Complex processing of coal combustion flue ash. RU patent 2502568.

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

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

It is known that at the present time to obtain highly dispersed silicon dioxide is used treatment of river sand calcium fluoride with 75-80% sulfuric acid [Rakov EG, Teslenko V.V. Pyrohydrolysis of inorganic fluorides. M - Energoatomizdat, 1987 - 151 C.]. The processing of sand calcium fluoride and sulfuric acid is carried out in rotary kilns at a temperature of 200-250°C. the Disadvantages of this method is the necessity of using sulfuric acid of high concentration and clean river sand.

There is a method for processing man-made and natural siliceous raw materials to produce silicon dioxide by treatment with solutions of hydrofluoric acid [Leaching with hydrofluoric acid aluminosilicate raw materials to produce cetarehhloristam silicon, aluminum fluoride, and fluorides and oxides of accompanying metals - US patent No. 5242670, IPC SW 033/08, No. 907854 from 02.07.92., publ. 7.09.93.]. The method consists in that the aluminosilicate raw materials are treated with a concentrated solution of HF. Released gaseous tetrafluorosilane (SiF₄) is skipped is via a series of cooling traps, which remains a large part of the impurities. The cleaned gas is in contact with an aqueous solution of sodium fluoride, with formation of a suspension of fertilitate sodium (Na₂SiF₆), from suspension of isolated crystals fertilitate sodium on the filter media, after which they are dried, and then calcined at 600-650°C. the Gas tetrafluorosilane released during the calcination condense, receiving liquid net tetrafluorosilane. At the same time in the residue from the calcination get powdered sodium fluoride (NaF). The suspension remaining after the decomposition of the raw materials and the distillation of tetrafluorosilane and containing aluminum fluoride and a small amount of secondary metal salts, dilute with water going into the liquid phase soluble salts. The resulting solution is separated from the insoluble residue. After evaporation receive the remainder trihydrate aluminum fluoride mixed with salts of related metals contained in the feedstock. After evaporation of the solution emitted gaseous Forestwood (HF) condense and return to the cycle. Equation flowing reactions:

SiO₂+4HF→SiF₄↑+2H₂O

2NaF+SiF₄→Na₂SiF₆↓

Na₂SiF₆→2NaF+SiF₄↑

The disadvantage of this method is the use of 50%hydrofluoric acid which is the substance of the first class of danger that makes the proposed technology environmental the ski threat.

A method of refining aluminum silicates on the aluminum fluoride which is processed ash Ekibastuz coal [inventor's certificate SU # 1668301, MKI C01F 7/50, application No. 4671911 from 03.04.89, publ. 07.08.91 - Way processing of silicates on the aluminum fluoride. / L.D. Shapiro, V.I. Shapoval, Mac, S.O. Akhmetov, VA, Gubenko]. Ash from the combustion of high-ash coal 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 the amount of 100-120% of the stoichiometrically required for the formation of aluminum fluoride and crematoria ammonium at 300-600°C. the Resulting gases tetrafluorosilane, ammonia (NH₃), water and hydrogen fluoride is distilled off and absorb in the sink with water, to obtain silica and ammonium fluoride (NH₄F). Silicon dioxide is filtered off and the remaining solution evaporated. Eye-catching ammonium fluoride return in the cycle. The resulting spectra, containing up to 90% of aluminum fluoride, can be used in the aluminum industry. Further use of the resulting hydrolysis of silicon dioxide is not considered [Copyright certificate №1668301, USSR C01F 7/50].

Equation flowing reactions:

SiO₂+4NH₄F→4NH₃↑+SiF₄↑+2H₂About

SiO₂+6NH₄F→(NH₄)₂SiF₆+4NH ↑+2H₂O

NH₄F→NH₃↑+HF↑

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

Closest to the claimed method is hydrochemical obtain highly dispersed silicon dioxide from man-made materials [Patent RU No. 2261841, application No. 2004109475 from 29.03.04, publ. 10.10.05 - Barbat VF and others].

