ES2397018B1 - METHOD OF TREATMENT OF WASTE WITH LOAD IN SULFATES AND VALUATION OF THE MUDS OBTAINED. - Google Patents

Abstract

Waste treatment method with sulfate loading and recovery of the sludge obtained # Method for the treatment of sulphate containing waste and recovery of the by-products obtained. It comprises a phase, of adding variable proportions of tricalcium aluminate (3CaO-Al {sub, 2} O {sub, 3}), for the production of a main reaction (2), the sulfates being eliminated by precipitation, obtaining a byproduct that incorporates the precipitates; and a pretreatment phase (2) of said residues (1), in which varying proportions of calcium hydroxide, calcium oxide and / or calcium carbonate are added, prior to the addition (4) of tricalcium aluminate, obtaining a precipitate containing calcium sulfate or hydrated calcium sulfate. Said by-product comprises sludge (5) containing etringite. The sludge can be used as a coagulant and / or neutralizer in a wastewater treatment plant (9).

Description

"Waste treatment method with sulphate loading and recovery of the sludge obtained"

Technical sector of the invention

The present invention relates to the treatment of waste, solid, liquid or gaseous, containing sulfates, for example, wastewater with sulfate contents exceeding the limit determined by the legislation in force in the country of application. Specifically, the present invention relates to a method of treating sulfate-laden waste, as! as a valuation of the sludge obtained.

Background of the invention

Sulfate treatment processes can be classified into four categories:

chemical treatments with mineral precipitation

membranes

ion exchange

biological removal of sulfates

There are several methods of chemical treatment: Calcium carbonate / lime; barium salts, SAVMIN method; melod CESR.

In neutralizing upor calcium carbonate I cal-, it is neutralized with hydroxide or calcium carbonate, precipitating gypsum or anhydrite. It is an economic system but, given the solubility of calcium sulphide, the sulfate concentration cannot be lowered below about 1200-1500 ppm. the corresponding reactions are:

Ca (OH), + H, SO, - + CaSO, · 2H, O CaC03 + H2S04 + H20 -1> CaS04'2H20 + C02

If barium salts are used, it is neutralized with carbonate barium salts,

barium hydroxide or sulphide, obtaining insoluble 8a sulfate. 105 sulfate levels are reduced to about 200 ppm. It is a relatively expensive system given the cost of reagents. the corresponding reactions are:

BaC03 + H2SO. - + BaSO ,, + H2C03 Ba (OHh + H2SO "- + BaSO ,, + 2H20 BaS + H2SO "- + BaSO ,, + H2S

The SAVMIN method consists in the elimination of sulfates by precipitation of Etringite. This is achieved by the reaction of aluminum hydroxide with the sulfate ion in the presence of lime. Low levels of output sulfates are obtained but for waters with low sulfate levels. On the other hand, investment costs are also high. the corresponding reactions are:

3S0, H, + 6Ca (OH), + 2AI (OH), + 19H, O -> 3CaO-3CaSO, -AI, O, -31H, O

The CESR method also uses sulfate removal by precipitation of etringite but using calcium aluminate (CaAI20 ,,) as the reagent. Sulfates are reduced below 200ppm but with a high production cost. The reaction of the process can be written as:

3S0, H, + 5CaO + CaAl, O, + 28H, O -> 3CaO-3CaSO, -AI, OJ-31 H, O

The membranes are used in two important water treatment processes: reverse osmosis and electrodialysis. These processes achieve high purities with quality for mouth water. In return, investments, maintenance and operating costs are high. In addition, these systems generate a salt concentrate, continuously, more or less concentrated depending on the initial sulfate concentration and the level of quality required, difficult and expensive management_

Another method of sulfate removal is through the use of ion exchange resins.

They are composed of a high concentration of polar, acidic or basic groups, incorporated into a matrix of a synthetic polymer (styrenic resins, aquatic resins, etc.) and act by taking ions from the solutions (usually water) and yielding equivalent amounts of other ions . The main advantage of ion exchange resins is that they can recover their original exchange capacity, by treating with a regenerating solution.

