ES2423708B1 - RECOVERY OF TOXIC METALS PRESENT IN THE BIOMASS OF FITORREMEDIATION PROGRAMS - Google Patents

Abstract

We deposit the biomass from phytoremediation in an incinerator oven (1). The fumes generated in said process are processed by bubbling in a sulfuric acid solution (2) with the consequent dissolution of the metals present in the received fumes. Said solution is basified (2) by means of potassium hydroxide -KOH- from an auxiliary tank (3). With this gasification we achieve the precipitation of the previously dissolved metals and can be removed in the form of hydroxides and leaving a solution of potassium sulfate - K {sub, 2} SO {sub, 4} - that can be used as fertilizer when diluted directly in waters for agricultural use (4). The solid waste generated in the incineration is subjected to a stream of carbon dioxide –CO {sub, 2} - (4), generating carbon monoxide -CO- and basic metal oxides (6) from which we can extract the corresponding metals .

Description

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DESCRIPTION

Recovery of toxic metals present in the biomass of phytoremediation programs.

The use of plants to extract contaminants, including heavy metals from the soil is known as phytoremediation. The present invention relates to the plant material treatment system resulting from these treatments by incineration at temperatures of eight hundred degrees and atmospheric pressure and the consequent treatment of the generated fumes. The present procedure must be framed within the environmental sector since the contamination of the soil by heavy metals is an important problem at present.

BACKGROUND OF THE INVENTION

Soil metals do not degrade, so they can accumulate in the ecosystem over time causing high concentrations of adverse effects in most organisms; Therefore, techniques have been developed to remedy contaminated soils. Traditional soil remediation techniques generally involve conventional civil works practices. In these physical, chemical and thermal processes, it is mainly necessary to physically remove. Due to the limitations presented by traditional remediation technologies, there is a need to use alternative techniques such as phytoremediation and with it the need to find possible treatments for plant residues that are generated and that have high concentrations of heavy metals. The advantages of phytoremediation are several, the amount of disturbed soil, the amount of waste destined for confinement and the spread of air pollution are reduced

or water But in turn it has limitations, since it is restricted to surface contamination, it can take years to repair a contaminated site, it depends on the climatic conditions and the biomass of the plants harvested in the phytoextraction treatment can be classified as a hazardous waste. These plants known as hyper accumulators are capable of growing in soils contaminated with toxic metals and accumulating them at extraordinarily high levels. By definition, hyper-accumulators of lead -Pb-, Niquel -Ni-, Cobalto-Co- and Copper -Cu are capable of accumulating more than one thousand milligrams of metal per kilogram of dry weight in the aerial part and more have been described of four hundred different species of hyper accumulator plants.

In the last two decades, energy from biomass has become the second renewable source of electricity, after hydroelectric power. As with hydroelectric power, biomass facilities can provide electricity to utility companies, which operate wherever electricity is needed. If properly grown and harvested, biomass can be a renewable source that can be used to generate electricity when needed, without a net contribution to global carbon dioxide.

In the current state of the art, the biomass generated in these programs once the plants have absorbed the accumulated contaminants, can be harvested and disposed of. If organic chemical contaminants degrade in molecules such as carbon dioxide, plants may not require any special method of disposal.

Controlled incineration is the most common method for disposing plants that have absorbed large amounts of contaminants. This process produces ashes, which can be discarded at the sites intended for that purpose. This generation of waste is a major problem for the complete development of this remediation technique. Therefore the present process involves the incineration at temperatures of approximately eight hundred degrees Celsius and at atmospheric pressure of all the vegetal matter and the consequent treatment of the fumes and the ashes generated. With this treatment, more than sixty-five percent of the toxic elements remain in the ashes and it is necessary to process the fumes in a special way to avoid the possible spillage of toxins in suspension or in the released vapors.

With the proposed process it is possible to remove metallic elements such as aluminum, nickel, copper, zinc, mercury and lead from the effluent gases from the incineration.

