RU2757115C1 - Method for obtaining a sorbent for cleaning waste gases - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention relates to a method for obtaining a sorbent for cleaning waste gases, characterized by the fact that the waste from the production of lime milk is used as a feedstock, which is classified with the release of a paste-like mass containing solid particles with a size of less than 5 mcm, followed by washing the paste-like mass in a counterflow with acetone at a ratio of 1:1 by weight, at a temperature of 50-70°C for 1-3 minutes, the washed mass is subjected to ultrafiltration with the release of permeate containing particles of microcrystalline calcium hydroxide with a size of less than 50 nm, with its subsequent air drying at a temperature of 110-130°C.

EFFECT: expansion of the range of waste gas cleaning.

1 cl, 2 tbl, 1 ex

Description

The invention relates to the field of environmental protection, namely to the production of a powder sorbent from the waste of lime milk production, which can be used to purify toxic high-temperature gas emissions from industrial equipment, in particular waste incineration plants, installations for burning associated petroleum gas, landfill biogas, etc.

For the purposes of the present invention, the following terms mean:

"Quicklime" is a crystalline substance, the chemical composition of which is based on calcium oxide CaO, most often obtained by thermal decomposition of limestone - calcium carbonate CaCO ₃ ;

"Slaking" - the process of interaction of quicklime with water;

"Slaked lime" - calcium hydroxide Ca (OH) ₂ - a product obtained as a result of slaking;

"Milk of lime" - a suspension formed in the process of slaking quicklime with excess water;

"Hydrator" - an apparatus for slaking quicklime.

With the development of production, the protection of the air from pollution by industrial emissions is becoming an increasingly urgent task, which is part of the complex of global problems of environmental protection.

The composition of the waste gases of various industries and even of one production may differ in time, for example, in recent years, the chemical composition of municipal solid waste (MSW), which is processed at incineration plants, has changed, and, accordingly, the composition of their waste gases changes, which entails the need for continuous improvement of gas cleaning systems. The main components of the composition of the exhaust gases are oxides of sulfur, nitrogen, carbon, hydrogen sulfide, heavy metals (mercury, cadmium, lead, chromium, nickel, arsenic, manganese, cobalt, etc.), organochlorine compounds, acids - hydrogen chloride, hydrogen fluoride, etc. ...

One of the methods for increasing the degree of purification of waste gases is sorption purification, which, under conditions of changes in the chemical composition of waste gases, dictates the need to expand the range of sorbents used.

Lime milk is widely used in the chemical, construction and other industries, in particular, for the production of sugar and the preparation of mixtures for combating plant diseases. During the production of milk of lime by slaking quicklime with excess water, non-utilizable waste is formed - a sludge containing, among other things, CaO particles that have not reacted with water. The lime slaked sludge in the production of milk of lime belongs to the IV class of hazard and, as a rule, is subject to burial, which requires the alienation of land for its burial and leads to environmental pollution.

From the prior art there is a known method of obtaining sorbent S-KP for purifying atmospheric air from nitrogen oxides, carbon, sulfur, formaldehyde and butyl mercaptan by fluffing silted, granular and dried clay with pyrolusite, followed by calcining at 1150-1200 ° C [Patent RU 2336945, B01J 20 / 06, 2008].

The disadvantage of the sorbent obtained by the known method is the low sorption of acid gases, such as hydrogen sulfide, sulfur dioxide and carbon dioxide.

A known method of producing a sorbent for air purification from nitrogen oxides, sulfur and hydrogen sulfide, obtained by mixing finely ground 100 g of Portland cement-500, 100 g of the flask of the Astrakhan region with 100 cm ^{3 of a} 10% aqueous solution of sodium chloride and the formation of granules with a size of 0.5 cm up to 5 cm in diameter. After hardening, the formed mass is placed in running water and kept until a negative reaction to chloride ions. After drying, the granules are placed in a 40% aqueous solution of diethanolamine for 1 hour. The sorbent has the following chemical composition, wt%: CaO - 40.0, SiO ₂ - 35.0, Al ₂ O ₃ - 15.0, NH (CH ₂ ) ₄ OH ₂ - 5.0, H ₂ O - 5, 0. The invention provides for the production of a sorbent for effective air purification of residential premises, working areas of industrial enterprises and drilling areas [Patent RU 2452561, B01J 20/16, B01J 20/30, 2012].

