SK284700B6 - Method of cleaning alkaline water and its desalination - Google Patents

Abstract

A method of cleaning and desalination of alkaline waters, which are polluted especially by sodium, potassium, aluminium and eventually by chromium, is described, so that the cleaned water fulfils legislative requirements for surface waters purity. The cleaning of water is accomplished by adding fluoride and aluminium compounds to water in such an order, that a cryolite precipitate (Na3AlF6) is formed, which after its filtering out and drying becomes a quality commercial raw material. Advantageously, a technology of cleaning water from hexavalent chromium may be introduced to the process of desalination, by which chromic hydroxide (Cr(OH)3) is obtained, captured into separate product or according to the used reducing agents in a mixture with ferric hydroxide (Fe(OH)3), the precipitated cryolite and clear water.

Description

Technical field

Treatment and cleaning of industrial waters, environment.

BACKGROUND OF THE INVENTION

At present, industrial, waste, alkaline waters, regarded as alkaline hydroxide-carbonate solutions, are treated and purified in principle in two ways. Neutralization and electrodialysis. By neutralization, the solutions are only rendered harmless by adjusting their alkalinity and electrodialysis to clean and desalinate them. The neutralization of the solutions is essentially carried out with acids. Sulfur or hydrochloric acid. By using them, foreign anions are introduced into the solution which additionally contaminate it and increase its salinity. Usually no precipitates are formed. Such solutions are diluted many times before being discharged into the recipient. Modification in this way consists only in solving the problem of alkalinity of the solution, which must comply with the legal standard and do not solve the desalination of the solution. Electrodialysis belongs to a group of membrane processes which ensure separation of the purified solution into two solutions. Concentrated to which most of the dissolved salts are transferred from the original solution and to a purified solution containing residual amounts of dissolved salts. The concentrated solution is treated in the next process step by concentration and crystallization to give a mixed salt. Depending on its purity, it is deposited or used commercially. The desalinated solution must be brought to standard alkalinity and salt content before being discharged into the recipient. The solution desalination process using electrodialysis is a costly, energy intensive and complicated process due to the need for a number of accompanying technological operations.

SUMMARY OF THE INVENTION

The solution relates to the purification and desalination of alkaline waters from sodium, potassium, aluminum and chromium by a precipitation process based on a chemical reaction. During this, an insoluble sodium compound, cryolite (Na ₃ AlF ₆ ), is formed, which precipitates from the aqueous solution as a bulk precipitate. It binds quantitatively the sodium, potassium and aluminum present in alkaline water. Separation of the cryolite from the solution provides desalination and purification of the water from sodium, potassium and aluminum. Due to the composition of the alkaline solutions, which usually contain not more sodium than aluminum and contain no fluorine, it is necessary to add stoichiometric amounts of aluminum, taking into account the original aluminum, and fluorine, in the form of suitable compounds, to the sodium present in the solutions. Preferably in the form of a solution of aluminum fluoride (AlF ₃ ) and hydrofluoric acid (HF) to form a cryolite precipitate (Na ₃ AlF ₆ ). Use aluminum hydroxide (A1 (OH) ₃ ) and concentrated hydrofluoric acid (HF) to prepare a solution of aluminum fluoride (A1F ₃ ). Advantageously, a hexavalent chromium (Cr ^{+ 6} ) alkaline water purification sequence can be incorporated into said main desalination process.

The removal of chromium from the solution consists in reducing it from the hexavalent (Cr ^{+ 6} ) form to the trivalent (Cr ⁺³ ) form and then precipitating it as chromium hydroxide (Cr (OH) ₃ ). The reduction can only take place in an acidic environment. The reduction is achieved by suitable reducing agents, in particular iron sulphate (FeSO ₄ ) in the form of green vitriol (FeSO ₄ .7H ₂ O) and sodium sulfite (Na ₂ SO ₃ ) in the form of hydrate (Na ₂ SO ₃ .7H ₂ O). The acidic medium is formed by adding hydrofluoric acid (HF) to the alkaline solution in excess to form a solution with a basicity from pH 3 to 6. Reactions (1) and (2) take place between the main components of the solution:

