RU2426699C1 - Method of treating recycled water from metallurgical production - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods of treating recycled water from metallurgical production with high content of heavy metal phosphates and salts thereof, and can be used in metallurgical production. The method involves further reactant precipitation with lime milk and flocculant at pH=10.5-11.5, followed by packing the precipitate after selling in a sludge densifier and drying the precipitate on a press filter. Suspended substances are removed from the water on a filter with granular filler. The water is then demineralised on an ion-exchange filter filled with weakly acidic cation-exchange resin in Na form. The water is then purified on a fine filter and then desalinated on a two-step reverse osmosis apparatus at pH=7-7.5. Process water pressure at the first step of the reverse osmosis apparatus is equal to 20 kgf/cm² and 55 kgf/cm² at the second step. The purified water used is filtrate from the first step. The filtrate from the second step is returned for re-purification at the first step.

EFFECT: method increases quality of the purified water and efficiency of the process owing to low consumption of chemical agents.

4 cl, 2 dwg, 1 tbl

Description

The invention relates to methods for treating recycled water of metallurgical production with a high content of phosphates from heavy metals and can be used in metallurgical industries associated with the processing of heavy metals.

The well-known "Method of wastewater treatment from heavy metal ions" (patent RU No. 2048453, publ. 20.11.1995). In this method, the wastewater is pre-filtered, concentrated in a reverse osmosis apparatus. The concentrate is subjected to electrochemical treatment, heavy metals are precipitated and returned to production. In the known method there is no purification from fine suspended solids that are present in the circulating waters of metallurgical enterprises and which clog the pores of the membranes, as a result of which frequent replacement of the membrane modules occurs. In the known method, the reverse osmosis unit uses recirculation of the concentrate, which increases the yield of the filtrate, however, this requires the installation of increased productivity, this approach also reduces the quality of the filtrate and does not allow to obtain a high degree of concentration of impurities.

The well-known "Method of deep wastewater treatment from heavy metals" using combined treatment methods (patent RU No. 2085518, publ. 07.27.1999), adopted as a prototype. It includes reagent purification, sorption on aluminosilicates, desalination in a reverse osmosis apparatus, processing of concentrate on aluminosilicates, evaporation of the concentrate to dry residue and reuse of purified water in the enterprise’s closed water circulation system. In the known method, sorption on aluminosilicates is used at the preliminary purification stage, which cope well with heavy metals and organic impurities, but at the same time finely dispersed suspended substances are poorly retained. In the known method, before installing reverse osmosis, the remaining suspended solids, which clog the pores of the membranes, are not removed, which entails a decrease in the productivity of the installation and, as a result, an increase in pressure on the membrane, a decrease in the intervals between washes, and more frequent replacement of the membrane modules.

The technical result of the invention is to improve the quality of purified water and increase the efficiency of the process by reducing the consumption of chemicals.

The technical result is achieved due to the fact that in the method for purifying recycled water of metallurgical production with a high content of phosphates from heavy metals and their salts, which includes a preliminary purification step by reactive precipitation with the addition of an alkali solution, clarification of water after reagent purification in a sump, dewatering of the sediment after settling, neutralization of water before reverse osmosis with an acid solution, desalination at a reverse osmosis unit, regeneration of reverse osmosis membranes is carried out in the mode desalination in an alkaline environment and reuse of purified water, characterized in that the reagent precipitation is additionally carried out with milk of lime and flocculant at pH 10.5-11.5, then the sediment is condensed after settling in the sediment compactor and the precipitate is dried on a filter press, and water is purified from suspended solids is carried out on a filter with a granular charge, then water is softened on an ion exchange filter filled with a weakly acid cation exchange resin in Na-form, then water is purified on a fine filter, then desalination is carried out at a 2-stage reverse osmosis unit at pH 7-7.5, the working pressure of water at the first stage of reverse osmosis is 20 kgf / cm ² , and at the second stage - 55 kgf / cm ² , the filtrate is used as purified water of the first stage, the filtrate of the second stage is returned for re-purification to the first stage.

In the filter with a granular charge, quartz sand and hydroanthracite can be used as a filter material.

The washing of the filter with a granular charge can be carried out by a reverse flow of water supplied to the cleaning.

