RU2502568C2 - Complex processing of coal combustion flue ash - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to processing of wastes, particularly, to ash-and-slag wastes of thermal electric power stations. Coal combustion ash is placed in reaction zone to add carbon sorbent thereto in amount of 10-25 kg per ton of ash. Then, it is processed by the mix of ammonium fluoride and sulfuric acid, heated to 120-125°C and held thereat for 30-40 minutes. Tetrafluorosilane resulted from said processing is absorbed by ammonium fluoride. Solution of ammonium hydroxide is introduced into that of ammonium tetrafluorosilicate to precipitation of silicon dioxide. Then, concentrated sulfuric acid in double surplus is added to aluminium residue, held at 250°C for 1.5 h and processed by water. Solid residue is calcined at 800°C.

EFFECT: production of highly-dispersed silicon dioxide, aluminium sulphate, concentrate of rare and rare-earth elements.

2 cl, 6 tbl, 2 ex

Description

The invention relates to the field of processing silicon-containing raw materials, including ash and slag waste of thermal power plants with the aim of obtaining a number of products: highly dispersed silicon dioxide, aluminum sulfate, concentrate of rare and rare earth elements.

It is known that at present, for the preparation of highly dispersed silicon dioxide, treatment of river sand with calcium fluoride with 75-80% sulfuric acid is used [Rakov E.G., Teslenko V.V. Pyrohydrolysis of inorganic fluorides. M. - Energoatomizdat, 1987 - 151 p.]. Processing of sand with calcium fluoride and sulfuric acid is carried out in drum furnaces at a temperature of 200-250 ° C. The disadvantages of this method is the need to use sulfuric acid of high concentration and clean river sand.

A known method of processing man-made and natural silicon-containing raw materials to obtain silicon dioxide by treatment with hydrofluoric acid solutions [Leaching of hydrous alumina silicate with hydrofluoric acid to produce silicon tetrafluoride, aluminum fluoride, as well as fluorides and oxides of related metals - US patent No. 5242670, IPC С01В 033/08, No. 907854 dated 02.07.92., Publ. September 7, 93.]. The essence of the method lies in the fact that the aluminosilicate raw materials are treated with a concentrated solution of HF. The gaseous tetrafluorosilane (SiF ₄ ) released is passed through a series of cooling traps in which most of the impurities remain. The purified gas is contacted with an aqueous solution of sodium fluoride to form a suspension of sodium fluorosilicate (Na ₂ SiF ₆ ), crystals of sodium fluorosilicate are isolated from the suspension on a filter press, after which they are dried and then calcined at 600-650 ° C. The tetrafluorsilane gas liberated during calcination is condensed to obtain pure liquid tetrafluorsilane. At the same time, powdered sodium fluoride (NaF) is obtained in the residue from calcination. The suspension remaining after the decomposition of the feedstock and distillation of tetrafluorosilane and containing aluminum fluoride and a small amount of salts of the accompanying metals is diluted with water and the soluble salts are brought into the liquid phase. The resulting solution was separated from the insoluble residue. After evaporation, the resulting residue is three-water aluminum fluoride with an admixture of salts of the accompanying metals contained in the feedstock. After evaporation of the solution, the resulting gaseous hydrogen fluoride (HF) is condensed and returned to the cycle. The equations of the proceeding reactions:

SiO ₂ + 4HF → SiF ₄ ↑ + 2H ₂ O

2NaF + SiF ₄ → Na ₂ SiF ₆ ↓

Na ₂ SiF ₆ → 2NaF + SiF ₄ ↑

The disadvantage of this method is the use of 50% hydrofluoric acid, which is a substance of the first hazard class, which makes the proposed technology environmentally hazardous.

A known method of processing aluminosilicates into aluminum fluoride in which the ash of Ekibastuz coal is processed [Copyright certificate SU No. 1668301, MKI C01F 7/50, application No. 4671911 from 04/03/89, publ. 08/07/91 - A method for processing aluminosilicates into aluminum fluoride. / L.D. Shapiro, V.I. Shapoval, M.M.Maldabekov, S.O. Akhmetova, V.A. Zhabenko]. The ash from the combustion of high-ash coals is calcined at 550-750 ° C in a closed reactor, and then subjected to magnetic separation. The non-magnetic fraction is treated with ammonium fluoride in an amount of 100-120% of the stoichiometrically necessary for the formation of aluminum fluoride and ammonium silicofluoride at 300-600 ° C. The resulting gases tetrafluorosilane, ammonia (NH ₃ ), water and hydrogen fluoride are distilled off and absorbed in water absorbers to produce silicon dioxide and ammonium fluoride (NH ₄ F). Silica was filtered off and the remaining solution was evaporated. Ammonium fluoride released is recycled. The resulting cake containing up to 90% aluminum fluoride can be used in the aluminum industry. Further use of silica resulting from hydrolysis is not considered [Copyright certificate No. 1668301, USSR C01F 7/50].