NaOH _(a) + HF _(a) NaF = _(a) + _H2O (1)

Na ₂ CO _{3 (a)} + 2HF _(a) = 2NaF _(a) + H ₂ O + CO 2g ₎ (2)

After acidification of the solution and subsequent addition of reducing agents, the hexavalent chromium is reduced to trivalent. The solution changes from yellow to green, which at the same time serves as an indication of the progress of the reduction, further addition of reducing agents is ineffective. Subsequently, the addition of sodium hydroxide solution (NaOH) changes the alkalinity of the purified solution to a pH value from 9 to 12, precipitating chromium hydroxide (Cr (OH) ₃ ) and, depending on the reducing agent used, ferric hydroxide (Fe (OH) ₃ ), which is separated from the solution. As a result, the solution is purified from chromium.

The basic solution, after removal of the chromium, is again neutralized with hydrofluoric acid (HF) according to reaction (1). The treated solution contains only sodium fluoride (NaF), which is mixed in a stoichiometric amount with a specially prepared aluminum fluoride solution (AlF ₃ ), whereupon a quantitative precipitate of cryolite (Na ₃ AlF ₆ ) is precipitated at normal temperature according to reaction (3).

3NaF _(a) + AlF _{3 (a)} = Na ₃ AlF ₆ (3)

The precipitate formed is separated from the solution and dried. This gives a pure, well-defined salt, which is a commercial product. In this way, the desalination of the alkaline water and its purification from aluminum and chromium are achieved. The clear, clear water is discharged into the recipient.

At the node of the aluminum fluoride solution (A1F ₃ ), hydrofluoric acid (HF) is mixed with aluminum hydroxide (A1 (OH ₃ )) in stoichiometric quantities, and reaction (4)

3HF _(a) + A1 (OH) _} = AlF _{J (a)} + 3H ₂ O (4), a solution of aluminum fluoride (AlF _{3 (a)} ) is formed which has to be processed while hot to avoid heat exothermic reaction to crystallize.

The alkaline water purification and desalination process of the present invention represents an advance over previously known processes in that the simple method of chemical reactions achieves purification of water from carcinogenic hexavalent chromium and subsequent precipitation of the insoluble sodium salt, cryolite (Na ₃ AlF ₅ ), which is a commercial product. Purified, initially alkaline, water is suitable for being discharged into a recipient, meeting all legislative standards.

When using neutralization procedures, the desalination of water is not achieved but only the basicity is reduced. Desalting and changing the alkalinity of the water are achieved by membrane processes, but the salt formed is mixed and its composition is volatile, depending on the composition of the water to be treated. Salt should not be deposited because of its high solubility. The process is energy-intensive, technically complex and technologically complicated.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

For the purification and desalination of alkaline water, contaminated tailings water, the composition of which is given in Table 3, was used:

SK 284700 Β6

Tab. 3 Chemical composition of alkaline water (mg / 1)

sample NaOH Na ₂ CO ₃ The Al V-1 14960 6680 140 900

Hydrofluoric acid (HF) was added to the alkaline water to reduce its basicity from pH = 13.6 to pH = 7. A pure sodium fluoride solution (NaF) was obtained. Stoichiometric addition of preformed aluminum fluoride solution (AlF ₃ ) to sodium fluoride solution (NaF) precipitated a bulk white precipitate of cryolite (Na ₃ AlF ₆ ). The precipitate was filtered off and dried at 120 ° C. The products whose chemical analysis is shown in Table 4 were obtained.

Tab. 4 Chemical analysis of purified and desalinated water (mg / l) and cryolite (Na ₃ AlF ₆ ) (wt.%)

sample On the The Al Water 253 26 0.13 cryolite 32.94 0.27 12.49

The cryolite phase (Na ₃ AlF ₆ ) was confirmed by X-ray diffraction analysis.

