HU188884B - Process for the treatment of arsenous water - Google Patents

Abstract

Waters contg. arsenic are treated by first adding an amt. of oxidising agent appropriate to the chemical oxygen deficiency and the ammonia content of the water, followed by an amt. of water soluble iron-III cpd. equiv. to 2-10 g iron per cubic metre or an amt. of water soluble aluminium cpd. equiv. to 5-15 g aluminium per cubic metre as coagulant, and opt. 5-20 g calcium-hydroxide per cubic metre water as clarifying agent. The water is now filtered, and contacted with granular, opt. carrier born manganese-dioxide. Opt. 0.5-4.0 g per cubic metre manganese-VII cpds. may be added to the water before filtration and contacting with manganese-dioxide.

Description

The present invention relates to a process for the treatment of water containing arsenic. The present invention relates to a process for reducing the content of arsenic, iron and optionally manganese in water.

The arsenic content of natural water often exceeds the level permitted for residential or technological use, and it is therefore necessary to decontaminate the water. The food industry is particularly demanding in this respect. These needs can be met by industrial water treatment processes that are suitable for reducing the arsenic content to below 0.05 mg / liter.

In Hungary, the arsenic content of water often exceeds this value; water with an arsenic content of 0.1 mg / liter or higher is not infrequent. Arsenic is also accompanied by iron in the waters to be purified, and is often found in manganese. These pollutants also need to be removed from the water because they are detrimental to their industrial use.

Several methods are known for removing these three contaminating ions. Their effectiveness is determined by their arsenic removal efficiency, since satisfactory removal of the other two ions does not cause much difficulty. Arsenic is known to exist in the form of arsenite and arsenate ions in water. Trivalent ions are more difficult to remove, and pentavalent ions are more easily removed by precipitation or adsorption.

The known processes based on precipitation are the formation of a precipitate of iron and optionally manganese hydroxide or oxide in water. Iron and manganese ions already present in the water are used for this purpose, but significant amounts of precipitating agents such as ferrous sulfate or chloride are also added to the water to form a fluffy precipitate sufficient to adsorb the arsenic-containing compounds. If a precipitating additive of about 20-30 g / m ³ is used, 90% of the average arsenic water can be removed. The disadvantage of this process is that a large amount of chemicals is required for a satisfactory decontamination of arsenic and that large quantities of precipitate are removed and treated.

Another known process is to render the arsenic compounds insoluble by raising the pH. To this end, lime milk is added to the water to raise the pH to 10.8 or higher. At this time, the arsenic compounds precipitate, but the precipitate is difficult to filter.

They also experimented with an ion exchange process. The anion exchanger removes arsenic completely from the water. However, the process is not widespread in industrial practice because regeneration of the ion exchanger takes a long time and results in increased environmental pollution due to the high consumption of chemicals.

It is an object of the present invention to provide a process which enables the arsenic content of water to be reduced to meet the requirements of industrial use and drinking water supply and the complete removal of iron and optionally manganese ions in the water and can be easily carried out on an industrial scale.

In our studies, it has been found that the desired object can be achieved by treating water with soluble iron (III) or 2 to 10 g / m ³ iron as coagulant after treatment with oxidizing agent corresponding to chemical oxygen demand and ammonia content. water-soluble aluminum compound corresponding to g / m ^{3 of} aluminum and optionally 5-20 g / m ^{3 of} calcium hydroxide as a clarifying and clarifying agent, filtering the water, then contacting it with particulate manganese dioxide, optionally supported on a support. 0.5 to 4 g / m ^{3 of} manganese (VII) is added before filtration or contact with manganese dioxide.

The present invention is based on the discovery that if the water's arsenic content is substantially reduced by coagulation and clarification, a small amount of residual arsenic can be removed by contacting the water with manganese dioxide to provide a suitable oxidizing medium. Manganese dioxide chemically binds arsenate ions, catalyzing the chemical transformation leading to the incorporation of arsenate. However, this feature can only be exploited when working with guaranteed arsenite-free water. The ion of trivalent arsenic is the catalyst poison of manganese dioxide. Therefore, during the pre-oxidation treatment, which is intended to remove oxidizable impurities, we provide the oxidative medium for the conversion of arsenic arsenate. However, if necessary, the conversion is completed with a manganese (VII) compound before filtration or contact with manganese dioxide. Contact with manganese dioxide also removes the arsenic from the water and yields less than 0.05 mg / l of arsenic.

