ITMI20101443A1 - PROCESS FOR THE TREATMENT OF A MATERIAL CONTAINING ASBESTOS - Google Patents

Description

"PROCESS FOR THE TREATMENT OF A MATERIAL CONTAINING ASBESTOS"

The present invention relates to a process for the treatment of a material containing asbestos, so as to transform the asbestos itself and allow a possible reuse of the products deriving from said treatment.

Asbestos (also known as Asbestos) is the trade name attributed to some natural minerals with a fibrous structure, belonging to the class of silicates. In modern times some of these minerals have been widely used for their excellent technological properties: they have good resistance to heat and fire, to the action of chemical and biological agents, to abrasion and wear, and have a high mechanical resistance. , good flexibility, bond easily with building materials and have good sound-absorbing and heat-insulating properties. For all these properties and the low cost, asbestos has been widely used in industrial products and applications, in buildings, in means of transport and in the home. In particular, the raw fiber was processed to obtain various products adaptable to multiple uses. In these products the asbestos fibers can be either free, or strongly or weakly bound. If they are weakly bound, we speak of friable materials, which can be crumbled with the only manual action due to the poor internal cohesion. If they are strongly bound, we speak of compact materials, which can only be crumbled into powder with the aid of mechanical machinery. The materials in friable matrix are certainly the most dangerous, as the fibers can be dispersed in the air with extreme ease and therefore be inhaled. Compact matrix asbestos, on the other hand, by its nature, does not tend to release fibers and a dangerous situation exists only if it is abraded, deteriorated or sawn.

There is a very large number of types of Asbestos Containing Materials (MCA) that have extremely varied and differentiated characteristics and uses. The American Federal Register has over 3,000 finished items that contain asbestos. FCMs can be classified into three categories:

(a) Surface materials: this includes MCAs sprayed or distributed by trowel on surfaces (load-bearing elements, walls, ceilings) for sound-absorbing, heat-insulating and decorative purposes;

(b) Thermal Insulation Materials: this includes MCAs used to prevent the formation of condensation in pipes, ducts, boilers, tanks and in various components of water cooling systems, but also in heating, ventilation and air conditioning systems. 'air;

(c) Various materials: all the other MCAs are grouped in this category, such as false ceilings, sheaths, fabrics, etc.

The most widespread use of asbestos certainly affects the construction sector, in particular in the form of composite of asbestos itself with cement, the so-called asbestos-cement. Furthermore, in order to avoid or limit damage to structures in the event of a fire, asbestos has been widely used as a covering through spraying and troweling techniques on beams or floors. The heat-resistant mixture was composed of variable percentages of asbestos and other materials (vermiculite, sand or cellulose fibers) and binder materials (gypsum and / or calcium carbonate): the result was a continuous and soft to the touch layer of varying color from gray dark to white. Asbestos minerals were used as additives in cement conglomerates to improve their mechanical characteristics: usually the phases were Portland cement, water, aggregates, Chrysolite fibers, Crocidolite and / or Amosite (more rarely), until the only Chrysotile. The asbestos content was variable and could reach 50% by weight depending on the type of product to be obtained.

Today it is universally recognized that asbestos is one of the most dangerous materials for human health among those present in the living and working environments, a danger which translates into very serious diseases affecting mainly the respiratory system. Although an etiological link between the inhalation of airborne asbestos fibers and the onset of specific pathologies had already been hypothesized at the beginning of the last century, it was only in the 1990s that legislation consistent with the danger of material.

The assessment of the damage that this raw material caused to workers has forced governments of all nations of the world to tackle the problem, in consideration of the very significant social costs resulting from the occupational diseases developed by operators in the sector over the years.

It should be remembered that the accumulation of Waste Containing Asbestos (RCA) in landfills does not solve the problem, but simply passes it on to future generations: it is therefore important to study a strategy that allows to transform the RCA and subsequently exploit them as materials in production. of new products, which are totally safe from an environmental point of view.

In addition to the "inertization" and "isolation" of RCAs, various processes suitable for their transformation are currently in use, with the aim of completely eliminating their danger. The "inertization" processes include conditioning processes in matrices of different nature that avoid the dispersion of the asbestos fibers in the environment, while the "transformation" processes intervene directly on the fibrous structure of the mineral itself, transforming it into other non-dangerous phases for human health.

