RU2082681C1 - Method for removing heavy metal ions from concentrated process solutions and waste water - Google Patents

Abstract

FIELD: waste water purification. SUBSTANCE: invention relates to purifying industrial waste water from heavy metal ions and galvanic production waste water. Predetermined volume of process solution or waste water with known concentration of heavy metal ions is treated with excess of ferric sulfate, then alkalized to pH 9-10 and, and into thus formed suspension, ferritization catalyst is introduced. Suspension is heated to 60-90 C and maintained at that temperature with simultaneous bubbling with compressed air to completion of ferritization process. Suspension is finally cooled and separated into liquid and solid phases by a known technique. As ferritization catalyst, alkali metal persulfates, permanganates or perchlorates are used, their quantity being 0.001- 0.002% based on ferric sulfate weight. Completion of ferritization process is judged of by minimum solubility of solid phase in excess of 0.0001 N sulfuric or hydrochloric acid. EFFECT: enhanced efficiency of process. 3 cl, 2 tbl

Description

 The invention relates to methods for purifying industrial wastewater from heavy metal ions (ITM) and is intended to remove these ions mainly from concentrated technological solutions obtained in the wastewater treatment processes of galvanic plants by physicochemical methods, in particular by ion exchange, electrodialysis and ultrafiltration methods. In addition, the invention can be used to remove ITM directly from concentrated wastewater of metallurgical, engineering, electrical and other industries that are not amenable to cleaning by physicochemical methods.

где Me тяжелый металл (никель, медь, кадмий, цинк, железо и др.), а X - его валентность. Далее суспензии смесей гидроксидов тяжелых металлов, образовавшихся по уравнению (1), подвергают механическому разделению на жидкую и твердую фазы (чаще всего отстаиванием и фильтрованием), причем остаточное содержание ИТМ в жидкой фазе (т.е. в очищенной воде) находится в пределах десятых долей миллиграмма в литре.In the process of wastewater treatment of galvanic plants from ITM by the ion-exchange method, along with purified water (90-95% of the volume of the initial wastewater), technological solutions (eluates of ionite filters) with a high concentration of ITM (more than 1 g / l each) are obtained. It is recommended to remove ITM from such concentrated solutions by a widespread chemical (“reagent”) method called hydroxide. The essence of this method consists in alkalizing weakly acidic wastewater to a certain optimum pH value, which is within 8 10 units of the universal acidity scale. As a result of such wastewater treatment, the bulk of the ITM contained in them is deposited as a mixture of their slightly soluble hydroxides in water according to the equation

where Me is a heavy metal (nickel, copper, cadmium, zinc, iron, etc.), and X is its valence. Further, suspensions of mixtures of heavy metal hydroxides formed according to equation (1) are subjected to mechanical separation into liquid and solid phases (most often by settling and filtering), and the residual content of ITM in the liquid phase (i.e., in purified water) is within tenths fractions of a milligram in a liter.

Precipitation mixtures of heavy metal hydroxides obtained by treating wastewater from ITM using the hydroxide method described above, until relatively recently, were dumped by industrial enterprises into urban landfills. However, in recent decades, the pH of the medium in rain and melt snow waters has universally decreased to 4 5 units due to the mass release of acidic industrial gases into the atmosphere (Zaikov G.E. Maslov S.A. Rubaylo V.I. Acid rains and the environment. M Chemistry, 1991, p. 144). This led to the risk of spontaneous removal of ITM from landfills into the environment with a drain of acid rain and melt snow water, in which heavy metal hydroxides easily dissolve with the formation of the initial ITM according to the equation
Me (OH) _X + X • H ⁺ __ → Me ^{X +} + X • H ₂ O. (2)
In this regard, at present, in Russia and other CIS countries, hydroxide precipitation of wastewater from galvanic plants is allowed to be stored for a long time (“buried") only at special landfills that exclude the risk of ITM being released into the environment by acidic natural waters (the procedure for accumulation, transportation, and neutralization) and burial of toxic industrial wastes. Sanitary rules. M. Ministry of Health of the USSR, 1985, 37 pp.). However, the construction of such landfills requires significant investment and land (landfills for the disposal and disposal of toxic industrial waste. Design provisions. SNiP 2.01.28-85. M. Gosstroy of the USSR, 1985, 13 pp.). In addition, any artificial structures for the burial of toxic industrial wastes have a boundary volume, upon completion of which it is necessary to build other similar structures with all the ensuing consequences.

