More specifically, applications of hydrogen peroxide - H2O2 can be defined in:
- Paper industry
- Textile industry
- Dairy (food) industry
- organic chlorine compounds
- decolouration of synthetic dyes
- reduction of chemical oxygen demand
- organic effluents
- Extra oxygen intake:
- biological treatment plants
- elimination of odour from anaerobic decomposition of municipal and industrial effluents
- Detoxification of cyanides and nitrites
- Oxidation of sulphur compounds
- Oxidation of organic effluents
Detoxification of cyanides and nitrites
In the treatment of industrial effluents, one of the main problems is the removal of cyanides that are toxic to humans and animals. In conventional cyanide oxidation procedures only sodium hypochlorite is used, resulting in the introduction of excessive salt in the water and the formation of carcinogenic, chlorinated organic compounds. Therefore, in Europe, it is seldom used and has been replaced by H2O2 oxidation.
In many cases, especially in waste waters from hardening furnaces, as well as cyanides there are also nitrates, which are highly toxic to many plants and aquatic animals. With the help of H2O2 they are oxidized into the less toxic nitrates, which are later removed in biological treatment plants.
Oxidation of sulphur compounds
In many industrial effluents there are also sulphur compounds (sulphides, sulphites, poly-sulphides, mercaptors, etc.), which are corrosive and toxic. They reduce oxygen, so before discharging them into the sewer system they must be oxidised. This can be achieved simply and efficiently by using H2O2. The final product of the breakdown is, in most cases sulphate, which is not toxic.
H2O2 can be used in various ways, even for municipal effluents that contain considerable amounts of sulphur compounds. In effluents remaining too long in sewage pipes, particularly at elevated summer temperatures, decomposition starts, leading to the formation of hydrogen sulphide. H2S is released from the water and causes strong odours, and in certain circumstances it may be even poisonous. In sewage systems H2S, in combination with air, converts into sulphuric (VI) acid, which causes corrosion of up to 0.6 mm per year to the upper circumference of concrete and metal pipes.
The result of anaerobic decomposition is the lack of oxygen in the water. Even small quantities of H2O2 can quickly and easily eliminate odours, and at the same time prevent corrosion of sewage pipes.
H2O2 is also used in biological sewage treatment in cases where there is a lack of oxygen due to overload of biological treatment plants, which causes disruption to their operation. By adding H2O2 water gets extra oxygen, which reduces short-term overload.
Oxidation of organic effluents
Many organic substances are not inherently bio-degradable, so they must be chemically treated. An extremely powerful oxidant is a mixture of H2O2 and di-valent iron salts, known as Fenton’s reagent. When processing effluents through this process, a mixture of organic compounds is created that is biologically less dangerous and can be bio-degraded in further processes.
Recently, in oxidation processes the most usual activation of H2O2 is by ultraviolet (UV) light. This procedure is particularly suitable for oxidation of organic substances in leachates from refuse dumps and for the breakdown of organic dyes and decolouration of textile effluents. It can successfully decompose a number of persistent and stable compounds and organic toxic substances, which also include pesticides.
Use of H2O2 in paper industry
H2O2 consumption in the pulp and paper industry has significantly increased in recent decades. The reasons for this are lower production costs, improved paper quality, increased yield of wood mass, as well as ecological efforts to remove chlorine compounds in pulp bleaching processes and their replacement with environmentally friendly bleach products.
In the pulp and paper industry, hydrogen peroxide is used in three areas: for delignification and bleaching of cellulose, pulp bleaching (pulp of high yield), and for re-cycling waste paper (de-inking).
- Bleaching of cellulose
- Bleaching of wood pulp
- Recycling of waste paper - de-inking (removal of ink)
Bleaching of cellulose
Because of the negative ecological impacts of chlorine use in cellulose bleaching processes (chlorine together with lignin degradation products forms environmentally unfriendly chlorine compounds: polychlorinated di-benzo-dioxins and polychlorinated di-benzo-furans, which are included on lists of mutagenic and carcinogenic substances), there have been long-standing efforts to partially or totally replace it.
Ecologically controversial bleaching processes using chlorine whiteners of CEH (C - bleaching with chlorine, E - alkaline extraction, H - bleaching with hypochlorite) are replaced by bleaching processes with oxygen-based materials. The switching of the paper industry to use oxygen whiteners is also encouraged by legislation, which specifies permissible limits of AOX in waste water generated in cellulose production.
Today, there are two forms of cellulose in the market: ECF-cellulose that still contains a certain amount of chlorinated organic compounds (‘Clor arm’ cellulose) and TCF cellulose, which is totally chlorine free (‘Clor frei’ cellulose). Therefore, paper products already have ‘Clor frei’ labels, which mean that this cellulose was bleached without chlorine. This, of course, also affects buyer choice.
Bleaching of wood pulp
By 1962, wood pulp was produced only in grinders (SGW pulp), but today, this type of pulp represents less than half of the world pulp production. It has been substituted by TGW pulp, RMP pulp (refiner mechanical pulp), TMP pulp (thermo-mechanical pulp), and CTMP pulp (chemo-thermo-mechanical pulp) which is supposed to become the wood pulp of the 21st century. Thermal and chemo-thermal pulps are, due to their mechanical properties, closer to cellulose (chemical pulp) and have been replacing it in many products.
The current global policies favour production of these special ingredients of wood pulp of high yield, which must approach the cellulose not only in mechanical properties, but also in whiteness. Nowadays, H2O2 is vital in bleaching pulp, since it helps us achieve high degrees of whiteness, and improved whiteness stability.
Recycling of waste paper - deinking
The recycling of waste paper in the paper industry is increasing globally, because of savings in wood, water, and energy, as well as protecting the environment.
De-inking requires the following steps: conversion of the ink into a form that is easily removable, separation of ink from the fibres, and bleaching of fibres. Even this field is increasingly dominated by H2O2. There also exist combined bleaching processes with H2O2.
Use of H2O2 in the textile industry
H2O2 consumption in the textile industry is declining. In full bleaching, H2O2 is used before dyeing and for the oxidation of reductive dyes in dyeing. For a completely white cloth and yarn, combined bleaching is still used sometimes: hypochlorite in pre-bleaching, and then peroxide for anti-chlorination, yellowing, and bleaching. In most companies a completely white cloth is achieved only by an alkaline boil-out procedure, hot peroxide bleaching, which is repeated if necessary, and the addition of optical whiteners.
Textiles that are usually bleached are yarn, knitwear, and textile cloths primarily from cotton, less so from viscose, and some from cotton/polyester mixtures. In the future, it is predicted that cotton will have precedence over regenerated and artificial fibres.
Combined bleaching, using hypochlorite and peroxide, is decreasingly used. The transition to solely peroxide bleaching requires an appropriate choice of an optical whitener and organic stabilizer. Instead of hypochlorite, some companies in the world have introduced a combined PAA (per-acetic acid): H2O2 method of bleaching.
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