DESCRIPTION OF THE INVENTION

The present treatment consists of the treatment of the fumes and ashes generated in the controlled incineration processes to which the biomass generated by the plants used in the remediation of soils contaminated with heavy metals is subjected. Among the elements that can be found in these generated fumes are: aluminum –Al-, nickel –Ni-, copper –Cu-, zinc –Zn-, cadmium –Cd-, mercury -Hg- and lead - Pb-

The process consists of two main stages.

The first (stage 1), bubbling the effluent gases from the incineration, in a solution of concentrated sulfuric acid. The volatile metal residues when passing through the concentrated sulfuric acid solution, all metals remain as sulfates, with the exception of lead, which could precipitate as hydrogen lead sulfate

(II) - Pb (HSO4) 2- according to the process:

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PbSO4 + H2SO4 → Pb (HSO4) 2

The bubbling of fumes may be carried out for several successive incineration processes until an optimum concentration of toxic metals is reached.

Likewise, the generated fumes contain between thirty and thirty-five percent of water vapor that will dilute sulfuric acid –H2SO4- in the successive cycles of system operation. This could eventually require regeneration of the acid solution at its initial concentration.

Stage two, basification with potassium hydroxide –KOH-, begins when the sulfates reach a sufficient concentration. A double system can be set up in which the reactors work alternately. Thus, while one of the reactors is in stage one, the other would be in both and then vice versa.

When basifying the medium with potassium hydroxide, the precipitation of nickel –Ni-, copper –Cu- and mercury –Hg as hydrated oxides will take place. In addition, all acidity of the sulfuric medium must be neutralized by the reaction:

2 KOH + H2SO4 → K2SO4 + 2 H2O

This reaction produces water, which will help dilute the resulting mixture.

The reaction that takes place between nickel metal sulfates –Ni-, copper –Cu-, zinc –Zn-, cadmium –Cd-, mercury -Hg- and lead –Pb- with potassium hydroxide is:

MSO4 + 2 KOH → M (OH) 2 + K2SO4

It will be necessary to control the pH, which cannot be higher than pH 11, so that zinc precipitation - Zn - is effective, since an amphoteric hydroxide is formed that is redissolved in excess of basic medium.

In the case of aluminum -Al- it would precipitate according to the reaction:

6 KOH + Al2 (SO4) 3 → 3 K2SO4 + 2 Al (OH) 3

Cadmium -Cd- would require an additional process, such as an electro-reduction, given the solubility of cadmium hydroxide -Cd (OH) 2- which is two hundred sixty milligrams per liter at twenty degrees Celsius.

Once all the toxic metals in solution have been removed, and removed in the form of hydroxides, the resulting supernatant will consist of a solution of potassium sulfate - K2SO4 - that can be conditioned to be used as fertilizer when diluted in irrigation water, taking into account that the maximum sulfate concentration for this purpose has been established at one point five grams per liter.

Subsequently, the solid residue composed of ash from the incineration will react with a stream of carbon dioxide to remove carbon debris producing carbon monoxide according to the reaction:

CO2 + C → 2 CO

As a result, it will be a mixture of basic metal oxides from which the different metals present on one side can be extracted, and carbon monoxide -CO- on the other. In these ashes the oxides of the following metals will remain in variable percentages: aluminum –Al-, nickel –Ni-copper –Cu-, zinc –Zn-, cadmium –Cd-, mercury –Hg- and lead –Pb.

With the process described, all metals present in the biomass are recovered, either in the form of oxides or in the form of hydroxides.

BRIEF DESCRIPTION OF THE DRAWINGS

For the best understanding of what is described herein, a scheme is attached in which only by way of example, the process is represented.

Incinerator furnace where the biomass (1) is processed, fume bubble tank in sulfuric acid –H2SO4-concentrate and basification of the solution containing the metals generated in the incineration process (2), potassium hydroxide deposit –KOH- (3), potassium sulfate solution dilution tank - K2SO4- so that the supernatant is reusable (4), ash combustion furnace where the solid waste generated is treated (5), carbon monoxide current output - CO- and basic metal oxides (6).

DESCRIPTION OF THE PREFERRED EMBODIMENT FORM.