 Meanwhile, it is well known that some natural chemical compounds of heavy metals (ores), in particular their ferrites, are resistant to an excess of dilute solutions of strong mineral acids and acidic natural waters, due to the special structure of their crystal lattice (spinel type). In accordance with this property of ferrites, cases of ITM removal in environmentally hazardous concentrations by acidic natural waters from places where ferrite ores occur (for example, in the Kursk Magnetic Anomaly region) to nearby water bodies have not yet been recorded. It follows that one of the most environmentally and economically acceptable ways to remove ITM from concentrated technological solutions and wastewater is to deposit them in the form of a mixture of heavy metal ferrites, since the resulting precipitates of ferrites can be subjected to long-term storage in the bowels of the earth without resorting to the construction of bulky and expensive special training grounds.

There is a method of removing heavy metal ions from wastewater in the form of ferrites of these metals, according to which ferrous iron and an alkaline reagent are introduced into the wastewater, and the resulting suspension of a mixture of heavy metal hydroxides is heated to 50 ^° C and bubbled with oxygen-containing gas for at least 1 hour. for the oxidation of iron. An alkali metal hydroxide or carbonate is used as an alkaline reagent, air is used as an oxygen-containing gas, and iron is introduced in the form of a sulfate salt in a molar ratio with heavy metal ions of 1: ≥2. As a result, a crystalline precipitate of a mixture of heavy metal ferrites is obtained, which is separated. The residual content of heavy metal ions in water after separation of the ferrite precipitate is in the range of hundredths-thousandths of a milligram per liter [1]
The disadvantage of this method is the long duration of the ferritization process and the resulting high consumption of thermal energy.

There is also known a method of treating wastewater from heavy metal ions, in which caustic soda solution is added to acidic wastewater containing heavy non-ferrous metal ions and iron to a pH of 10.4 to 11.0 and the resulting suspension of a mixture of heavy metal hydroxides is aged by stirring at 60 ± 1 ^o C. In this case, a suspension of a mixture of compounds of the type Me _x Fe _2x (OH) _{6x is formed} , which, when standing, exfoliates into liquid and solid phases much easier and faster than a suspension of the starting hydroxides. Further, the upper layer of the settled water is drained, and the lower layer of the concentrated suspension is bubbled with air for 1 h at a temperature of 60 ± 1 ^o C, or subjected to oxidative drying at a temperature not exceeding 120 ^o C. As a result of this treatment, a mixture of heavy metal ferrites of the general formula Me''Fe ₂ O ₄ , where Me '' is a divalent heavy metal [2]
The method [2] has the following disadvantages.

 The implementation of the wastewater treatment process in two stages unreasonably increases its duration, and consequently, the consumption of thermal energy.

 Full precipitation of ions of all heavy metals contained in the wastewater of galvanic plants in the form of mixtures of their hydroxides by alkalization of wastewater to a pH of 10.4 to 11.0 is not possible due to large differences in the acid-base properties of hydroxides of different heavy metals.

Closest to the proposed chemical essence is a method of treating wastewater from heavy metal ions, which consists in the fact that after the introduction of ferrous iron, the wastewater is heated to 60-80 ^o C with simultaneous continuous sparging with an oxygen-containing gas (air) and mixed with heated to 60 -80 ^o C alkaline reagent containing NH ions $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ and hco $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ after which the mixture is maintained at the same temperature and pH 7-9 for 6-12 minutes with continuous air sparging. The introduction of ferrous iron and mixing with an alkaline reagent is carried out in the following ratios, mg / l:
heavy metal ion: ferrous iron 1: (2.00-2.75);
ion Fe ²⁺ ion NH $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ HCO ion $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ 1: (0.4-0.9) :( 0.7-1.5). The residual content of iron ions in purified water ranges from 0.031-0.053 mg / l, and ions of heavy non-ferrous metals (in particular, copper and zinc ions) within 0.017-0.022 mg / l [3]
The disadvantages of the method [3] adopted as a prototype are:
high consumption (low efficiency) of the process catalyst (ammonium bicarbonate), which is due to the instability of ammonium ions in alkaline media and the rapid volatilization of ammonia formed during their destruction from the hot reaction mass;
the absence of an objective criterion for determining the end of the ferritization of mixtures of heavy metal hydroxides in suspensions.

