****!!!!****!!!!**** "The formation of chlorite increases with increasing of pH value whereas the formation of chlorate increases with decreasing the pH value, and the chloride ion is increased at a pH value below 3.4. The chlorite (ClO2–) exists as inert at above the ph 7. On the other hand this compound is very active below pH 7. However, the most efficient bleaching and maximum brightness are obtained in the first ClO2 stage (D0) at a pH level of about 3.5 – 4.0 while it is obtained in the second ClO2 stage (D1) at pH of 5.5 to 6. Normally, the pH varies from 7 to 3.5. So, less chlorine dioxide bleach is required for low pH than for high pH bleaching, but the temperature and retention time should be increased to speed up the reaction rate. At a lower pH (below 3.0) ClO2 can produced organic chlorine compounds which is not environment friendly. It is possible to take advantage of the chlorite with maintaining a rapid rate of bleaching by keeping the pH level high at starting position and then lower. Corrosion is kept under control by using tile lined or acid proof brick towers and plastic-lined or ceramic pipe. The residual ClO2 can create toxic fumes and corrosion at subsequent washing step. Therefore, SO2 or NaOH is used at the bottom of the tower to neutralizing it so that can be reduced corrosion and eliminated toxic fumes." "To eliminate explosion, the chlorine dioxide bleach should protect from uv light, keep lower concentration, uncontaminated and cool. Moreover, to reduce consumption pH would to be controlled carefully."

**** (2020) " ClO2 is produced on site in concentrations of... 8 to 12 thousand parts per million (ppm)" "Fortunately, ClO2 gas is self-alerting presenting a faint odor of bleach or chlorine at 0.1 ppm levels or less, thus necessitating area evacuation or use of respiratory protection. Based on ERPG data, such low-level exposure to ClO2 gas is not life-threatening. In fact, more recent studies indicate that ClO2 gas at low levels, well below the TWA17 of 0.1 ppm, could be useful as an antimicrobial in the presence of humans to mitigate and/ or prevent viral infection and possibly other related respiratory tract infections... ClO2 gas could be used in places such as hospitals, clinics, offices, hotels, schools and airport buildings without interrupting normal activities."

"ClO2. This is a reactive gas formed from chlorine and oxygen. It is a strong oxidizing agent with a tendency to open up unsaturated compounds and aromatic rings. In water solution it may be present as an ion with a single negative charge. Function: Papermakers who use bleached kraft pulp are likely to have heard about chlorine dioxide even if they have never set foot in a pulp mill. That's partly because it is very often present in the very last bleaching stage (D-stage), increasing the likelihood that some of it is carries over with the pulp into the paper machine system. Another reason is that it is one of the most popular biocide treatments for alkaline papermaking. Its use in that role is pretty much restricted to bleached grades, since any unbleached fibers will rapidly consume the chemical. Strategies for Use: Chlorine dioxide has the desirable attribute of "quick kill" of bacteria and fungi, and it also decomposes relatively rapidly. It is conventional practice to treat the pulp furnish and then monitor the residual activity at a later point in the process. Typically the addition rate is controlled to give a residual chlorine content of about 1 ppm."

Bleaching is a whitening process that is used in the paper industry to produce paper with high brightness. Chemicals—such as chlorine gas (C), sodium hypochlorite (H), oxygen (O), hydrogen peroxide (X), ozone (Z), and chlorine dioxide (D)— are used in various combinations to produce pulp with the desired properties

Eka chlorine dioxide is a preferable bleaching agent used in the production of elemental chlorine-free (ECF) bleached chemical wood pulp all over the world.