The disadvantage of the invention is the limited application temperature of the obtained sorbent - no more than 40 ° C, which is an obstacle for use in gas cleaning systems of industrial enterprises.

For the purification of exhaust gases, sorbents obtained on the basis of activated carbon are most often used, which combine the capabilities of chemisorption and osmotic adsorption interactions with sorbates such as chlorine, fluorine, sulfur, heavy metal ions, etc.

A known method of producing a sorbent based on activated carbon for cleaning gases from hydrogen sulfide. The original activated carbon is impregnated with an aqueous solution of potassium iodide with a concentration of 0.5-3.5 wt.%, Into which 0.035-0.075 wt.% Of monoethanolamine (MEA) is additionally introduced. The amount of water for preparing the impregnating solution is taken based on the mass of active carbon. The sorbent provides high productivity of the air purification process and avoids frequent replacement of the catalyst [Patent RU 2254916, B01J 20/30; B01D 53/02, 2005].

The disadvantage of the sorbent obtained by a known method is the limitation of its functionality due to the impossibility of purifying hydrogen sulfide in combination with other acid gases and heavy metals.

There is a known method for the production of highly efficient adsorbents based on activated carbon in the form of individual grains of activated carbon, which includes the treatment of the initial carbon-containing material by initial carbonization and subsequent activation, and the starting material includes sulfonated polystyrenes crosslinked with divinylbenzene in granular form, and where the activation is carried out in two stages, in during which the carbonated starting material in the first stage is subjected to activation in an atmosphere containing water vapor, and then in the second stage is subjected to activation in an atmosphere containing CO ₂ [Patent RU 2426591, B01J 20/20, 2011].

The disadvantage of this method is the production of a carbon adsorbent with insufficiently high adsorption capacity, lower than that of activated carbon, due to the presence of chemically bonded hydrogen and sulfur in the adsorbent structure, as well as its low mechanical strength.

There is a known method of producing a block nanoporous carbon material for the accumulation of natural gas methane, which consists in the fact that the carbon nanoporous material obtained from carbonized and activated solid raw materials of organic origin is crushed to an average fraction of granules from 700 to 1000 microns, a polymer binder is added to the crushed material in an amount from 3 to 12% wt. and distilled water in an amount from 5 to 80% wt., mix, produce molding at a pressure of 150 kgf / cm² up to 3000 kgf / cm², after which the molded blocks are dried at a temperature of 110 ° C to 150 ° C for 3 ÷ 48 hours. Latex or polyvinyl acetate is used as a polymer binder, and molding is performed using a press or an extruder [Patent RU 2625671, B01J 20/20, 2016].

The disadvantage of this method is that the resulting material is an insufficiently effective sorbent for cleaning exhaust gases, since it has an insufficient specific volume of micropores per sample volume, which does not exceed 0.45 cm of micropores / cm of sample volume, as well as the fact that the molding method, including one-stage pressing and drying after the pressure is released, does not allow obtaining samples of sufficient density, which characterizes low mechanical strength.

There is a known method of obtaining a molded nanostructured microporous carbon sorbent, including mixing a powdered carbon sorbent with an aqueous solution of a polymer binder and pressing, characterized in that polyvinyl alcohol is used as a polymer binder, and pressing is carried out with increasing load and temperature in four steps in the ranges from 25 kgf / cm ² to 1600 kgf / cm ² , from 75 to 190 ° C and from 10-130 min, respectively [Patent RU 2736586, B01J 20/20; B01J 20/30, 2019].

The disadvantage of powder sorbents obtained by various methods based on activated carbons is a rather low mechanical strength, which makes it difficult to use and regenerate them in a number of technologies.

The disadvantages of the above known technical solutions are the complex composition of the obtained sorbents with a low efficiency of their use for cleaning off gases from a wide range of toxic impurities in gas cleaning systems of industrial enterprises at real temperatures in gas cleaning systems from 150 ° C to 280 ° C.

In addition to sorbents with activated carbon, sorbents based on hydrated lime are widely used, which, with comparable porosity, have a much higher density than activated carbon, i.e. greater mechanical strength.