 In the method [3] as well as in the analogue methods [1 and 2], the minimum residual content of the removed ions in the liquid phase of the reaction mixture is taken as the criterion for ending the wastewater treatment with ferrites. However, this criterion is far from sufficient for assessing the quality (environmental safety) of the precipitate obtained, since, as follows from the analogue method [2], precipitates of the ferritization process that is not complete until the end of the process, along with heavy metal ferrites, can contain a significant amount of heavy metal hydroxide aging products easily soluble in dilute acids and acidic natural waters. Such mixed precipitation is an undeniable environmental hazard, and for their burial it is necessary to build special landfills (see above).

 The objective of the invention is to expand the range of catalysts for the process of ferritization of mixtures of iron hydroxides and non-ferrous heavy metals in concentrated suspensions and to ensure the completeness of the process while maintaining a high degree of removal of heavy metal ions from concentrated technological solutions and wastewater.

The goal is achieved by the proposed method for removing heavy metal ions from concentrated solutions and wastewater, including treating them with an excess of ferrous salt, an alkaline reagent and a catalyst, subsequent heating of the resulting suspension with simultaneous continuous introduction of an oxidizing gas, further cooling the suspension to room temperature and dividing it into liquid and solid phases, in which the differences are that a given volume of the initial technological solution or wastewater with known content aniem heavy metal ions by bubbling compressed air is treated with an excess of a salt of bivalent iron, basified to pH 9-10 and the resulting suspension is administered catalyst ferritization process, then the suspension was heated to 60-90 ^o C and kept at this temperature until complete closure ferritization process, after whereby the precipitate of the mixture of heavy metal ferrites is separated from the water.

 The difference of the proposed method is that either persulfates, or permanganates, or alkali metal perchlorates, which are taken in an amount of 0.001-0.002% by weight of the ferrous sulfate taken, are used as catalysts for the ferritization process.

 In addition, the difference of the proposed method is that the complete end of the ferrification process is determined by the minimum solubility of the solid phase ferritizable in excess of 0.0001 N. sulfuric or hydrochloric acid at ordinary room temperature.

 The use of these catalysts for the process of ferritization of intermediate mixtures of heavy metal hydroxides in concentrated suspensions expands the range of technically available chemicals to accelerate this process.

 Determination of the completeness of the course of the ferritization process by the minimum solubility of the solid phase of the ferritized suspension in 0.0001 N. sulfuric or hydrochloric acid (pH 4) provides the chemical resistance of the resulting precipitation (a mixture of heavy metal ferrites) to dissolve in acid rain and melt snow waters, as well as the possibility of environmentally safe long-term storage of such precipitation in the earth's interior (most expediently in developed quarries for the extraction of polymetallic ore), without resorting to the construction of bulky and expensive special training grounds.

где источником O₂ является воздух, а Ктл катализатор;
в) термокаталитическая дигидратация смесей гидроксидов двухвалентных цветных тяжелых металлов с гидроксидом трехвалентного железа, приводящая к образованию смеси ферритов тех же металлов по уравнению

где Me" двухвалентные цветные тяжелые металлы (никель, медь, цинк и др. );
г) термокаталитическая окислительная дегидратация избыточного гидроксида двухвалентного железа, приводящая к образованию феррита железа (магнетита) по уравнению