Ionic exchange resins have a fixed radical and a mobile ion or replacement ion. The mobile ion is the ion that is exchanged for ions that wish to be removed from the solution and this exchange only works between ions of equal electric charge: cations for cations and anions for anions.

In general, ion exchange resins operate in columns, to favor the exchange process, similar to distillation or distillation in trays. The exchange reaction moves in the resin bed, generally towards the

lower levels.

The weak-based anionic resins remove the anions of strong acids, such as sulfates, nitrates and chlorides, with great efficiency.

It is a method of high efficiency but high cost, both maintenance and management, also generating a significant amount of waste.

Biological treatments for sulfate removal are an alternative to the different methods described above. The main technologies currently used are: bioreactor, artmcial wetlands for wastewater treatment, alkalinity producing systems (toxic drainage through limestone and vertical flow systems) and vertical reactive barriers. All of them, except the bioreactor, are passive treatment systems and therefore of low treatment speed. On the other hand they are influenced by the use of substrates with the presence of other anaerobic bacteria, with the toxicity due to the presence of H2S and the dissolution of metals, which even at trace level seem to inhibit the reduction of sulfates.

Although the above methods generally have correct functionality, they present individually and together some drawbacks that limit the application in certain fields.

In this sense, the techniques of membranes and Ionic exchange resins, produce excellent quality but with low performance and high cost. Biological techniques, in general, are very slow and critical in the maintenance of battery strains.

Thus, the most suitable candidates are precipitation techniques. Within them the precipitation by calcium carbonate and lime is an effective system but with the disadvantage that given the solubility of the precipitated plaster, the sulphate level cannot be lowered below 1200 ppm. Of all forms, given the low cost, it can be used as a pretreatment.

Precipitation by barium salts has the disadvantage of the high cost of the reagents in addition to the risk in the use of a highly toxic heavy metal, which in general discourages its use.

The other two SAVMIN and CESR methods present high investment and / or production costs, either due to the complexity of the system or due to the cost of reagents.

The purpose of the present invention is to solve all these inconveniences simultaneously.

Explanation of the invention.

For this purpose, the object of the present invention is a method for the treatment of residues containing sutphates, whether solid, liquid or gaseous, and of recovery of the by-products obtained, again, characterized in that it comprises a phase, or main reaction , to add variable proportions of tricalcium aluminate (3CaO · AI203), the sulfates being eliminated by precipitation, obtaining a by-product that incorporates the precipitates.

Preferably, the method comprises a pretreatment phase of said residues, in which varying proportions of calcium hydroxide, calcium oxide and / or calcium carbonate are added, prior to the reaction, obtaining a precipitate containing calcium sulfate or sulfate of hydrated calcium.

Said by-products may consist of sludges containing etringite or some other phase of calcium sulfoaluminate.

In such a case, the invention contemplates the possibility of the additional phase of conditioning said sludges.

Said sludge can be valued as a component for the manufacture of cement 0, alternatively or in addition, as a coagulant-neutralizer in a sewage treatment plant, the inventors having observed that, surprisingly in both uses, the characteristics of cement significantly improve, in one case, and the coagulant-neutralizing functionality, in the other case ..

The inventors have found after many tests that the provided mixture of C3A with specific amounts of Ca sources improves sulfate precipitation, and this may be due, at least partially, to both the excellent precipitation efficiency and the good characteristics of the sludge as a cement component and coagulant.

The proposed method uses as a reagent tricalcium aluminate instead of calcium aluminate, in the CESR method or aluminum hydroxide in SAVMIN. The advantages that this entails are in its better reactivity and greater economy of the product, in the case of CESR. Calcium aluminate is the main component of aluminous cement that is synthesized by fusion at temperatures of the order of 16000c while tricalcium aluminate is synthesized at temperatures of about 300 ° C lower, that is to say about 13QOOC and by solid state synthesis. This implies a greater economy in the product. On the other hand, the reactivity of tricalcium aluminate is much higher since it reacts with sulfates just by coming into contact, thus being used as part of the reaction of the Partland cement retarding process.

