, which has the characteristics of large contact surface with air and high online temperature, the headbox is also the key part of scale formation and has the greatest impact on production. Therefore, the method of local soaking with concentrated nitric acid is adopted.
Soaking with concentrated nitric acid for treatment
Nitric acid is a strong oxidant, except for a few metals such as gold and platinum, almost all metals can be oxidized. Some metals such as Fe, Al and Cr are soluble in dilute nitric acid but insoluble in cold concentrated nitric acid. Metals treated with concentrated nitric acid are not easy to interact with dilute acid again. Therefore, treating scale with concentrated nitric acid can prevent corrosion of equipment.
Matters needing attention in concentrated nitric acid treatment
1) soaking with concentrated nitric acid is simpler than sulfamic acid treatment in the cyclic cleaning process, but due to the corrosiveness of strong acid, it must be considered in the operation, and the protection of operators is well done, preparation such as tool 选择.
2) the object cleaned with concentrated nitric acid must have a large exposure to facilitate the pouring of liquid and the observation of descaling effect.
3) when the cleaning solution is emptied, the pH value must be measured. If the acidity is too high, necessary alkali neutralization should be carried out to prevent environmental pollution. Comparison of cleaning effects of two cleaning methods
Concentrated nitric acid is suitable for cleaning local or individual equipment of the system. The object required for cleaning is of single structure and simple structure, without complicated instrument components, sealing rings, packing and other components that may react with strong acid, such as headbox, the pressure screen and single vacuum pump, etc. are mainly static soaking during cleaning. In addition, the volume of the cleaning object should not be too large, otherwise the washing residue needs to be neutralized with a large amount of alkali before it can be discharged, resulting in a large amount of waste, and direct emissions will affect the environment.The characteristic of concentrated nitric acid method cleaning is that the cleaning process is relatively simple, the reaction is fast, the time is short, and the effect is good, but the operation must be careful, because the concentrated nitric acid has strong corrosivity.
Sulfamic acid method is suitable for comprehensive cleaning of pipeline process, including pipeline and equipment can be carried out at the same time, and its cleaning liquid circulates in the system with the aid of flow equipment such as slurry pump, such as short process, the circulating water system of the vacuum pump, etc., and requires heating, its process is relatively complex, the action time is long, because the cycle cleaning range is large, so the amount of chemicals is large, but the operation is relatively simple.
Sulfamic acid method can also adopt the method of soaking to clean the equipment, but it needs a longer reaction time.
Nitric acid method can clean almost all metal oxides and calcium, magnesium carbonate scale; Aminosulfonic acid method is suitable for cleaning calcium, magnesium carbonate scale and hydroxide scale, and has poor ability to clean iron scale.
After chemical cleaning of the paper machine's flow system and key equipment, the descaling effect is good. The loss rate of finished products such as pulp spots and dust caused by scale before cleaning, after the heat exchange equipment passes through cleaning, the heat exchange efficiency is obviously improved, and the water consumption returns to the normal consumption level;
The scaling on the surface of the drying cylinder and the soft calendering roller is not suitable for chemical cleaning. If the scraper condition is good, it can be handled clean, but the scaling of the soft calendering roller is difficult to handle and needs to be handled by special polishing equipment.The paper machine polluted by scale basically resumed normal production after being thoroughly cleaned by the above two methods. Prevention of scale
Descaling chemicals are generally highly corrosive and will cause a certain degree of damage to the equipment. The cleaning work lasts for a long time and consumes a lot of production time. Scaling prevention is more important than cleaning and descaling. There are mainly two methods to prevent scaling:
1) Use Scale Inhibitor
In professional water treatment process, the method of adding scale inhibitor is usually used to achieve scale inhibition effect. The effect of scale inhibition is mainly to prevent the formation of crystal nucleus of CaCO3, prevent the crystal growth of CaCO3, and disperse CaCO3 crystals so that they do not produce condensation. It can also be said that scale inhibitor increases the solubility of calcium salt to a certain extent.
In the production process, the net used to basically take a large amount of water as the carrier, which can ensure the solubility of calcium salt so that it does not precipitate, but after reaching the dry part, most of the water is removed, with the evaporation, calcium salt is concentrated to a great extent, and finally precipitated and stuck on the surface of the equipment, so the scaling degree is more serious than before.
The scaling treatment method of the drying part is relatively simple. It can be removed by physical methods without chemicals, but it has to be handled almost every time it is stopped, and the workload is large. Therefore, the use of scale inhibitor only partially solves the scaling problem and is not thorough.
2) reduce water hardness (water softening)professional Water softening methods include boiling method, reagent softening method, ion exchange method, electrodialysis and ultrafiltration technology. Not all methods are suitable for the paper workshop, and the processing cost and the operability of the production workshop must be considered.
Compared with the above methods, ion exchange method is more suitable for softening water quality in papermaking workshop. Complete sets of equipment can be used to soften all workshop water, or simple equipment can be used to soften water for a single equipment.
Water quality softening by ion exchange refers to the mutual exchange between Ca2 and Mg2 in aqueous solution and cation exchange groups ionized by solid resin, so that Ca2 and Mg2 are adsorbed on the resin surface, the failed resin can be regenerated with salt or acid, and then can continue to be put into use.
When formulating the softening water quality standard, it should be noted that reducing the hardness to less than 180 mg/L(CaCO3) should be able to meet the production needs, and excessive softening should be avoided to save costs.Paperbridge, the partner you can trustzhiqiao technology, a partner you can trust
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