The process of the invention will be described in detail below.

Water containing arsenic, iron and optionally manganese is first subjected to oxidation treatment to remove oxidizable impurities. To the water is added an amount of oxidizing agent based on the chemical oxygen demand and the ammonia content, preferably a chlorine-containing substance such as chlorine-saturated water or sodium hypochlorite. In most cases, an amount of about 1-5 g / m ^{3 of} chlorine was sufficient in our experiments. Oxidation is an intermediate oxidation of divalent iron and manganese ions to form water-insoluble oxides and hydroxides, partly due to the presence of other ions in water. However, the trivalent arsenite ions are also converted to the pentavalent arsenate ions.

The coagulant is then added to the water. The iron (III) compound is preferably iron (III) chloride and the aluminum compound is preferably aluminum sulfate. The former is added in an amount of 2 to 10 g / m ^{3 of} iron and the latter in an amount of 5 to 15 g / m ^{3 of} aluminum. Upon addition of the coagulant, a precipitate is formed which can be removed by filtration, which also adsorbs most of the arsenic compounds. The pollution

For a more complete removal of 188 884, a clarifying agent may be used as needed. For this purpose, 5-20 g / m ^{3 of} calcium hydroxide is added to the water, preferably in the form of lime milk. If it is still necessary to provide an oxidizing medium for the complete oxidation of the arsenite ions, a manganese (VII) compound, preferably potassium permanganate, is added in an amount of 0.5-4 g / m ³ .

The precipitate is then removed from the water by filtration. The filter material is preferably quartz sand having a particle size of 1 to 5 mm, preferably 2 to 3 mm. The water was passed through a 1.2 to 1.8 m thick filter bed.

After filtration, the water is contacted with manganese dioxide if it no longer contains arsenite ions. If the presence of arsenite ions is still to be expected, the water at this stage of the process is treated with a manganese (VII) compound as mentioned above. The manganese dioxide is used in the form of particles of 0.2 to 3 mm, preferably 0.5 to 0.8 mm. It may be granular brown stone, or it may be manganese dioxide deposited on a carrier of suitable particle size, preferably on quartz sand. In the latter case, the process is carried out in the usual manner, that is, the carrier is first contacted with a solution of a manganese (III) salt, preferably manganese (II) chloride, and then treated with potassium permanganate. The manganese dioxide treatment can be carried out in any way that allows contact; however, it is expedient to pass the water through a granular layer of manganese dioxide having a thickness of about 0.9 to 1.5 m. The water leaving the manganese dioxide contains less than 0.05 mg / l of arsenic and is free of iron and manganese.

The process of the invention may be carried out using devices and equipment used in water treatment. Most preferably, the operations described above are carried out under continuous flow of water. If the water is flowing at a rate of about 3 to 10 m / h, filtration and contact with manganese dioxide can be successfully performed on the above-mentioned filter layer or manganese dioxide contact layer. The need for regeneration of the latter would in principle limit the cycle time of the continuous process; however, according to the invention, the manganese dioxide contact layer may be regenerated in operation with potassium permanganate. The use of potassium permanganate does not hinder the process of the present invention and, as indicated, may be advantageous or particularly desirable for the process.

An advantage of the present invention is that it allows simple removal of arsenic from water without the need for special chemical chemicals or special equipment. Due to its simplicity it is suitable for processing large volumes of water, for industrial water treatment. The most important advantage of the present invention is that it allows the removal of arsenic from the water to meet the most stringent requirements.

The invention is illustrated by the following examples.