Comparing it to the SAVMIN process, this is a much more complex process, as can be seen in the literature on the subject and in the description of the corresponding patent and in the economy of the reagent, which in this case must be an aluminum hydroxide high reactivity like amorphous aluminum hydroxide.

In the case of the present invention, a much more economical product is used that has greater reactivity; This implies a lower complexity of the process and a greater economy of both the product and the investment necessary for its application and maintenance.

Thanks to this, the present invention allows to solve the aforementioned drawbacks in an efficient way.

Brief description of the drawings

The following is a detailed description of preferred, but not exclusive, embodiments of the method of treating sulfate-laden waste and recovery of the by-products obtained object of the invention, for which better understanding is accompanied by drawings in which a non-limiting example is illustrated as an embodiment of the present invention, in which only Fig. 1 shows a schematic flow diagram of the method of the invention.

Detailed description of the drawings

The method of treatment of sulfate-laden residues and recovery of the by-products obtained are described below.

To obtain C3A, the following are used as raw materials:

Any product (natural, synthetic or residue) in which the content of

CaO, after calcination, is greater than 25%, such as calcium carbonate.

Any product (natural, synthetic or residue) in which the content in A120 3 • after calcination. is greater than 25%, such as alumina.

The process consists of weighing the raw materials and mixing them intimately in order to obtain a mixture with the stoichiometric proportions of C3A, at least 50%, for example by mixing 74.6 grams of calcium carbonate with 25.4 grams of alumina.

The mixture is heat treated at an elevated temperature, between 3000c and 1700oC, for example at 13500c, so that the radiation between the components occurs and C3A is obtained.

The product obtained is finely ground until it passes, in its entirety, through a 5mm mesh of light, for example when the average size is also 0.050mm.

According to the present invention, the application of C3A in the treatment of sulfate-containing waters is proposed, for example, with sulfate contents exceeding the limit allowed by the legislation in force in the country of application. For example in Espafla, with sulfate contents greater than 1000 ppm. By applying the product in appropriate proportions, to waters with high levels of sulfates, the proportion of sulfates is reduced to levels below the maximum pennitide values. It is also applicable in the case of drinking water purification, in which case the maximum recommended limit for Espai "la, is 250

ppm.

In water with sulfate contents (above the solubility limit of calcium sulfate dihydrate) (very high), calcium hydroxide, or some other substance that precipitates the sulfate ion, can be used first for a first precipitation of sulfates and Subsequently, C3A is used to bring sulfate content to levels below the maximum established by the country's environmental laws. In certain cases, a source of calcium ions can also be used to improve process performance, mixed with C3A.

Sulfate removal by precipitation. In this case sulfates are also removed by precipitation of etringite, but the reagent used is aluminate

tricalcium (3CaO-AI, O,) (C3A) _

The method consists of a previous treatment with calcium carbonate, calcium oxide and calcium hydroxide with precipitation of gypsum or anhydrite, depending on the conditions of composition and the temperature existing in the reactor.

Maintaining a basic pH, C3A is added by reacting with sulfate ions and precipitating etringite.

This process can be schematized in the flowchart of Fig. 1.

The sulphate liquid (block 1) is subjected to a pretreatment 2, where the following reactions occur:

Ca (OH), + H, SO. -> CaSO.-2H, O CaC03 + H2SO ", + H20 -...) or CaSO", '2H20 + C02

with obtaining a precipitate 3.

The main reaction 4 with the sulfate follows:

3H, SO. + 3Ca (OH) + 3CaO-AI, O, + 25H, O -> 3CaO-3CaSO.-AI, O, -31 H, O

with production of etringite (3CaO · 3CaSO ", · AI20 3 · 31H20), incorporated in the mud

5_

The water 7 obtained in the main precipitation reaction of etringite is a water with a low sulphate content, for example less than 100 ppm, and suitable for pouring

Do or consumption in the mouth.