Процессы окислительного гидролиза ионов Fe²⁺ по уравнению (3) и окислительной дегидратации гидроксида двухвалентного железа по уравнению (5) протекают и в отсутствие катализаторов, но для их завершения при 50-60^oC требуется более 1 ч [1,2]
Пример 1. Модельные технологические растворы (сточные воды) с высокой концентрацией ионов тяжелых металлов, необходимые для проведения опытов, готовят растворением химически чистых сульфатов никеля, меди и цинка в дистиллированной воде. Концентрации ионов этих металлов в приготовленном растворе контролировались унифицированными методами анализа (Унифицированные методы анализа вод. /Под ред. проф. Ю.Ю.Дурье. М. Химия, 1971, 375 с.) и составляли, мг/л:
Ni²⁺.1761; Cu²⁺.1906; Zn²⁺.1961,1.In the process of processing concentrated technological solutions and wastewater by the proposed method, the following basic chemical reactions proceed in them:
a) hydrolysis of ions of divalent and trivalent heavy metals (including Fe ^{2+ ions} introduced with ferrous sulfate) to a mixture of the corresponding hydroxides according to equation (1);
b) thermocatalytic oxidative hydrolysis of Fe ^{2+ ions} with the formation of ferric hydroxide according to the equation

where the source of O ₂ is air, and Ktl catalyst;
c) thermocatalytic dihydration of mixtures of ferrous hydroxides of non-ferrous non-ferrous heavy metals with ferric hydroxide, leading to the formation of a mixture of ferrites of the same metals according to the equation

where Me is divalent non-ferrous heavy metals (nickel, copper, zinc, etc.);
d) thermocatalytic oxidative dehydration of excess ferrous hydroxide, leading to the formation of ferrite iron (magnetite) according to the equation

The processes of oxidative hydrolysis of Fe ^{2+ ions} according to equation (3) and oxidative dehydration of ferrous hydroxide according to equation (5) also occur in the absence of catalysts, but it takes more than 1 hour to complete them at 50-60 ^o C [1,2]
Example 1. Model technological solutions (waste water) with a high concentration of heavy metal ions, necessary for the experiments, are prepared by dissolving chemically pure nickel, copper and zinc sulfates in distilled water. The concentration of ions of these metals in the prepared solution was controlled by unified analysis methods (Unified methods of water analysis. / Ed. By prof. Yu.Yu. Durye. M. Chemistry, 1971, 375 p.) And amounted, mg / l:
Ni ²⁺ .1761; Cu ²⁺ .1906; Zn ²⁺ .1961.1.

 The following specific salts are used as catalysts for the ferritization of non-ferrous heavy metal and iron hydroxide mixtures in suspensions: sodium persulfate (TU 6-09-2869-78), potassium permanganate (GOST 20490-75) or sodium perchlorate (MRTU 6-09-3250 -76). In the experiments of the control series, conducted by the prototype method [3], ammonium bicarbonate (GOST 3762-75) was used as a catalyst.

 Serial experiments on the removal of heavy metal ions from a model solution of the above composition (3-5 experiments in each series) are carried out according to the following general procedure.

In a flask equipped with a bubbler for supplying compressed air and a gas outlet tube, 3 l of model technological solution are placed and with continuous bubbling with compressed air (air flow 1.5-2.0 l / min), an excess of ferrous sulfate (FeSO ₄ • 7H _{2 is} added O, GOST 4148-78), after which it is alkalinized to pH 9 and a predetermined amount of a ferritization catalyst is introduced.

The resulting suspension of a mixture of hydroxides of heavy non-ferrous metals and iron is heated to 80 ± 1 ^o C and maintained at this temperature and continuous sparging with air, periodically taking samples of the reaction mass for analysis to ensure the completeness of the ferritization process. For this purpose, samples of reaction suspensions cooled to room temperature are filtered through a blue ribbon paper filter (MPTU 6-09-2411-75) and in the filtrate (purified water)), the residual content of heavy metal ions is determined by the same methods as in the original technological solution. The filter cake is transferred to a 1000-fold amount of 0.0001 N. sulfuric and hydrochloric acid (pH 4), stirred for 10 min, again filtered through a blue ribbon paper filter and the content of heavy metal ions was determined in the secondary filtrate (acid extract from the precipitate). At the end of the ferritization process, the minimum content of heavy metal ions in the acid extract from the precipitate is taken.