The main transformation processes of RCA are based on chemical treatments by the action of acids and on thermal and mechano-chemical treatments, although biochemical and microbiological methods have recently been studied.

As regards acid treatments, various methods have been developed that involve the use of both organic and mineral acids for the transformation of RCA, obtaining recyclable and often reusable secondary materials in the ceramic industry. In particular, the effects of mineral acids, such as hydrofluoric, hydrochloric and sulfuric acids, but also the effects of organic acids, such as formic and oxalic acids, were studied.

As for heat treatments, asbestos materials are known to be unstable at high temperatures. Chrysotile, for example, has a tendency to lose hydroxyl groups at 500-600 ° C and to be transformed into a different inert mineral phase, Forsterite, which recrystallises at 820 ° C. The application of this principle allows to obtain inert materials from RCA, as such or ground, treated in ovens at a temperature of 800-950 ° C. Furthermore, if firing is preceded by a compacting of the material, the consequent disorientation of the crystals allows the final product to be used as an electrical insulator or refractory material. This process is called ceramization. It is also possible to carry out a vitrification of RCA through various processes that are based on the melting of waste containing asbestos, carried out with the addition of different additives in a wide temperature range (1300-1800 <0> C), followed by rapid cooling with production of inert material with an amorphous vitreous structure. However, this solving method requires a lot of energy to bring the melting furnaces to extremely high and constant temperatures.

In vitroceramization, on the other hand, the waste is melted at temperatures between 1300 and 1400 ° C together with particular additives, such as blast furnace slag or industrial sludge, forming a mixture with a high metal content. The resulting slag is crystallized at a controlled temperature: in this way products with very high mechanical resistance are obtained and are particularly suitable as coating and protection surfaces in the construction, mechanical and chemical industries.

Another technique is constituted by the so-called lithification, which is based on the fusion of RCA, deriving from the decoibentation of railway carriages, at a temperature of 1300-1400 ° C. Slow cooling determines the crystallization of pyroxenes, olivine and iron oxides. The final result of the treatment is the production of inert materials, which can be recovered for various applications.

As regards biological treatments, the microbiological action of mosses and lichens on various rocky substrates containing asbestos fibers has been studied both in vivo and in vitro: the hyphae of lichens and fungi are able to penetrate and secrete chemical compounds (oxalic acid is one of the primary metabolites), some of which can alter the mineralogical structure of asbestos fibers (see for example the article by S.E. Favero-Longo, M. Girlanda, R. Honegger, B. Fubini, R. Piervìttori; Mycological Research, Vol. Ili, Issue 4, pp. 473-481 (2007)).

Microbiological methodologies have also been developed for the transformation of asbestos through the use of bacteria, in particular Lactohacillus casei and Lactobacillus plantarum (see for example the article by I.A. Stanik, K. Cedzyhska, S. Zakowska; Fresenius Environmental Bulletin, Vol. 15, Issue 7, pp. 640-643 (2006)). The method is based on the disintegration of the crystalline planes of Brucite (oxygen-magnesium) present within the crystalline planes of Chrysotile, due to the indirect metabolism of the bacterial cultures used. The decomposition of the crystalline planes seems to be due to the acidification of the reaction environment, thanks to the presence of metabolites secreted by the bacteria, including lactic acid. The hypothesized reaction mechanism is achieved through a substitution of the Mg <2+> ions by the H <+> ions, present in strong excess. The magnesium thus released reacts with the lactic acid present to form soluble salts.

However, all the processes that use biochemical and microbiological methodologies have highlighted a low degree of transformation of the asbestos fibers, sometimes only superficially, without reaching a complete transformation: therefore these methods have not found up to now applications that can be proposed on a scale. industrial.

Unfortunately, the methods of transformation of materials containing asbestos (MCA) known to date have significant disadvantages. In particular, the acid treatment leads to the accumulation of a large quantity of waste products, also to be disposed of. Furthermore, it should be borne in mind that, for the treatment of millions of tons of RCA (the approximate estimate only for the Italian territory is between 20 and 30 million tons), it would be necessary to use huge quantities of reagents, which would entail environmental risks. not negligible and very high costs. As far as the heat treatment is concerned, the main disadvantage, in addition to the enormous energy demand to bring the ovens to very high and constant temperatures, is given by the fact that suitable systems are very expensive and therefore not very widespread in the area, so it is necessary transporting RCAs over long distances, with consequent environmental risks and logistical costs.