The regeneration of the ion exchange filter can be carried out with a 15% NaCl solution.

One of the effective methods for purifying water from impurities is reagent treatment. Large particles of impurities are deposited quite quickly under the action of gravity, and coagulation is used to deposit fine particles with sizes less than 10 microns - the process of enlargement of small particles as a result of their interaction and association into aggregates. The use of milk of lime (Ca (OH) ₂ ) as a coagulant is due to the fact that Ca is an active metal that displaces heavy metals from soluble compounds, converting them to insoluble, and also precipitates various salts, including phosphates, sulfates, chlorides . Moreover, such an amount of milk of lime is required so that with the minimum excess, all necessary reactions proceed.

To ensure a high degree of purification of water from heavy metals and their salts, it is necessary to achieve a minimum solubility of salts and hydroxides, at which they precipitate. The solubility is primarily affected by the pH of the medium, the optimal value of which in terms of efficiency / cost is pH 10.5-11.5. A pH adjustment is achieved by adding caustic soda (NaOH).

The use of flocculant after coagulation increases the capture of particles, accelerates the formation of flakes and makes the flakes more dense and rapidly deposited. The use of a flocculant also allows you to limit the dosage of coagulant to a minimum amount, since an excess of coagulant is not required to form a suspension that can precipitate.

After sedimentation, the water still contains quite a lot of suspended substances, which must be removed before membrane treatment. To do this, use filters with granular loading, while the most effective and economical multilayer filters, which consist of materials with different densities and particle sizes. Larger and lighter particles are on top of the filter, and smaller and heavier ones on the bottom. In this case, large water pollution is retained in the upper layer, and the remaining small ones in the lower layer, i.e. the entire load volume works.

The use of quartz sand and hydroanthracite allows you to create a two-layer filter granular loading. Hydroanthracite is a mechanically strong and chemical resistant material having a lower density than that of quartz sand, therefore it is placed in the upper layer of the filter. The heterogeneity of the hydroanthracite granules in size and shape allows suspended particles to penetrate into the layer of filter material to a greater depth, which leads to an increase in the dirt capacity of the filter layer, the duration of the filter cycle and a decrease in the resistance of the layer. Quartz sand is also a mechanically strong and chemically resistant material, but more dense than hydroanthracite, so it retains smaller suspended particles and is located in the lower layer of the filter.

As the filter operates, the amount of trapped substances increases and at a certain moment the filter layer becomes so dirty that the impurities begin to slip into the filtrate, while the filtration resistance increases sharply, and the productivity decreases. The filter is stopped and regenerated by reverse current. For regeneration of the filter, chemical reagents are not required, it is carried out by water supplied for cleaning.

The duration of the filter cycle depends on the rate of contamination of the filter and is determined as the pressure loss on the filter changes. Changing the duration of the filter cycle leads either to excessive pollution of the filter, difficulty in washing it, deterioration of the quality of the filtrate and subsequent problems when clogging the pores of reverse osmosis membranes, or is impractical due to the increased downtime of the filter. Changing the washing time leads either to incomplete cleaning of the filter material, or is impractical due to the increase in the downtime of the filter.

After mechanical and reagent cleaning, some excess calcium remains in the water, which must be removed, since calcium is an undesirable component for reverse osmosis membranes, it contributes to the deposition of hardness salts on their surface. To soften the water, an ion-exchange filter is used, filled with a weakly acidic cation-exchange resin in the Na-form, the purpose of which is primarily to extract calcium, which is replaced by more active Na.

An ion exchange resin is an insoluble high molecular weight compound with functional ionic groups capable of entering into exchange reactions with solution ions. Extraction of calcium cations (softening) requires a weakly acidic resin in the Na form, since it has a higher exchange capacity than strongly acidic and is better regenerated, reacts only in an alkaline environment and acts selectively, without reducing the total salinity, which also significantly increases the intervals between regenerations.

When the limit value of the working exchange capacity of the filter material (resin) is reached, regeneration of cation exchangers is necessary. The regeneration process consists of the following sequential operations: loosening the cation exchanger layer with an ascending stream of feed water, lowering the water cushion, regenerating the cation exchanger by filtering it with a 15% sodium chloride solution (NaCl) through an ion-exchange layer. The concentration of sodium chloride is selected based on the regeneration rate of the filter material and the utilization rate of the regenerating solution. On the one hand, the higher the concentration of the solution, the higher the rate of regeneration, on the other hand, the higher the concentration, the lower the utilization rate of the regenerating solution, the optimal concentration from this point of view is a 15% sodium chloride solution. Spent regenerating solutions - eluates, are sent for disposal.