A known method of obtaining a powdery lime reagent composition, including mixing CaO with water in an amount sufficient to obtain milk of lime with a solid content of 10-50 wt.%, Separating water from milk of lime and drying it, while before, during or after mixing an alkali metal compound is added to water and / or milk of lime [Patent RU 2425000, C04B 2/06, 2011].

The disadvantage of this method is the dependence of the reactivity of the alkaline composition containing milk of lime on the reactivity of the additional alkali metal compound. Alkali metals and their compounds, except for Ca (lithium Li, sodium Na, potassium K, rubidium Rb, cesium Cs, francium Fr) have a high fire and explosive hazard and significant cost, and therefore, to obtain them, special additional protected production facilities are required and significant costs.

A known method of producing slaked lime having a large surface area and small particle size by hydrating lime with an aqueous hydrate solution of an organic solvent and preferably washing the resulting hydrate with an aqueous solution of an organic solvent before drying in order to further increase the surface area and reduce the particle size. The high surface area hydrates of the invention are excellent sorbents for removing SO ₂ from gas streams [US Pat. No. 5,492,685, C01B 17/56, 1996].

The disadvantage of this method is its high cost due to the fact that quicklime is quenched mainly in the presence of a large amount of alcohol, while the complete removal of alcohol in the future is difficult to carry out, which requires an expensive installation for maximum recycling of the alcohol used.

The problem, the solution of which is provided by the implementation of the invention, is to reduce the risks of harm to the environment by obtaining an inexpensive sorbent that allows you to increase the degree of purification of exhaust gases, while simultaneously disposing of the milk lime production waste.

The technical result from the use of the proposed invention is to obtain a sorbent with a developed system of micropores, high sorption capacity, increased mechanical strength and the ability to multiple regeneration.

The problem is solved, and the technical result is achieved due to the fact that in the method of obtaining a sorbent for purifying exhaust gases, the waste of lime milk is used as a raw material, which is classified with the release of a pasty mass containing solid particles with a size of less than 5 microns, followed by washing in countercurrent with an organic solvent at a temperature of 50-70 ° C for 1-3 minutes, the washed mass is subjected to ultrafiltration with the separation of the permeate containing particles of microcrystalline calcium hydroxide with a size of less than 50 nm, followed by drying it with air at a temperature of 110-130 ° C.

It is preferable that the washing of the pasty mass with an organic solvent is carried out at a ratio of 1: 1 by weight.

It is preferable that acetone or acetic acid of 25-30% concentration or citric acid of 50-65% concentration is used as the organic solvent.

Preferably, air drying is carried out for 1.5-5 hours.

The method is carried out as follows.

Waste from the production of milk of lime is classified with the release of a pasty mass containing solid particles with a size of less than 5 microns, followed by washing in a countercurrent with an organic solvent - acetone or acetic acid 25-30% concentration, or citric acid 50-65% concentration at a ratio of 1: 1 by weight at a temperature of 50-70 ° C for 1-3 minutes, the washed mass is subjected to ultrafiltration with the release of permeate containing particles of microcrystalline calcium hydroxide (MCA) with a size of less than 50 nm, followed by drying it with air at a temperature of 110-130 ° WITH.

In industrial processes, burnt ground quicklime is used to produce milk of lime. The actual percentage of CaO and other inclusions may vary depending on the grade of the starting material. Quicklime may contain impurities of magnesium oxide, sulfur oxide, aluminum oxide, silicon dioxide, etc. The process of slaking lime is most rapid when using pure lime, calcined at a moderate temperature, with a high CaO content. Quicklime, crushed to particles no more than 5-10 mm in size, is fed into the hydrator. Slaking consists in treating lime with water to convert calcium oxide into hydrate. Usually, when quenching occurs spontaneous disintegration of lump lime pieces into fine particles no more than 5-20 microns in size [Volzhensky A.V. and other Mineral astringents. M .: Stroyizdat, 1979, p. 99]. Calcium oxide hydrate is usually formed in the form of hexagonal plates with a layered crystal lattice / ibid, p. 102 /.

In the process of slaking the original lumpy quicklime by stirring with excess water at the outlet of the hydrator, a solution of lime milk is separated from the sediment, which contains mainly small particles of calcium compounds (including silicates, aluminates and ferrites of calcium), magnesium, sand and others that are not dissolved in the slaking process. inclusions. The sediment is a pasty mass, which is classified by particle size distribution to a particle size of less than 5 microns. The larger particles are waste rock from the lump lime deposit, are chemically inert and must be disposed of.