The Applicant has therefore set itself the problem of studying and developing a process for the treatment of materials containing asbestos in relatively mild conditions, which is capable of combining effectiveness with both ecological and economic sustainability.

The Applicant has thus considered it convenient to couple a heat treatment to an acid treatment in which the material containing asbestos in divided form is subjected not to the action of a common acid chemical reagent, but of an acid industrial waste product, namely the whey, which, in addition to creating an acidic environment, brings bacterial components that are believed to favor the attack of the material itself. After this treatment, which frees the asbestos fibrils from the matrix in which they are incorporated, the asbestos is then inerted through a high temperature and high pressure hydrothermal process. At the end of this process, a solid phase is obtained essentially consisting of silicates and oxalates together with an organic component resulting from the thermal transformation of the bacterial component, and a liquid phase which is rich in various metal ions (in particular magnesium, nickel, manganese, potassium and calcium), which can possibly be recovered through electrolytic processes.

The present invention therefore relates to a process for the treatment of a material containing asbestos, which comprises:

treat the material with whey so as to obtain an acid liquid phase and a solid phase containing asbestos;

subject the solid phase containing asbestos to a hydrothermal process at a temperature from 120 ° C to 250 ° C and at a pressure from 5 bar to 20 bar.

In a preferred embodiment, the whey is preferably made up of exhausted whey.

In a preferred embodiment, the hydrothermal process is carried out at a pH value from 1 to 7, more preferably from 2 to 6.

As regards the material containing asbestos, this can be any product that includes asbestos in fibrillar form, in friable matrices or in cementitious matrix, or even in compact polymeric matrices (generally synthetic or cellulosic polymers). If materials containing asbestos are used in compact non-cementitious matrices, a pre-treatment of the material that is able to free the fibers from the matrix itself is preferably carried out, as better illustrated below.

For example, asbestos-containing material can be:

(a) a surface covering material for the purpose of imparting sound-absorbing, heat-insulating, flame-retardant and / or decorative properties, in which asbestos fibers are mixed with inorganic components (e.g. silicates, gypsum and / or calcium carbonate) and applied by spraying or spatulating (for example to obtain fire barriers in ducts for electrical systems, or for the insulation of railway carriages, ships, buses);

(b) a material in the form of ropes, tapes or sheaths obtained by weaving asbestos fibers, generally mixed with other natural and synthetic fibers, which can be used for wrapping pipes to be thermally insulated or electrical cables close to heat sources intense, or for the production of fireproof fabrics or with properties of resistance to the corrosive action of acids and bases.

(c) a material in which the asbestos fibers are included in a cellulose-based matrix, which by pressing allows to obtain sheets to be used, for example, as fire barriers or as coatings for supporting surfaces, cupels or fiber panels raw compressed, used for the insulation of pipes for the transport of high temperature steam, or as industrial filters;

(d) a material obtained by mixing asbestos with cement (known as cement-asbestos), in which asbestos is present in variable quantities up to 50% by weight, generally between 15 and 20% by weight, compared to the total weight of the material, for the construction of building products (for example tiles, partitions, pipes, roofing elements, tubs, flat or corrugated sheets, etc.).

The aforementioned asbestos-containing materials are generally delivered as waste to be disposed of, and for their safe transport they are preferably wrapped with a protective film, which is preferably made up of a biodegradable polymer (for example starch derivatives, polylactic acid, polyhydroxyalkanoates).

In order to avoid the volatilization of the asbestos fibers, the material to be treated is preferably kept wrapped in said film also during the crushing process, which is carried out to obtain said material in divided form. The crushing has the purpose of obtaining fragments of variable size within wide margins, such as to make the material itself compatible with the specific plant used for the subsequent phases and to increase the exchange surface and therefore reduce treatment times. This size can therefore vary from micrometric dimensions to dimensions of a few centimeters, for example between 1 and 5 cm. The crushing process can be carried out with various types of machinery, depending on the specific material to be treated.