The equations of the proceeding reactions:

SiO ₂ + 4NH ₄ F → 4NH ₃ ↑ + SiF ₄ ↑ + 2H ₂ O

SiO ₂ + 6NH ₄ F → (NH ₄ ) ₂ SiF ₆ + 4NH ₃ ↑ + 2H ₂ O

NH ₄ F → NH ₃ ↑ + HF ↑

The disadvantage of this method is the use of high temperature, as well as the production of tetrafluorosilane contaminated with ammonia, which necessitates the stage of separation and utilization of ammonia by absorption by water. Also, further use of the solid residue after fluorination of the ash is not considered.

Closest to the claimed is a method of hydrochemical production of highly dispersed silicon dioxide from industrial raw materials [Patent RU No. 2261841, application No. 2004109475 from 03/29/04, publ. 10.10.05 - Borbat V.F. and etc.].

In this method, calcium fluoride and sulfuric acid or man-made waste containing fluorides and sulfuric acid are used as a fluorinating agent. In the method, fluorination of the ash is carried out with hydrogen fluoride, which is directly released in the reaction zone. In this case, the reactions proceed:

$${\text{Caf}}_{\text{2}}+{\text{H}}_{\text{2}}{\text{SO}}_{\text{four}}\to {\text{CaSO}}_{\text{four}}+\text{2HF}\uparrow \left(\text{one}\right)$$

$${\text{SiO}}_{\text{2}}+\text{4hf}\to {\text{SiF}}_{\text{four}}\uparrow +{\text{2H}}_{\text{2}}\text{O}\left(\text{2}\right)$$

$${\text{SiF}}_{\text{four}}+{\text{NH}}_{\text{four}}\text{F}\to {\left({\text{NH}}_{\text{four}}\right)}_{\text{2}}{\text{SiF}}_{\text{6}}\left(\text{3}\right)$$

$${\left({\text{NH}}_{\text{four}}\right)}_{\text{2}}{\text{SiF}}_{\text{6}}+{\text{2NH}}_{\text{four}}\text{OH}\to {\text{SiO}}_{\text{2}}\downarrow +{\text{4NH}}_{\text{four}}\text{F}+\text{2HF}\left(\text{four}\right)$$

The reaction of sulfuric acid with inorganic fluoride in a Teflon reactor produces hydrogen fluoride (1), which interacts with ash components to form various fluorides and gaseous tetrafluorosilane (2). Tetrafluorosilane is absorbed with a 15% solution of ammonium fluoride (3) in 3 polypropylene absorbers connected in series. Then the resulting solution is poured from the absorbers and neutralized with a 20% ammonia solution (4). In this case, highly dispersed silicon dioxide is released, which is filtered off. In the inventive method, silane tetrafluoride is not contaminated with ammonia, which is an advantage of the method. The disadvantage of this method can be considered that it considers the possibility of obtaining only finely dispersed silica from ash, but the large amounts of aluminum in the ash and expensive rare and rare-earth metals remain in solid products after fluorination and are not utilized. The use of calcium fluoride or industrial waste will lead to difficulties in the further processing of solid ash fluorination products due to contamination with gypsum and other products, in the case of industrial waste.

The objective of the present invention is to develop a method for the integrated processing of ash from burning coal using a mixture of ammonium fluoride and sulfuric acid as a fluorinating agent, while a carbon sorbent is additionally introduced into the reaction zone. To achieve this result, it is proposed to use a mixture of ammonium fluoride, sulfuric acid, silicon-containing waste from industry and energy - ashes of thermal power plants since the ash contains up to 60% silicon dioxide, which can be seen from the chemical composition of the ash according to the main (% wt.) and microcomponent (gram / ton) components presented in tables 1 and 2, and the process should be carried out at a temperature of 120 ± 5 ° С.

Table 1 SiO ₂ Al ₂ O ₃ Fe ₂ O ₃ Cao MgO TiO ₂ K ₂ O Na ₂ O P ₂ O ₅ MnO ₂ SO ₃ Ppp 61.5 27.4 5.65 1.17 0.49 1.49 0.42 0.32 0.52 0.17 0.57 5.1

table 2 Sr Ba Y La Xie Yb Tb Dy Th U Zr Cu V Ga Sc 420 200 42 fifteen 67 6 9 10 7 2 330 57 140 43 90

To obtain a concentrate containing rare and rare-earth elements - Sc, Y, Ce, La, Ga, during fluorination of ash, carbon sorption material is additionally introduced into the fluorination reactor in an amount sufficient to sorb rare and rare-earth elements. Rare and rare-earth elements will partially remain in the form of fluorides, and will partially concentrate on a carbon sorbent, as a result of which they can be isolated in total in the form of a concentrate.

