RU2107037C1 - Method of treatment of sewage waters of production of cellulose nitrates - Google Patents

Abstract

FIELD: chemical technology, mainly, production of cellulose nitrates and relating to treatment of sewage waters of this production to remove suspended matter. SUBSTANCE: method includes reagent neutralization of sewage waters, separation into liquid and solid phases, mixing of clarified water with hydrolyzed coagulant - aluminium sulfate and final treatment to remove suspended matter by settling. In this case, the time taken for mixing coagulant with water to obtain homogeneous solution in the entire volume of liquid amounts to not less than 3 s with velocity gradient of 1200 ss^-1. EFFECT: higher efficiency of treatment up to 93!3%, increased intensity of flocculation in water coagulation treatment and reduced time of coagulation from 15 to 10 min, that results in reduced volume of coagulation structures and capital outlays for their construction. 3 tbl

Description

 The invention relates to chemical technology, mainly to the production of cellulose nitric esters, and for the treatment of wastewater from this production of suspended solids.

 At certain stages of the production of cellulose nitrate esters, in particular nitration and pre-stabilization, wastewater with the same pollution pattern is formed. They are combined into one common stream and taken out of production.

The total runoff of cellulose nitrate (SC) production includes NC suspension (up to 300 mg / dm ³ ) and free mineral acids, nitric and sulfuric, in total up to 5 g / dm ^3, as the main pollutants.

 A known method of cleaning the total runoff from the phases of nitration and preliminary stabilization of the production of SC [1], including reagent neutralization of effluent with lime and gravitational separation into liquid and solid phases. The neutralized water is sent from the reaction chamber to the sludge traps, which serve as clarifiers of water and collectors of the precipitate.

 This method of wastewater treatment is characterized by a long residence time of neutralized wastewater in sludge collectors (up to 30 days), due to the low rates of natural deposition of calcium sulfate dihydrate and residual cellulose nitrates in water. Sludge collectors used for the separation of solid and liquid phases are bulky, capital-intensive structures, occupying large production areas. In the process of operation of sludge collectors, as they are filled with sludge and the useful volume decreases, the degree of wastewater treatment decreases.

 A known method of wastewater treatment from suspended solids under the action of centrifugal forces created by the rotational movement of a liquid in a hydrocyclone [2]. Due to the tangential supply of the initial fluid to the cylindrical part of the hydrocyclone, the entire mass of water in the apparatus is in rotational-translational motion. In this case, centrifugal forces directed along the radius from the axis of rotation to the periphery act on suspended particles moving in the flow. When the fluid rotates in hydrocyclones, centrifugal forces exceed gravity; accordingly, the speed of particles in a centrifugal field also increases. The duration of the process of separation of solid and liquid phases is reduced to the same extent. However, the dispersion composition of suspended solids has a significant effect on the performance of hydrocyclones.

Thus, an experimental verification of this method showed that the duration of purification from suspended solids of a pre-neutralized total runoff of SC production from the nitration phases and preliminary stabilization in a battery of three pressure hydrocyclones decreased from 15-30 days to 6-8 hours, the cleaning efficiency was 60-70 % and was close to the level of purification in the sludge traps, while clarified water contained up to 200 mg / dm ^{3 of} suspension.

 The insufficient degree of wastewater treatment in pressure hydrocyclones is due to the fact that up to 25% by weight of suspended solids present in water, including SC, are represented by fine fractions of less than 45 microns.

Additional treatment of water clarified in the hydrocyclone battery with hydrolyzable coagulant aluminum sulfate (coagulant dose 25 mg / dm ³ , mixing time 40 s, settling 30 min) allowed to increase the degree of purification from suspended solids from 60-70% to 90%, to reduce their content in purified water from 200 to 70 mg / dm ³ .

However, taking into account the large volume of discharged water at each plant manufacturing SC (10-12 thousand m ³ per day), the achieved level of purification does not guarantee exclusion of pollution of water bodies by the effluents of SC production.

 Significant disadvantages of the considered cleaning method also include the long duration of the coagulation stage (15-20 min), which, in turn, requires increased volumes of coagulation facilities.

 The objective of the invention is to increase the degree of purification of the succulent water formed at the nitration phases and the preliminary stabilization of the SC production from suspended solids, the intensification of the flocculation process during the treatment of wastewater with a coaculant aluminum sulfate.

The problem is solved in that in the method of purification of wastewater produced by SC, discharged from the nitration and preliminary stabilization phases, including reagent neutralization of effluents, separation into liquid and solid phases in the field of centrifugal forces, mixing clarified water with hydrolyzable coagulant aluminum sulfate and final purification from suspended substances by sedimentation, the coagulant is mixed with water until a homogeneous solution is obtained in the entire liquid volume at a velocity gradient of more than 1200 s ^-1 for less than 3 s.

 The positive technological effect of the rapid mixing of coagulant with water can be explained by the extremely high rate of formation of hydrolysis products that destabilize the particles of contaminants and contribute to the simultaneous agglomeration of primary aggregates in the entire water volume, which, in turn, contributes to the formation of larger and denser flakes [3].