After preliminary filtration, the water contains the minimum amount of suspended solids, which are extremely undesirable for reverse osmosis membranes, because clog the pores of the membranes, which entails a decrease in productivity, and as a result, an increase in pressure on the membrane, a decrease in the interval between flushing and more frequent replacement of the membrane modules. As a result, a fine filter is installed before installing reverse osmosis. A mesh filter element with a mesh size of 10-20 microns is installed in the filter. The filter is designed to remove residual contaminants, it has a backwash system, which is launched as the pressure drops on the filter. The flushing process works in filtering mode.

After preliminary treatment, the water has a pH of 10.5-11.5, then it enters the neutralization unit, where, by adding sulfuric acid (H ₂ SO ₄ ), the water reaches a pH of 7-7.5 and enters the reverse osmosis unit. The choice of pH is associated, firstly, with the creation of conditions to reduce the likelihood of precipitation of hardness salts on the surface of the membranes, secondly, with the formation of a filtrate with a neutral pH and the absence of its subsequent neutralization, and thirdly, with a decrease in the amount of neutralization reagent. There is no need to lower the pH to acidic, since the likelihood of precipitation is minimal due to preliminary softening of the water.

The number of concentration steps for reverse osmosis is associated with the need for maximum return of water to the closed water circulation system. From one stage of reverse osmosis, it is usually possible to obtain 60-70% of the filtrate, from two stages - about 85-95% of the filtrate, but at the same time its quality decreases. The maximum degree of concentration depends on the osmotic pressure of the dissolved salts and the resulting concentration of hardness salts, which are harmless at low concentrations, but can be deposited on the membrane surface when the solubility limit is reached. In addition, an increase in the number of concentration steps increases the complexity of the hardware design of the desalination stage, which leads to an increase in capital costs and reduces the reliability of the entire installation, because requires coordinated work of all steps. The concentrate (the remaining 5-15% of the initial water volume) is used in production, so there is no need to concentrate more strongly. Optimal in terms of the number of concentration steps is a 2-stage reverse osmosis unit.

As the concentration increases, the amount of dissolved salts in the concentrate increases, which leads to an increase in the osmotic pressure, therefore, at the 2nd stage, to continue the filtration, it is necessary to increase the water pressure. The productivity of the installation increases with increasing pressure, the limiting factor is the maximum allowable pressure that the membrane withstands. Also, with increasing pressure, the costs of the process increase. The optimal working pressure in the first stage is 20 kgf / cm ² , and in the second - 55 kgf / cm ² .

During the filtration process, reverse osmosis modules become contaminated, as a result of which the installation productivity decreases and the selectivity of the membranes decreases, therefore, with a significant drop in productivity, as well as with an increase in the conductivity of the filtrate, indicating a decrease in selectivity, it is necessary to regenerate the membrane installation. The regeneration of reverse osmosis membranes is carried out in desalination mode at pH 10.5-11.5, for the regeneration no additional reagents are required, it is necessary to suspend the neutralization process for the duration of it. Processing time depends on the degree of contamination of the membranes; if the duration of the reagent is insufficient, complete regeneration of the membrane is not achieved, however, with prolonged regeneration, there is a risk of deposition of hardness salts on the membrane surface, as well as the risk of destruction of the selective membrane layer.

As a result of preliminary cleaning, a precipitate is formed, which settles in a thin-layer sedimentation tank, then is compacted in a sediment compactor and dehydrated in filter presses. The amount of dry sediment obtained is less than 2% by weight of the source water. With a content of 1 kg of dry sludge, 300 g of phosphates and a drier sludge are used in the production of phosphoric acid.

After filtering the water at the reverse osmosis unit, a concentrate is formed, which is 5-15% of the volume of the source water. When the concentration of phosphates in the concentrate is not more than 1 g / l, sulfates not more than 16 g / l, fluorides not more than 10 g / l and a pH of 6.5-8.5, the concentrate is used to prepare the raw material mixture in the production of Portland cement.