In the processing of biomass with high metal content

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from phytoremediation programs, by controlled incineration at eight hundred degrees Celsius and at atmospheric pressure (1), a stream of toxic fumes and solid waste with a high metal content is obtained. The smoke stream, where remains of metals such as aluminum –Al-, nickel –Ni-, copper - Cu-, zinc –Zn-, cadmium –Cd-, mercury –Hg- and lead –Pb- are found, are transported to a tank containing sulfuric acid-H2SO4-concentrate (2) where it is bubbled. In this way the metals, when forming the corresponding sulfates, will be dissolved in the medium with the exception of lead - Pb - which could precipitate as lead hydroxide sulfate II - Pb (HSO4) 2. Thirty-five percent of water vapor contained in the fumes will dilute sulfuric acid in the successive cycles of the system. Subsequently, the medium is basified by potassium hydroxide –KOH-, in the same tank (2). In this phase the precipitation of the 10 metals as hydrated oxides will take place, as well as the neutralization of the acidity of the sulfuric medium, with the consequent formation of water which will allow diluting the resulting mixture. In the case of some metals such as zinc -Zn- and aluminum -Al-, it will be necessary to control the pH for the precipitation to be effective. Once all the toxic metals in solution have been removed, they will be removed as hydroxides. The resulting potassium sulfate solution - K2SO4, can be used as fertilizer when diluted in irrigation water. Subsequently the solid residue

15 from the incineration is subjected to a stream of carbon dioxide -CO2- in an ash combustion furnace (5) where carbon debris is removed producing a carbon monoxide stream -CO- that can be stored. The result (6) is the carbon monoxide stream -CO- and a mixture of basic metal oxides from which the metals can be extracted.

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Claims (1)

image 1 1.- The biomass that has captured during its vegetative growth heavy metals from contaminated soils, it must be subjected to a phytoremediation process, to recover the metals and avoid environmental pollution, which is the object of the invention, whose technical characteristics are the bubbling in an aqueous solution of concentrated sulfuric acid from effluents and metals from the combustion of biomass. As a result of the sulfuric acid solution, sulfates are formed with respect to each metal, which, when soluble, are dissolved in the solution. By basifying the solution with potassium hydroxide, precipitation of metals, such as hydrated oxides, occurs. In the solution is the potassium sulfate - K2SO4 - which is a fertilizer that is diluted in irrigation water. The characterizing part whose technical characteristics are characterized by passing volatile metals through the concentrated sulfuric acid solution. Among the elements that can be found in these generated fumes are: aluminum –Al-, nickel –Ni-, copper –Cu-, zinc –Zn-, cadmium –Cd-, mercury -Hg- and lead –Pb- All metals they remain as sulfates, with the exception of lead, which could precipitate as hydrogen lead sulfate (II) - Pb (HSO4) 2- according to the process: PbSO4 + H2SO4 → Pb (HSO4) 2 When the sulfates reach a sufficient concentration, the solution is basified with potassium hydroxide -KOH-. When basifying the medium with potassium hydroxide, the precipitation of nickel –Ni-, copper –Cu- and mercury –Hg- will take place as hydrated oxides. In addition, all acidity of the sulfuric medium must be neutralized by the reaction: 2 KOH + H2SO4 → K2SO4 + 2 H2O The reaction that takes place between nickel metal sulfates –Ni-, copper –Cu-, zinc –Zn-, cadmium –Cd-, mercury -Hg- and lead –Pb- with potassium hydroxide is: MSO4 + 2 KOH → M (OH) 2 + K2SO4 It will be necessary to control the pH, which cannot be higher than pH 11, so that zinc precipitation - Zn - is effective, since an amphoteric hydroxide is formed that is redissolved in excess of basic medium. In the case of aluminum -Al- it would precipitate according to the reaction: 6 KOH + Al2 (SO4) 3 → 3 K2SO4 + 2 Al (OH) 3 Cadmium -Cd- would require an additional process, such as an electro-reduction, given the solubility of cadmium hydroxide -Cd (OH) 2- which is two hundred sixty milligrams per liter at twenty degrees Celsius. image2