According to the proposed method, wastewater treatment of the production of SC from suspended solids is carried out as follows. Wastewater from the phases of nitration and preliminary stabilization is neutralized with a 5% solution of milk of lime to a pH of 7-7.2, in a pressure hydrocyclone unit at a working pressure of 0.4-0.45 MPa, the precipitate is separated, clarified water is sent to an apparatus with an intensive axial and tangential mixing, for example, into an apparatus with a propeller stirrer and a guide tube. A 1% solution of aluminum sulfate is introduced thereto at the rate of 25 mg / dm ³ , the duration of the complete mixing of the coagulant with the water being purified in the entire liquid volume is less than 3 s, the velocity gradient is more than 1200 s ^-1 . Next, the water is kept under slow stirring (speed gradient 20-90 s ^-1 ) for 10-12 minutes, during which intensive flocculation occurs, the mixer is turned off, the water is left to stand for 30-40 minutes and drained.

Example 1 (prototype). Wastewater from the phases of nitration and preliminary stabilization of the SC production was neutralized with a 5% solution of milk of lime to pH 7, in a two-stage pressure hydrocyclone unit at a working pressure of 0.4 MPa, a precipitate was separated and clarified water was obtained with a suspended matter content of 195 mg / dm ³ .

 Next, the water was processed in the following sequence: mixing aluminum sulfate with water, coagulation, sedimentation of suspended solids. All stages of the process were carried out in an apparatus equipped with a paddle mixer with an adjustable speed. The ratio of the diameter of the apparatus to the diameter of the stirrer was 3.

Mixing performance parameters: coagulant dose 25 mg / dm ³ , stirrer speed 200 per minute, corresponding speed gradient 50 s ^-1 , travel time 1-50 s. Operating parameters of coagulation: the number of revolutions of the mixer 40 per minute, the corresponding gradient of speed of 20 s ^-1 , the duration of mixing 15 minutes

 The precipitation of suspended solids was carried out with the mixer off for 30 minutes.

 The results of the analyzes are presented in table. one.

The completeness of mixing of the coagulant with the treated water can be judged by the change in the alkalinity of water and its comparison with the alkalinity of a neutral sample. According to the table. 1, the duration of mixing the coagulant with water in the case under consideration was 40 s. With subsequent coagulation and sedimentation, water was obtained with a suspended matter content of 65 mg / dm ³ , which corresponds to a degree of purification relative to the original 195 mg / dm ^{3 of} 66.5%.

 Example 2 (the proposed method). The source wastewater, its composition, treatment sequence were the same as in example 1.

The difference was that the coagulant was mixed with water in an apparatus equipped with a propeller stirrer and a guide tube, providing intense axial and tangential fluid flows. The ratio of the diameter of the apparatus to the diameter of the mixer was 3, the angle of inclination of the blades 40 ^o .

 Coagulation and sedimentation of suspended solids was carried out in an apparatus equipped with a paddle mixer with an adjustable speed, the technical data of which correspond to those given in Example 1.

Mixing performance parameters: coagulant dose 25 mg / dm ³ , stirrer speed 1500 per minute, corresponding velocity gradient 1200 s ^-1 , coagulant mixing time with water was 1-10 s and was controlled by changing the feed rate of the ammonium sulfate solution.

 The operational parameters of coagulation and sedimentation did not differ from those adopted in example 1. The results of water analyzes are presented in table. 2.

According to the table. 2 when mixing the coagulant with water for 1-3 s, subsequent coagulation and sedimentation, water with a suspended matter content of 13-14 mg / dm ^{3 is} obtained, which corresponds to the cleaning efficiency relative to the original 195 mg / dm ³ 92.4 - 93.3% .

 With a longer mixing time of the coagulant with water, the content of suspended solids in the purified water increases, and the efficiency of its purification decreases.

 Example 3. The source of wastewater, its composition, treatment sequence were the same as in example 1.

 The difference was that the coagulant was mixed with water in the apparatus with a paddle mixer and in the apparatus with a propeller mixer and a guide tube. The technical data of the apparatuses correspond to those given in examples 1 and 2. Coagulation and sedimentation of suspended solids was carried out in an apparatus equipped with a paddle mixer with an adjustable speed, the technical data of which correspond to those given in example 1.

Mixing performance parameters: coagulant dose 25 mg / dm ³ , stirrer speed 1500 per minute, corresponding velocity gradient 1200 s ^-1 , mixing time 5-15 minutes. The precipitation of suspended solids was carried out with the mixer turned off. The results of water analyzes are presented in table. 3.

 From the data table. 3 it follows that when mixing the coagulant with water in a known manner (40 s), the duration of the coagulation stage is 15 minutes, when mixed by the proposed method (up to 3 s), the duration of coagulation is reduced to 10 minutes, i.e. 1.5 times.

 Thus, the proposed method allows to increase the efficiency of wastewater treatment from the nitration phases and preliminary stabilization of the SC production from suspended solids from 66.5 to 93.3%, to intensify the process of flocculation during coagulation water treatment and to reduce the duration of the coagulation stage from 15 to 10 minutes. This, in turn, allows to reduce the volume of coagulation facilities and capital costs for them.

Claims (1)

Method of wastewater treatment for the production of cellulose nitrates removed from the nitration and pre-stabilization phases, including reactive neutralization and separation of the liquid and solid phases, characterized in that the separated liquid phase is additionally mixed with aluminum sulfate at a speed gradient of 1200 s ^{- 1} for less than 3 s with subsequent sedimentation and separation of sediment.

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