In a preferred embodiment, the crushing step is carried out on the material immersed in a tank containing the whey. In this way, the dispersion of asbestos fibers into the environment is avoided during crushing. This crushing can preferably be carried out by means of diamond cutters, which are able to work in immersion in whey.

In the event that the material to be treated includes asbestos in fibrillar form dispersed in a compact polymeric type matrix (generally synthetic or cellulosic polymers, for example paper or cardboard), before treatment with whey, the material containing asbestos is preferably subjected to a heat pre-treatment in order to cause the combustion of the polymeric matrix and free the asbestos fibrils.

As for whey, as is well known, this is made up of the liquid part that separates from the curd following the coagulation process of the caseins present in the milk. It is therefore a by-product of the production of cheese or casein from milk. Whey can be considered as the aqueous phase of milk. This phase is formed by all the substances dissolved in water (including soluble proteins), whatever their molecular size, and by substances with low molecular weights. The composition of whey is very complex and mainly includes water, lactose, proteins, fats, vitamins. Thanks to the presence of high quantities of proteins and other nutritionally valid compounds, whey is further used in various ways, in particular for the production of cheeses and other dairy products. The liquid phase of waste that derives from this second process is the so-called "exhausted whey", which is considered a special non-hazardous waste. The following table shows a typical composition of used whey:

Average quantity

Present substances

(wt%)

Lactose 4.0 Ã · 4.6

Proteins 0,10 Ã · 0,15

Fats 0,15Ã · 0,30

Salts 0,9Ã · 1,1

Organic acids 0,20 ÷ 0,25

PH 3,5Ã · 5, 2

It should be noted that the used whey can have a variable composition according to the specific processes carried out within the dairy, which are different from product to product and also have seasonal variability.

It should be emphasized that exhausted whey, despite being completely free of toxic agents, cannot in any case be discharged directly into water bodies due to its high organic content and the presence of a complex bacterial flora. For the same reason, its treatment using the classic biological purification systems turns out to be remarkably difficult and expensive. A particularly innovative and economically relevant aspect of the present invention is therefore precisely that of providing a process that allows not only to treat waste containing asbestos so as to make it no longer dangerous for health, but also to dispose of large quantities of whey. of milk, and in particular of used whey, which is a serious ecological problem for the dairy industry.

Thanks to the acidity of the whey, and in particular of the exhausted whey, having pH values generally from 3.5 to 5.5, and to the concomitant presence of bacteria (in particular Lactobacilli), the cement matrix in which the asbestos in the form of fibrils is gradually transformed with the production of carbon dioxide, so as to release the asbestos fibrils, which remain in suspension in the liquid inside the tank where the treatment is carried out.

In the event that the pH of the whey is not sufficiently acidic for the optimal realization of the process, it is possible to add the whey with at least one organic acid, in particular lactic acid or oxalic acid, so as to bring the pH back to a value between 1 and 3.

Preferably, the quantity of whey used in the process according to the present invention can vary from 2 to 100 times by weight, preferably from 20 to 40, with respect to the weight of the overall material to be treated. The treatment with whey is carried out under stirring for a time ranging from 12 to 120 hours, more preferably from 48 to 72 hours, at a temperature from 20 to 90 ° C, more preferably from 30 to 50 <0> C.

During the treatment of MCA in a friable or cement matrix with the exhausted whey, carbon dioxide is formed, as a result of the action of the acidic environment on the carbonates present in the cement. This phase of the process is preferably carried out in a closed tank, so as to be able to collect and convey the carbon dioxide that develops in a plant that uses it without emission into the atmosphere: for example a plant for the production of methane or biogas or for the production of biopolymers (for example polyhydroxyalkanoates) through the use of specific cell cultures.

At the end of the treatment with the whey, the solid phase containing asbestos is conveyed, for example by suction, into a hydrothermal reactor (preferably in steel) where the hydrothermal treatment is carried out at a temperature from 120 ° C to 250 ° C, preferably from 160 ° C to 200 ° C, and at a pressure from 5 to 20 bar, preferably from 8 to 15 bar.

As indicated above, the hydrothermal process is preferably carried out at a pH value from 1 to 7, more preferably from 2 to 6. In the event that the pH of the solid phase is not sufficiently acidic for the optimal realization of the process, it is possible add to the reaction medium at least one organic acid, in particular lactic acid or oxalic acid, so as to bring the pH back to the desired value.