According to the proposed method, a sample of ash from burning coal, such as Ekibastuz, is placed in a Teflon reactor, carbon sorbent is added in an amount of 10-25 kg per ton of ash, which is 1-2.5% by weight of ash. The consumption of carbon sorbent less than 1% leads to a decrease in the content of REE in the solid residue to 0.6%, and an increase in the consumption of carbon sorbent more than 2.5% does not increase the content of REE in the resulting solid residue. Then this sample is mixed with ammonium fluoride at a mass ratio of silicon dioxide in ash to the fluorine content in ammonium fluoride 1: 1, 50-55% sulfuric acid is poured, heated to 120-125 ° C and kept for 30-40 minutes, while distilling off the resulting tetrafluorosilane into absorbers filled with a solution of ammonium fluoride using a vacuum pump.

The reaction of sulfuric acid with ammonium fluoride in a Teflon reactor produces hydrogen fluoride, which interacts with ash components to form various fluorides and gaseous tetrafluorosilane. Tetrafluorosilane is absorbed with a 15% solution of ammonium fluoride in 3 series-connected polypropylene absorbers. Then the resulting solution is poured from the absorbers and neutralized with a 20% ammonia solution. In this case, highly dispersed silicon dioxide is released, which is filtered off.

After filtration of the silica, the solutions were evaporated to give ammonium fluoride, which was returned to the absorption of tetrafluorosilane. Table 3 shows the dependence of the yield of silicon dioxide on the excess of the estimated amount of fluorine in ammonium fluoride and sulfuric acid at a temperature of 120 ° C.

Table 3 Excess H ₂ SO ₄ Excess fluoride The output of silicon dioxide,% 1: 1 1: 1 21.0 1: 2 1: 1 54.0 1: 3 1: 1 90.0

The table shows that at a temperature of 120 ° C, a 3-fold excess of 50% sulfuric acid and a fluorine consumption of 1: 1 ammonium fluoride, up to 90% silicon dioxide is recovered. The yield calculation is based on the initial content of silicon dioxide in the ash. The excess of sulfuric acid or ammonium fluoride is presented as the ratio of the amount of the substance of ammonium fluoride or sulfuric acid necessary for the complete interaction of the macro components of the ash (aluminum oxide, silicon oxide) contained in this sample, calculated theoretically by reactions 5, 6, 7 to the amount of the substance of ammonium fluoride or sulfuric acid added in the experiment.

5.2NH ₄ F + H ₂ SO ₄ = (NH ₄ ) ₂ SO ₄ + 2HF

6. SiO ₂ + 4HF = SiF ₄ + 2H ₂ O

7. Al ₂ O ₃ + 6HF = 2AlF ₃ + 3H ₂ O

A larger excess of sulfuric acid does not lead to a significant increase in the extraction of silicon from ash. So, with a 4-fold excess, the degree of extraction of silicon increases to 92%, which makes such an excess of sulfuric acid inappropriate.

Tetrafluorosilane is distilled off and processed into silicon dioxide as in the method of [Patent RU No. 2261841, application No. 2004109475 of 03.29.04, publ. 10.10.05 - Borbat V.F. and etc.]. And the residue is processed in order to isolate aluminum sulfate and a concentrate of rare and rare earth elements. To this end, aluminum fluoride is converted from the obtained fluorination solid into a soluble form by adding a 2-fold excess of concentrated sulfuric acid and heating to a temperature of 250 ° C for 1.5 hours. Data on the extraction of aluminum sulfate into the solution from the consumption of acid at a temperature of 120 ° C and a processing time of 30 minutes are shown in table 4.

Table 4 The ratio of T: W (solid residue: sulfuric acid) The degree of extraction of aluminum sulfate,% 1: 1 32.6 ± 1.8 1: 2 51.5 ± 2.6 1: 3 54.5 ± 2.8

From table 4 it follows that you should take a 2-fold excess of sulfuric acid. A higher acid consumption does not lead to a significant increase in the degree of extraction of aluminum sulfate from the solid fluorination residue. Data on the dependence of the degree of extraction of aluminum sulfate in solution on temperature and processing time with a twofold excess of concentrated sulfuric acid are shown in table 5.