Example 2

For purification and desalination of the alkaline water, water from the tailings pond, the composition of which is given in Table 1, was used.

Tab.1 Chemical composition of alkaline water (g / l)

sample NaOH Na ₂ CO ₃ The Al Cr V-1 14960 6680 140 900 50

Hydrofluoric acid (HF) was added to the alkaline water to reduce its basicity from pH = 13.6 to pH = 6. After homogenization of the solution, green vitriol (FeSO ₄ .7H ₂ O) was added as reducing agent. for hexavalent chromium (Cr ^{+ 6} ) in such a quantity as to discolour the solution from yellow to green. Subsequently, chromium and iron hydroxides were precipitated by changing the basicity of the solution from pH = 6 to pH = 10.5 by addition of sodium hydroxide (NaOH) solution. A dark rust precipitate formed and was filtered from the solution. Subsequently, the solution was neutralized with hydrofluoric acid (HF) to pH = 7. A pure sodium fluoride solution (NaF) was obtained. Stoichiometric addition of preformed aluminum fluoride solution (AIF ₃ ) to sodium fluoride solution (NaF) precipitated a bulk cryolite precipitate (Na ₃ AlF ₆ ). The precipitate was filtered off and dried at 120 ° C. The products whose chemical analysis is shown in Table 2 were obtained.

Tab. 2 Chemical analysis of purified and desalinated water (mg / l) and cryolite (Na ₃ AlF ₆ ) (wt.%)

The cryolite phase (Na ₃ AlF ₆ ) was confirmed by X-ray diffraction analysis.

Industrial usability

The present invention is industrially applicable wherever it is necessary to purify and desalinate alkaline waters containing mainly sodium, potassium, aluminum and chromium.

Claims (2)

PATENT CLAIMS A process for the purification and desalination of chromium-free alkaline waters, characterized in that reagents are added to the alkaline water in a controlled sequence such that hydrofluoric acid - HF is used as the first reagent in such a quantity that sodium is present in the a second reagent, a separately prepared solution of aluminum fluoride - A1F ₃ from aluminum hydroxide - A1 (OH) ₃ and hydrofluoric acid are added to the solution thus formed, taking into account the content initially present of aluminum in alkaline water, causing the stoichiometric cryolite-Na ₃ AlF ₆ compound to precipitate from the solution, thereby achieving purification and desalination of the alkaline waters. 2. A process for the purification and desalination of chromium-containing alkaline waters, characterized in that reagents are added to the alkaline water in a controlled sequence by adding hydrofluoric acid-HF as the first reagent in excess to change the acidity of the purified solution from pH values of 8 to 13 to a pH of 3 to 6 required for the reduction of hexavalent chromium to trivalent chromium, the reduction being carried out by the subsequent addition of reducing agents such as iron (II) sulfate - FeSO ₄ , green vitriol - FeSO ₄ .7H ₂ O, sulfite sodium - Na ₂ SO ₃ or sulfite septahydrátu -Na ₂ SO ₃ H 7 H ₂ into the thus prepared solution was added in the third, agent, sodium hydroxide - NaOH in an amount to change in pH from 3-6 to 9-12, thereby precipitating chromium hydroxide - Cr (OH) 3, or a mixture of chromium hydroxide and ferric hydroxide precipitate - Fe (OH) ₃ , which u removed by conventional procedures, as in turn, the fourth reagent is used hydrofluoric acid to neutralize the alkaline solution to give a pure sodium fluoride-NaF solution, to which a separately prepared aluminum fluoride-AlF ₃ solution is then added, adding when adding this reagent, it will take into account the aluminum content initially present in the alkaline water, causing the stoichiometric cryolite compound - Na ₃ AlF ₆ to be precipitated from the solution, thereby achieving purification and desalination of the alkaline waters, in particular from Cr, Al and Na. sample On the The Al Cr fe Water 287 24 0.16 0,098 o, l cryolite 33.01 0.29 12.69 0,006 0,001