In this way as a fluorinating agent used calcium fluoride and sulfuric acid, or industrial wastes containing fluoride and sulfuric acid. In the method of fluoridation ash produce hydrogen fluoride, directly emitted in the reaction zone. Run reaction:

$${\text{CaF}}_{\text{2}}+{\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}\to {\text{CaSO}}_{\text{4}}+\text{2HF}\uparrow \left(\text{1}\right)$$

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

$${\text{SiF}}_{\text{4}}+{\text{NH}}_{\text{4}}\text{F}\to {\left({\text{NH}}_{\text{4}}\right)}_{\text{2}}{\text{SiF}}_{\text{6}}\left(\text{3}\right)$$

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

The reaction of sulfuric acid with an inorganic fluoride in a Teflon reactor produces hydrogen fluoride (1), which interacts with components of the ash, forming various fluoride and gaseous tetrafluorosilane (2). Tetrafluorosilane absorb 15%solution of ammonium fluoride (3) 3-the series-connected polypropylene sinks. Then pour the resulting solution from the absorber and neutralize 20%ammonia solution (4). It stands a highly dispersed silicon dioxide, which is filtered off. In the present method chetyrehhloristy silane is not contaminated with ammonia, which is an advantage of the method. The disadvantage of this method is the fact that it discusses the possibility of obtaining from the ashes only highly dispersed silica, but contained in large quantities in the ashes aluminum and expensive rare metals remain in the solid products after fluorination and not disposed of. The use of calcium fluoride or industrial waste will lead to difficulties in further processing of the solid products of the fluorination ash from contaminated gypsum and other products, in the case of man-made waste.

The present invention is to develop a method for integrated processing of ash from burning coal to use as a fluorinating agent of a mixture of ammonium fluoride and sulfuric acid, in the reaction zone additionally injected carbon sorbent. To achieve the specified result, it is proposed to use a mixture of ammonium fluoride, sulfuric acid, silicon-containing wastes of industry and energy - ash CHP as ash contains up to 60% silicon dioxide, which can be seen from hee the systematic composition of ash on the primary (% mass.) and microcomponents (grams/tonne) components, presented in tables 1 and 2, and the process is carried out at a temperature of 120±5°C.

To obtain a concentrate containing rare and rare - earth elements, Sc, Y, Ce, La, Ga, when fluoridation of ash in the reactor in the fluorination impose additional carbon sorption material in a quantity sufficient for sorption of rare and rare earth elements. Rare and rare-earth elements partially remain in the form of fluorides, and in part will focus on the carbon sorbent, resulting in total can be allocated in the form of a concentrate.

In the proposed method the sample of ash from coal combustion, for example Ekibastuz, placed in a Teflon reactor, add carbon sorbent in the amount of 10-25 kg per ton of ash, which is 1-2,5% by weight of the s ash. The consumption of carbon sorbent is less than 1% leads to a decrease in REE content in the solid residue to 0.6%, and increase the flow of carbon sorbent more than 2.5% does not increase the content of REE in the resulting solid residue. Then the sample is mixed with ammonium fluoride at a mass ratio of the silica in the ash content of the fluoride ammonium fluoride 1:1, poured 50-55%sulfuric acid, heated to 120-125°C and incubated for 30-40 minutes, simultaneously driving the formed tetrafluorosilane in the sink 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 components of the ash, forming various fluoride and gaseous tetrafluorosilane. Tetrafluorosilane absorb 15%solution of ammonium fluoride in 3 series-connected polypropylene sinks. Then pour the resulting solution from the absorber and neutralize 20%ammonia solution. It stands a highly dispersed silicon dioxide, which is filtered off.

The solution after filtration of silicon dioxide evaporated, receiving ammonium fluoride, is returned to the absorption of tetrafluorosilane. Table 3 presents the dependence of the yield of silicon dioxide from an excess of the estimated Koli is esta fluorine in the ammonium fluoride and sulfuric acid at a temperature of 120°C.

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

5. 2NH₄F+H₂SO₄=(NH₄)₂SO₄+2HF

6. SiO₂+4HF=SiF₄+2H₂About

7. Al₂O₃+6HF=2AlF₃+3H 2About

The greater the excess of sulphuric acid does not lead to a significant increase in the extraction of silicon from the ashes. So when a 4-fold excess of the degree of extraction of silicon increases to 92%, which makes this an excess of sulfuric acid is not appropriate.