The reverse osmosis filtrate meets all the requirements for the composition of the water for the reservoir of the first category of water use, and the content of some components is several times lower than the permissible maximum permissible discharge (table 1). This is achieved by the fact that the purified water is the filtrate of the first stage of the reverse osmosis unit, and the filtrate of the second stage is returned for repeated purification in the first stage.

The invention is illustrated by drawings, in which Fig. 1 shows a diagram of the pre-treatment of recycled water: 1 - the capacity of the source of recycled water, 2 - the pump for supplying the source water, 3 - the tank with milk of lime, 4 - the metering pump that supplies the milk of lime, 5 - the tank with caustic soda solution, 6 - dosing pump supplying caustic soda, 7 - container with flocculant solution, 8 - dosing pump, supplying flocculant, 9 - thin-layer sump, 10 - sediment compactor, 11 - pump supplying compacted sludge, 12 - clarified water tank , 13 - filter press, 14 - pump, odayuschy clarified water, 15 - filter with granular media, 16 - pump, supplies water to the ion exchange filter, 17 - ion exchange filter, 18 - vessel with solution of common salt, 19 - a metering pump feeding table salt.

Figure 2 - scheme of neutralization and reverse osmosis: 20 - the capacity of softened water, 21 - the pump supplying softened water, 22 - the tank with sulfuric acid, 23 - the metering pump supplying sulfuric acid, 24 - the fine filter, 25 - pump a high pressure pump that supplies water to the 1st stage of reverse osmosis, 26 - the 1st stage of reverse osmosis, 27 - a tank with a reverse osmosis filtrate, 28 - a high pressure pump that supplies water to the 2nd stage of reverse osmosis, 29 - a pump, feed filtrate for reuse, 30 - 2nd stage of reverse osmosis, 31 - capacity ontsentrata reverse osmosis, 32 - pump feeding concentrate on the production of Portland cement.

The method of purification of recycled water of a metallurgical enterprise includes a preliminary purification (Fig. 1), where the initial recycled water is supplied to a tank 1, from where it is pumped to a pre-treatment unit where sequential treatment of effluents is performed: for the precipitation of phosphate ions from a tank 3 by a pump 4 a solution of milk of lime is fed; to adjust the pH from the tank 5, the pump 6 delivers a solution of caustic soda; to improve the process of sedimentation from the tank 7, the pump 8 serves a flocculant solution. Thus treated effluents are sent to the settler 9 to separate the suspension into the clarified part and sediment.

The clarified water is collected in a tank 12 and then for fine cleaning of suspended particles by a pump 14 it is directed to a filter with a granular charge 15, and then to remove the residual amount of calcium by a pump 16 to an ion-exchange filter 17.

Sludge (calcium phosphate, iron hydroxide, etc.) from the sump 9 is periodically discharged into the sediment compactor 10, from where it is pumped to the filter press 13. From where the dry sludge is sent to the production of phosphoric acid.

In the process of regeneration of the ion-exchange filter 17, a solution of sodium chloride from the tank 18 is pumped into it by a pump 19.

Schematic diagram of the site of neutralization and reverse osmosis desalination is presented in figure 2. Softened water from the pre-treatment unit is collected in the tank 20 and pump 21 is supplied to the fine filter 24, mixing to adjust the pH with a solution of sulfuric acid supplied from the tank 22 by the pump 23.

After the fine filter, the clarified water is pumped 25 to the first stage of reverse osmosis 26. The filtrate is collected in a tank 27 and pump 28 is fed to feed the closed-loop system. The concentrate of the first stage is pumped to the second stage of the membrane unit 30 by a pump 29. The filtrate is discharged to a tank 20, where it is mixed with the feed stream, and the concentrate is collected in a tank 31, from where it is sent by a pump 32 to the raw material charge preparation unit for the production of Portland cement.