The averaged results of serial experiments conducted under identical conditions are presented in table. 1. The same table for comparison presents the results of a series of experiments conducted by the prototype method [3]
As can be seen from the table. 1, at constant temperature (80 ± 1 ^o C) and the pH of the medium (9 units), the catalytic efficiency of permanganates, perchlorates and persulfates of alkali metals is almost the same, and the optimal amount of each of these catalysts is in the range of 0.001 0.002% of the mass of sulfate taken ferrous iron. Under these conditions, the minimum content of heavy metal ions in the acid extract from the precipitate (0.001 0.003 mg / l) is achieved in 27-30 minutes, and the residual content of heavy metal ions in purified water is also minimal and is within the same limits as in the acid extract.

In a series of experiments 8, carried out according to the prototype method [3] at a temperature of 80 ± 1 ^o C, pH 9 units and a ferritization time of 12 min, significant differences were noted in the content of heavy metal ions in purified water (0.017 0.022 mg / l) and in acid extract from the resulting precipitate (0.2 0.3 mg / l). This indicates the need for an objective criterion to determine the completeness of the process of ferritization. Such a criterion according to the proposed method is the minimum solubility of the solid phase of ferritizable suspensions in excess of 0.0001 N. sulfuric or hydrochloric acid.

Example 2. Serial experiments on the removal of heavy metal ions from a model technological solution are carried out according to the same procedure as in example 1, with a constant amount of catalyst (0.0015% by weight of FeSO ₄ • 7H ₂ O), but at variable temperatures (in between 50 - 100 ^o C) and the pH of the medium (within 8 to 11 units). The averaged results of serial experiments (3-5 experiments in each series) carried out under identical conditions are presented in table. 2.

As can be seen from Table 2, the optimal pH of the medium for ferritizing mixtures of non-ferrous heavy metals and iron hydroxides in concentrated suspensions is within 9 10 units of the universal acidity scale. An increase in the temperature of this process within 50 to 100 ^o C and other conditions being equal leads to a continuous decrease in its duration. This effect of temperature is more noticeable in the range of 50 to 60 ^o than in the range of 90 to 100 ^o C. The optimum temperature of the process of catalytic ferritization of mixtures of heavy metal hydroxides in concentrated suspensions should be considered 60 90 ^o C.

The above examples and tables illustrating them indicate the following advantages of the proposed method for removing heavy metal ions from concentrated technological solutions and wastewater over the prototype method [3]
the range of technically available chemicals is expanding, which are effective catalysts for the ferritization of mixtures of heavy metal hydroxides in concentrated suspensions (salts of higher oxygen acids of metals and nonmetals of groups VI or VII of the periodic system of elements);
it is possible to objectively determine the end of the ferritization process (according to the minimum solubility of the solid phase of the ferritizable suspension in excess of 0.0001 N sulfuric or hydrochloric acid, taken as the standard for the acidity of natural waters), which is essential for choosing sites for environmentally safe long-term storage of precipitates from here on until rational ways of their practical use.

Claims (3)

1. A method of removing heavy metal ions from concentrated technological solutions and wastewater, including treating with an excess of ferrous salt, alkalizing with a reagent in the presence of a catalyst for the ferritization process, heating the resulting suspension while bubbling oxygen-containing gas or air, cooling the suspension to room temperature and separating the liquid and solid phase, characterized in that the treatment is carried out in excess of ferrous sulfate, alkalization is carried out to pH 9 10, persulfates, or permanganates, or alkali metal perchlorates are used as a catalyst, and the resulting suspension is heated to 60 ^° -90 ^{° C.} with exposure at this temperature until the fermentation process is complete.  2. The method according to p. 1, characterized in that the catalyst is used in an amount of 0.001 to 0.002% of the amount of ferrous sulfate.  3. The method according to p. 1, characterized in that for the complete end of the ferritization process take the minimum solubility of the separated solid phase in excess of 0.0001 N. sulfuric or hydrochloric acid at room temperature.

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