EXAMPLE I Water in the amount of m ³ / h is treated with flow, the main quality characteristics of which are as follows:

Fe ' ⁺ 1.2 g / m ³

Mn ²⁺ 0.2 g / m ³

The chemical oxygen demand (COD) of NH ₄ ⁺ 6.0 g / m ^{3 is} 4.0 g / m ³

As 0.2 g / m ³

To the water was added a quantity of 5 g of chlorine saturated water, containing a total of 125 g / h of chlorine per hour, followed by 24 g / m ^{3 of} FeCl ³ · 6 H2O equivalent to 5 g / m ^{3 of} iron (III) ion. In addition to ferric chloride, 10 g / m ³ of total 250 g / h of lime milk is added to the water to completely iron out. After the chemicals have been added and thoroughly mixed, the water is passed through a 2200 mm closed pressurized pre-filter, which is filtered through an upper filter layer having a 1500 mm layer thickness of 2-3 mm mesh and a backing layer of about 200 mm thickness at a speed of 7 m / h. After pre-filtration, a specific amount of potassium permanganate (2 g / m ³ ), totaling 50 g / h, is added to the water. The water is then passed through the lower, so-called "plugged" filter layer of the pre-filter closed filter. The latter filter layer contains about 1500 mm thick MnO2 applied to a filter gravel of 1-2 mm mesh size and 200 mm thick backing layer.

The lower filter layer discharges high-quality purified iron and manganese-free purified water with a NH ₄ ⁺ content of 4 g / m ³ , a chemical oxygen demand of 3 g / m ³ and an arsenic content of less than 0.0.5 g / m ³ .

The filters become clogged after a filtration period of about 24 hours. The filters are then rinsed with water and air to remove any suspended solids from the filter layers. After about 1 hour of rinsing, the filters can be used again.

Example 2

The quality water of Example I is purified as in Example 1 except that a specific amount of 12 g / m ³ , i.e. 3700 g of aluminum sulfate per hour, is added instead of the iron (III) chloride mentioned therein. In the apparatus described in Example 1, the bottom filter layer is drained of iron and manganese-free purified water. Purified water has a chemical oxygen demand of less than 3 g / m ³ and an arsenic content of less than 0.05 g / m ³ .

Since larger amounts of coagulant are added this time, it is advisable to rinse the filter every 12 hours.

-3188884

Example ³ Water is purified in m ³ / h with the following qualitative characteristics:

Fe ²⁺ 1.9 g / m ³

Mn ²⁺ 0.3 g / m ³

NH ₄ * 0.3 g / m ³ Chemical Oxygen Demand (COD) 1.0 g / m ³

As 0.25 g / m ³

The chlorinated water containing 1 g / m ³ of unit amount (25 g / hour) of chlorine water, followed by 6 g / m ³ iron (III) -ion equivalent of 29 g / m ^3, FeCl ₃ .6 H ₂ O was added. After the chemicals are added and mixed, 2 g / m ^{3 of} potassium permanganate are added to the water and the water is passed through the filter apparatus of Example 1, which is passed through both filter layers. The lower filter layer discharges purified iron and manganese-free purified water with a NH ^ content of less than 0.3 g / m ³ , a chemical oxygen demand of less than 1 g / m ³ and an arsenic content of less than 0.05 g / m ³ .

The filter is preferably rinsed every 24 hours.

Example 4

The quality water of Example 3 was purified as described in Example 3. However, for filtration this time, a filtration apparatus is used which has a lower filter layer containing 2 to 3 mm mesh screened brown stone (manganese dioxide). Purified water, free of iron and manganese, leaving the lower filter layer has an NH NH content of 0.3 g / m ³ , a chemical oxygen demand of 1 g / m ³ and an arsenic content of less than 0.05 g / m ³ .

Claims (4)

Claims f. A process for reducing the arsenic, iron and, if applicable, manganese content of water by treating water with a soluble iron content of 2 to 10 g / m ^{3 of} iron as coagulant after treatment with oxidizing agent suitable for chemical oxygen demand and ammonia, preferably chlorine. Compound III), preferably ferric chloride, or water-soluble aluminum compound corresponding to 5-15 g / m ³ aluminum, preferably aluminum sulfate, and optionally 5-20 g / m ³ calcium hydroxide as the bleaching agent, and filtering the water, characterized in that the water is contacted, if desired after addition of 0.5 to 4 g / m ³ of manganese (VIII), with particulate manganese dioxide, optionally supported on a support. 2. The process of claim 1 wherein the manganese (Vn) compound is potassium permanganate. The process of claim 1, wherein the water is passed through a particulate manganese dioxide bed. contacting with manganese dioxide. A process according to claim 1, wherein all of its operations are performed under continuous flow of water.