As an example, consider the treatment of recycled water, the initial composition of which is presented in table 1. Note that the source of recycled water contains a fairly large amount of phosphates, sulfates, calcium and magnesium. In the process of purification, lime milk - 0.25 g, caustic soda - 0.8 g, flocculant like praestol - 2 mg are added to each liter of source water, while the pH is maintained in the range of 10.5-11.5. The volume of sediment after settling is 10-12%. The amount of dry sludge dried to a constant weight is 2.03 g with 1 liter of feed water, the phosphate content in the dry sludge is 51.2%. Filtration of the clarified part on a filter with a two-layer loading (quartz sand, hydroanthracite) is at a speed of 5-6 m / h. Next, ion-exchange filtration to remove the remaining calcium. The composition of the water after preliminary treatment is presented in table 1. As can be seen, most of the phosphates, calcium, potassium, magnesium, manganese and iron were filtered off. Then softened water is neutralized, 1.24 g of a 36% solution of sulfuric acid is added to each liter of water. After neutralization, water is fed to reverse osmosis filtration; the filtrate extraction rate is 85-95%. The results of the analysis of the composition of the filtrate and concentrate are shown in table 1.

Table 1. PDS for a reservoir of the first category and composition of recycled water at different stages of treatment. No. p / p Name of pollutants PDS Source water Water after pre-treatment Reverse osmosis filtrate Reverse Osmosis Concentrate one PH value - 4.82 10.68 5.53 8.18 2 Total hardness, mEq / L 3.83 15.0 0,07 0,03 2,8 3 Calcium, mg / L 41.6 250,0 1,0 0.7 26 four Magnesium, mg / L 21.0 30,0 0.24 0.2 eighteen 5 Aluminum, mg / L 0.04 0,03 0,03 0,03 - 6 Iron, mg / L 0.1 2.05 0.08 0.04 11.0 7 Potassium, mg / L 50,0 32,4 5,0 4,5 - 8 Manganese, mg / L 0.01 1,6 0.0023 0,0 - 9 Copper, mg / L 0.001 0,031 0,03 0,0 2,53 10 Sodium, mg / L 120.0 127.0 691.0 58.0 3913.0 eleven Ammonium nitrogen, mg / l 0.4 2.6 2.5 0,0 26.0 12 Nitrate Nitrate, mg / L 1.12 1.3 1.3 1,0 - 13 Nitrite Nitrogen, mg / L 0.02 0.05 0.05 0,0 - fourteen Sulfates, mg / L 79.0 830.0 807.0 30.8 6768.0 fifteen Phosphates, mg / L 0.2 620.0 71.0 0.2 645 16 Fluorides, mg / l 0.75 10.5 8.25 0,035 48 17 Chlorides, mg / L 64.0 29.0 25.0 5,6 174.0 eighteen Oil products, mg / l 0.05 0.23 0.21 0.05 - 19 COD 30,0 146.0 60.7 16,0 1532 twenty BOD 3.0 - - 0.31 - 21 Suspended Substances, mg / L 7.25 - - 3.2 - 22 The dry residue, mg / l 803.2 2570,0 3125 13 23400

Claims (4)

1. A method for purifying recycled water of metallurgical production with a high content of phosphates from heavy metals and their salts, including the stage of preliminary purification by reagent precipitation with the addition of an alkali solution, clarification of water after reagent purification in the sump, dewatering of the sediment after settling, neutralization of water before reverse osmosis with an acid solution , desalination at a reverse osmosis unit, regeneration of reverse osmosis membranes in desalination mode in an alkaline medium, and reuse of purification water, characterized in that the reagent precipitation is additionally carried out with milk of lime and a flocculant at pH 10.5-11.5, then the sediment is condensed after settling in the sediment compactor and the precipitate is dried on a filter press, the water is purified from suspended solids on a filter with granular loading, then water is softened on an ion-exchange filter filled with a weakly acidic cation-exchange resin in Na-form, then water is purified on a fine filter, then desalination is carried out on a 2-stage reverse installation MOCA at pH 7-7.5, water pressure in the first stage reverse osmosis is 20 kgf / cm ^2, and the second stage - 55 kgf / cm ² as purified water using the filtrate of the first stage, the filtrate is recycled to the second stage re-cleaning to the first step. 2. The method according to claim 1, characterized in that in the filter with a granular charge, quartz sand and hydroanthracite are used as filter material. 3. The method according to claim 1, characterized in that the washing of the filter with a granular charge is carried out by a reverse current of water supplied for cleaning. 4. The method according to claim 1, characterized in that the regeneration of the ion exchange filter is carried out with a 15% solution of NaCl.

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