Tetrafluorosilane distilled and processed on silicon dioxide as in method [Patent RU No. 2261841, application No. 2004109475 from 29.03.04, publ. 10.10.05 - Barbat VF and others]. And the remainder is processed with the purpose of selection of aluminum sulfate and concentrate of rare and rare earth elements. To do this, the obtained solid product fluoridation translate the aluminum fluoride in 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 in the solution from the consumption of acid is at a temperature of 120°C. and processing time of 30 minutes is shown in table 4.

From table 4 it follows that you should take a 2-fold excess of sulfuric acid. High consumption of acid does not significantly increase the degree of extraction of aluminum sulfate from the solid residue fluoridation. Data on the dependence of the degree of extraction of aluminum sulfate in solution temperature and processing time in a double excess of concentrated sulfuric acid are given in table 5.

Further increase in temperature is not advisable because of the high corrosivity of the reaction mixture. In the processing of solid products of fluorination of concentrated sulfuric acid in the above conditions, the aluminum fluoride enters sulfate. After cooling, the reaction mixture is treated with water when aluminum sulfate is dissolved, the remaining precipitate was separated by filtration and calcined. The solid product after calcination is a concentrate of rare and rare earth elements.

To eliminate contamination of the product aluminium by iron, it is recommended to conduct magnetic separation of ash for disposal of ash valuable product - magnetospheres [Anshits A.G. Selection of magnetic microspheres permanent part of the energy is evil, and the study of their physico-chemical properties // chemistry for sustainable development, 1999. - VIP. - s-118].

The results can be illustrated by the following examples.

Example 1: 59,7 g ash from the combustion of coal from Ekibastuz (selected fields 1-4 electrostatic CHP-4 Omsk) mix with is 129.3 g of ammonium fluoride (PM). Placed in a Teflon reactor (1), add 1.5 g of carbon sorbent BAU added 518 ml 50-55-Oh sulphuric acid, heated to 120°C and maintained at this temperature for 30 minutes, at the same time fending off the released gaseous tetrafluorosilane. Absorbed it with a 15%solution of ammonium fluoride in polypropylene sinks (2). The resulting solution of ammonium fluorosilicate preparation (NH₄)₂SiF₆pour into a polypropylene beaker and to it there is added a 20%solution of ammonia until the smell of ammonia.

Precipitated silica is filtered and dry at a temperature of 110 OC. The degree of extraction of tetrafluorosilane is 92%. The residue after distillation of tetrafluorosilane contains up to 75% of aluminum TRIFLUORIDE, up to 2% silicon dioxide. Else, sulfuric acid, nadareh contained in the original ash, fluoride, iron and REE. The specific surface of the obtained silicon dioxide determined by BET method is 400±15 m²/year

To the residue is poured 120 ml of concentrated sulfuric acid, heated to 250 for 1.5 hours, cooled, treated with water (800 ml), the precipitate filtered off. The solution is extracted 95% of the aluminum contained in the original ash.

Filtered the solid residue calcined at 800°C. the Mass of the solid residue is 2% by weight of the original ash, the composition of the residue is analyzed by the spectrophotometric method semi-quantitative atomic-emission analysis. Data on the chemical composition of the residue after Prokaeva the Oia in % are given in table 6.

This method allows you to expand the raw material base to obtain highly dispersed silicon dioxide due to the use of angry CHP

Example 2. Ash from burning coal is treated in the same conditions as in example 1, but additionally introduce nanostructured carbon sorbent derived IPPO SB RAS (Omsk). Removing the silicon and aluminum remains the same as in example 1. The REE content in the solid residue is 1.3%.

1. The method of complex overtime and ash from coal burning, in which the ash from burning coal is placed in a reaction zone, add carbon sorbent, providing absorption of 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 min, the resulting tetrafluorosilane absorb ammonium fluoride and the resulting solution tetraberlinia ammonium enter solution of ammonium hydroxide prior to the deposition of silicon dioxide, then add concentrated sulfuric acid in a two-fold excess contained in the remainder aluminum, maintained at a temperature of 250°C for 1.5 h and treated water, the solid residue is filtered and calcined at 800°C.

2. The method according to claim 1, characterized in that from the ashes previously removed iron magnetic separation.