Then the separated mass is washed in countercurrent with an organic solvent at a temperature of 50-70 ° C for 1-3 minutes.

Temperatures less than 50 ° C are insufficient to remove residual concentrations of the organic solvent, and washing at temperatures above 70 ° C is impractical, since it does not further reduce the residual concentrations of the organic solvent.

Washing in a countercurrent flow for less than 1 minute does not allow to displace excess water before drying, and at a washing time of more than 3 minutes, calcium particles can enter into chemical interaction with an organic solvent and precipitate, which is removed into a concentrate during subsequent ultrafiltration, while the amount of MGC in the permeate decreases.

The drying temperature of MGA below 110 ° C is insufficient to obtain a dry powder, which leads to an unjustified increase in drying time and an increase in energy consumption. And conducting the drying process at a temperature above 130 ° C is also economically inexpedient, since it does not lead to a further decrease in the moisture content of PCA.

The mass obtained after the drying process, analyzed on a transmission electron microscope under a magnification of 600,000 times, is a calcium hydroxide powder, consisting of strong colorless trigonal rhombic crystals with a layered structure and a particle size of 5 nm to 50 nm in the form of porous individual individuals and interconnected aggregates , while individual individuals and spliced aggregates are placed in trigonal rhombic crystals in such a way that the boundaries between them form an additional porous structure with a pore diameter of 0.3 nm-0.5 nm. At the same time, pores in individual individuals can only be structural defects, and in aggregates, this is also a consequence of fusion defects. For comparison, the mesopores of activated carbon particles have a diameter of 2 nm - 10 nm.

To increase the sorption properties of PCA, the process of hydration of quicklime is carried out so that the sorption surface of the microcrystals would be maximal.

The average crystal area and porosity of PCA are presented in Table 1.

The area of crystals, the porosity of which reaches about 50%, ranges from 25 to 700 ± 3.1 nm ² , while, as shown by the results of studies, the total pore volume of one aggregate is on average Vn = 36 cm ³ . As follows from microscopic measurements, at a magnification of × 200,000 times, each sample weighing 1 g contains from 1 × 10 ¹⁷ to 9.38 × 10 ¹⁹ aggregates. From this it follows that for many aggregates the mass of the aggregate is from 1.06 × 10 ¹⁷ to 1.06 × 10 ¹⁹ g, therefore, the specific surface area (porosity) of the aggregates is in the range from 50 to 60 m ² / g, which, when compared, corresponds to specific surface area of activated carbon.

An example of the implementation of the method

The original crushed lump lime of Ashikinsky (Ryazan region, Kasimovsky district) limestone was subjected to hydration. Content in lump lime CaCO ₃ (88.0-99.1)% wt .; MgCO ₃ (0.2-4.3) wt%; SiO ₂ + R ₂ O ₃ (0.2-8.31) wt.%, Where R are small residual components. In the hydrator (autoclave), quicklime and water, crushed to particles with a size of 5-10 mm, were fed in a ratio by weight of 1: 2, respectively. As a result of the exothermic reaction, a large amount of steam was formed, the condensate of which can be used to obtain milk of lime of the required quality. After releasing the pressure at the outlet of the hydrator, the formed sediment was separated from the milk of lime - a pasty mass with a moisture content of 60-65% with a solid dry matter content of 2 g per 100 g of mass, of which 1.968 g was the base - calcium bicarbonate, and 0.032 g - impurities. The sediment was classified on vibrating sieves with the release of solid particles less than 5 μm and then sent for washing in trays, where acetone was fed in countercurrent in an amount of 1: 1 by weight with the treated sediment at a temperature of 60 ° C for 2 minutes. Large particles, which were waste rock, were sent for burial. After washing, acetone was sent to regeneration for reuse. Then the precipitate was fed to the Dizzer XL ultrafiltration module, after which the permeate contains PCA particles less than 50 nm. After drying the permeate for 2 hours in air at a temperature of 130 ° C, MGC is a powder that can be used as a sorbent.

The MGC was tested as a sorbent in an experimental unit installed in the gas duct of an operating waste incineration plant. In the normal mode of incineration of municipal solid waste, exhaust gases were passed through samplers filled with PCA and activated carbon (AC). The concentration of trace elements in the PCA samples was determined by X-ray fluorescence analysis (XRF).