The application of C3A is considered in the treatment of sulphate-containing water,

for example, with sulfate contents exceeding the limit allowed by the legislation

S

current action. For example in Espal "ia, with sulfate contents higher than

1000 ppm By applying the product in the appropriate proportions, to

waters with high levels of sulfates, the proportion of sulfates to

levels below the maximum allowed values. It is also applicable in the ca

so of drinking water purification, in which case the recommended limit,

10

for Spain, it is 250 ppm.

In waters with sulfate contents (above the sulfate solubility limit

calcium dihydratadc »very high, hydroxide can be used first

calcium, or some substance that precipitates the sulfate ion, for a first price

fifteen

sulfate pite and then C3A is used to carry the content of

sulfates up to levels below the maximum established by the medioam laws

environmental factors and the solubility product of the plaster. In arrested cases,

a source of calcium ions can also be used to improve the performance of the

process, mixed with C3A.

twenty

For example, in the treatment of waters 1 with a sulfate content of

2,650 ppm., 2,800 gr.lm3 of C3A dislodged in the waters are used, reducing

sulfate content at <100 ppm.

25

For other cases of water 1 with a higher sulfate content, it is used

previously Ca (OH) 2 and subsequently C3A is applied.

According to the invention, a use of the sludge obtained in the precipitation of

sulfates with C3A.

30

After precipitation of the sulfates with the application of C3A, a sludge is obtained.

rich in sulfates, calcium and aluminum (etringite). This mud, conveniently

6, can be used as a component in manufacturing processes

of expansive cements (in block 8).

Due to its composition, the suitably conditioned sludge 6, can also be used in the treatment of wastewater (in block 9), as a coagulant, as a substitute for alumina sulfate, alumina polychloride, ferric chloride. etc., and

5 as a neutralizer and insulator of insoluble hydroxides, replacing calcium or sodium hydroxide and calcium or sodium carbonate.

The generated sludge, also conveniently conditioned, can be used to remove other compounds from water, such as boron metal.

Finally, it should be noted that the proportionate mixture of C3A with specific amounts of Ca sources improves sulfate precipitation. For this reason. the detailed study of each specific case, and the corresponding contribution and mixture of lime with C3A will result in an improvement of the process. In turn, the reuse of

15 sludge obtained (etringite) to eliminate sulfates, in certain cases, is very favorable.

Describing sufficiently the nature of the present invention, as well as the way of putting it into practice, it is stated that everything that does not alter, change or change its fundamental principle is subject to variations in detail.

Claims (7)

1.-Method for the treatment of sulfate-containing residues and recovery of the obtained by-products, characterized in that it comprises a phase, of adding variable proportions of tricalcium aluminate (3CaO'AI20 3), for the production of a main reaction (4): 3H, SO, + 3Ca (OH) + 3CaO "AI, O, + 25H, O .... 3CaO" 3CaSO, "AI, O," 31 H, O sulfates being eliminated by precipitation, with the obtaining of a by-product that incorporates the precipitates. 2. Method for the treatment of sulfate-containing residues according to claim 1, characterized in that it comprises a pretreatment phase (2) of said residues (1), in which varying proportions of calcium hydroxide, calcium oxide and / or calcium carbonate, prior to the main reaction (4) of tricalcium aluminate, obtaining a precipitate containing calcium sulfate or hydrated calcium sulfate. 3. Method for the treatment of sulfate-containing residues according to claim 1, characterized in that said by-product comprises sludge (5) containing etringite. 4. Method for the treatment of sulphate containing residues, according to claim 3, characterized in that it comprises the additional phase of conditioning said sludges (5). 5. Method for the treatment of sulfate-containing residues, according to claim 1 or claim 4, characterized in that it comprises the phase of adding said sludge in a mixture for the manufacture of cement (8). 6. Method for the treatment of sulfate-containing waste, according to claim 1 or claim 4, characterized in that it comprises the phase of pouring said sludge as a coagulant in a wastewater treatment plant (9) 7. Method for the treatment of sulfate-containing residues, according to claim 1, 6, claim 4, characterized in that it comprises the phase of pouring said sludge as a neutralizer in a wastewater treatment plant dual (9).