Table 5 Time of processing 120 ° C 250 ° C α (Al ₂ (SO ₄ ) ₃ ),% α (Al ₂ (SO ₄ ) ₃ ),% 0 0 0 10 20.1 ± 1.0 28.0 ± 1.4 twenty 36.9 ± 1.8 51.6 ± 2.6 thirty 51.5 ± 2.6 64.8 ± 3.2 fifty 60.0 ± 3.0 78.3 ± 3.9 70 66.4 ± 3.3 88.1 ± 4.4 90 70.2 ± 3.5 92.0 ± 4.6

A further increase in temperature is not advisable due to the high corrosion activity of the reaction mixture. When processing solid fluorination products with concentrated sulfuric acid under the above conditions, aluminum fluoride is converted to sulfate. After cooling, the reaction mixture is treated with water, while aluminum sulfate is dissolved, the remaining precipitate is separated by filtration and calcined. The solid product after calcination is a concentrate of rare and rare earth elements.

To eliminate the contamination of the aluminum product with iron, it is recommended that magnetic ash separation be preliminarily carried out in order to remove a valuable product - magnetospheres from the ash [Anshits A.G. Isolation of magnetic microspheres of constant composition from energy evils and the study of their physicochemical properties // Chemistry for Sustainable Development, 1999. - Issue 7. - p.105-118].

The results obtained can be illustrated by the following examples.

Example 1: 59.7 g of ash from the burning of Ekibastuz coals (taken from 1-4 fields of electrostatic precipitators of CHPP-4 in Omsk) are mixed with 129.3 g of ammonium fluoride (parts). We place it in a Teflon reactor (1), add 1.5 g of carbon sorbent BAU, add 518 ml of 50-55% sulfuric acid, heat it to 120 ° C and maintain it at this temperature for 30 minutes, while driving off the gaseous tetrafluorsilane that is released. We absorb it with a 15% solution of ammonium fluoride in polypropylene absorbers (2). The resulting solution of ammonium hexafluorosilicate (NH ₄ ) ₂ SiF _{6 is} poured into a polypropylene beaker and a 20% ammonia solution is added thereto until an odor of ammonia appears.

The precipitated silicon dioxide is filtered and dried at a temperature of 110 ° C. The tetrafluorsilane recovery is 92%. The residue after distillation of tetrafluorosilane contains up to 75% aluminum trifluoride, up to 2% silicon dioxide. The rest, sulfuric acid, underburn contained in the original ash, iron fluorides and REE. The specific surface area of the obtained silica determined by the BET method is 400 ± 15 m ² / g.

120 ml of concentrated sulfuric acid are added to the residue, heated to 250 for 1.5 hours, cooled, treated with water (800 ml), the precipitate is filtered off. 95% of the aluminum contained in the starting ash is recovered in the solution.

The filtered solid residue is calcined at 800 ° C. The mass of the solid residue is 2% of the mass of the original ash, the composition of the residue is analyzed spectrophotometrically by the method of semiquantitative atomic emission analysis. Data on the chemical composition of the residue after calcination in% are shown in table 6.

Table 6 Si Fe Al REE Wa Mn Zr Hf Co Ni V Sr 10-100 ~ 1 ~ 1 ~ 1 ~ 0.2 ~ 0.2 ~ 0.2 ~ 0.06 ~ 0.02 ~ 0.02 ~ 0.02 ~ 0.02

This method allows you to expand the raw material base to obtain highly dispersed silicon dioxide through the use of ash TPP

Example 2. Ash from coal combustion is treated under the same conditions as in example 1, but an additional carbon nanostructured sorbent obtained by IPPU SB RAS (Omsk) is additionally introduced. The extraction of silicon and aluminum remains the same as in example 1. The REE content in the solid residue is 1.3%.

Claims (2)

1. The method of complex processing of ash from burning coal, in which ash from burning coal is placed in the reaction zone, add a carbon sorbent that absorbs rare and rare earth elements, in the amount of 10-25 kg per ton of ash, then treated with a mixture of ammonium fluoride and sulfuric acid , heated to 120-125 ° C, incubated for 30-40 minutes, the resulting tetrafluorosilane is absorbed by ammonium fluoride, and a solution of ammonium hydroxide is introduced into the resulting ammonium tetrafluorosilicate solution before precipitation of silicon dioxide, after th added concentrated sulfuric acid in excess of twice to aluminum contained in the residue is heated at 250 ° C for 1.5 h and treated with water, the solid residue was filtered off and calcined at 800 ° C. 2. The method according to claim 1, characterized in that the iron is previously removed by magnetic magnetic separation.