In the investigated range of flow parameters and the duration of measurements, the mass of elements sorbed by PCA increases with an increase in the residence time of the sample in the gas duct, i.e. sorption saturation of PCA with respect to oxides and ions of heavy metals does not occur.

Unlike weighed portions of PCA with the duration of experiments 1, 4, and 5 min, after 40 min of the experiment, the following elements were sorbed on PCA: Ba, U, Th, Nb, Mo, W, Ta, Se, Ag, Sb, Sn, Te, In , Cd, Bi. Traces of Zr (0.001%), Υ (0.0003%), Sc (0.0003%), Ga (0.0003%) were also found.

Based on the elemental analysis data of the samples from the experimental setup, a calculation was made of the amount of impurities sorbed by the PCA in the process of passing the waste gas stream of the incineration plant through it.

Weight (m) of a sample of PCA before experiment = 1.968 (base) + 0.032 (impurities) = 2 g

Mass (m) of the PCA sample after 40 min of the experiment = 1.968 (base) + 0.0732 (impurities) = 2.0412 g.

The mass difference (Δm) = 0.0412 g is the amount of impurities that was captured by the PCA in 40 minutes of the experiment.

Comparing the content of elements and compounds in the samples before the start of the experiment and after 40 minutes of the experiment, the following was determined:

- the content of Na ₂ O increased by 8 times;

- the content of SiO ₂ increased 9 times;

- the MnO content increased by 1.5 times;

- SO ₃ content increased 17 times;

- the content of Cl ^- and F increased by 5 times;

- the Cu content increased by 4 times;

- Ni content increased 6 times;

- Zn content increased 16 times;

- PCA sorbed elements: Cr, Pb, V - which were absent in the sample before the start of the experiment.

Based on the experimental studies carried out, it was concluded that PCA can be used as a sorbent in gaseous media to trap oxides and heavy metals. At the end of all series of experiments, weighed portions of the MGC were placed in a muffle furnace with an air atmosphere at a temperature of 450 ° C-550 ° C, where they were regenerated for 2 hours. As shown by XRD, no traces of sorbates were found in the samples after regeneration; their complete desorption occurred. Repeated use of the same weighed portion of PCA under the same sorption conditions showed complete identity in terms of the masses of sorbates, which indicates the ability of PCA for multiple regeneration.

To determine the sorption properties of PCA, the sorption capacity was calculated in relation to Cl, F, S and the sum of metals.

To compare the sorption indices of MGC with the AC sorbent, the operational indices of the operating gas cleaning unit were taken.

Comparison of the sorption characteristics of MGC and sorbent from an industrial enterprise (AC) is given in Table 2.

The parameters of PCA exceed AC in fluorine (F) by 4.2 times, in sulfur (S) - 2.3 times, in sorption of metals by 47 times, and in chlorine (Cl) they are inferior to AC in 1.5 times.

As is known, the sorption capacity of powder adsorbents depends on the specific surface area of the powder particles. At the same time, AC due to the high specific porosity of particles (45-55 m ² / g) is an occlusive sorbent. The sorption properties of PCA with a specific porosity of particles from 50 to 60 m ² / g are mainly determined by chemisorption interactions with sorbates.

Thus, the proposed method makes it possible to obtain an inexpensive sorbent - microcrystalline calcium hydroxide, which has a developed system of micropores, high sorption capacity (physical and chemical sorption) and reagent properties, with increased mechanical strength and the ability to complete multiple regeneration. In addition, the production of the sorbent does not require the organization of special production and allows to utilize the waste from the production of milk of lime, thereby helping to reduce the risks of harm to the environment.

Claims (2)

1. A method of producing a sorbent for purifying waste gases, characterized in that the waste of lime milk production is used as a feedstock, which is classified with the release of a pasty mass containing solid particles with a size of less than 5 microns, followed by washing the pasty mass in a countercurrent with acetone at a ratio 1: 1 by weight, at a temperature of 50-70 ° C for 1-3 minutes, the washed mass is subjected to ultrafiltration with the release of permeate containing particles of microcrystalline calcium hydroxide with a size of less than 50 nm, followed by drying it with air at a temperature of 110-130 ° C ... 2. A method according to claim 1, characterized in that the air drying time is 1.5-5 hours.