As regards the times of the hydrothermal treatment, the Applicant has found that prolonging this phase too much can lead to an unexpected increase in the concentration of asbestos fibers. This is attributable to the formation of chrysotile crystals, mainly during the prolonged phases of the process and therefore to insufficient transformation. To avoid the crystallization of fibrous chrysotile during the hydrothermal process, it is preferable to add at least one aluminum and / or iron salt to the reaction bath. It is believed that these salts are able to favor, in case of recrystallization of the asbestos during the treatment, the formation of Lizardite and / or Antigorite, instead of the unwanted Chrysotile phase.

The hydrothermal treatment is preferably carried out for a time ranging from 12 to 168 hours, preferably from 72 to 100 hours, and preferably not exceeding 120 hours.

Since the hydrothermal process takes place in the presence of water, the solid phase containing asbestos deriving from the first phase of the process is generally added with water or, preferably, whey which is able to bring the quantity of water necessary.

In order to favor the passage from asbestos to non-toxic inorganic compounds, during the hydrothermal treatment it is advisable to add at least one chelating agent, chosen for example from: oxalic acid, lactic acid, formic acid, or their salts.

The chelation of magnesium ions is believed to promote the flaking of the crystalline planes of chrysotile. Once the structural order that characterizes it disappears, asbestos (and in particular chrysotile) will no longer be able to package itself according to its peculiar basic structural units, changing phase.

The use of at least one chelating agent in this phase could also be useful to extract some important constituent and substitute elements of asbestos, such as magnesium, nickel and iron, which can be recovered for example through electrochemical processes.

As illustrated above, the process according to the present invention leads to the obtaining of various by-products which can be individually intended for various uses. In particular, we obtain:

1) an inorganic solid phase deriving from the hydrothermal treatment, which mainly comprises oxalates, carbonates and silicates, usable as starting material for the ceramic industry and inert inorganic matrices;

2) an organic solid phase deriving from hydrothermal treatment, consisting mainly of thermally treated bacterial residues, usable as an agricultural fertilizer with a high nitrogen content;

3) a liquid phase deriving from the hydrothermal treatment, consisting of an aqueous solution of metal ions particularly rich in magnesium, nickel, manganese, potassium, iron and calcium, from which the various elementary metals with high purity and with substantial economic advantage;

4) a gas phase deriving from the treatment of the material containing asbestos with the whey of exhausted milk, which is mainly constituted by C02, usable, for example, through the use of specific bacterial strains, for the generation of methane or biogas ( which can at least partially support the energy needs of the treatment plant of the material containing asbestos) and of polymeric material with high technological value.

The present invention will now be further illustrated by means of some embodiment examples, which are provided purely by way of example but not limitative of the scope of the invention itself.

EXAMPLE 1.

4 g of previously crushed asbestos cement sample was weighed. These were added to 90 mL of exhausted whey at pH 4.16.

The mixture was stirred at a temperature of 30 ° C for a total time of 80 hours. At the end of this treatment the pH was 5.82.

The resulting solid phase, after filtration, was placed in a hydrothermal reactor together with 60 mL of exhausted whey, at a temperature of 150 ° C, a pressure between 7 and 9 bar for 72 hours.

At the end of this second phase, the resulting sample contained some constituent elements of the cement matrix (calcium carbonate, quartz and anorthite), while the percentage by weight of asbestos fibers had significantly decreased, passing from 12% of the untreated sample to 2% of the solid residue at the end of the treatment.

EXAMPLE 2

3.5 g of previously crushed asbestos cement sample was weighed. These were added to 80 mL of exhausted whey to which 10 mL of 0.5 M lactic acid was added. The pH was found to be 3.56.

The mixture was stirred at room temperature for a total time of 60 hours. At the end of this treatment the pH was 5.12.

The resulting solid phase, after filtration, was placed in a hydrothermal reactor together with 50 mL of exhausted whey, at a temperature of 190 ° C, a pressure between 8 and 12 bar for 60 hours.

At the end of this second phase, the resulting sample contained some constituent elements of the cement matrix (calcium carbonate, quartz and anorthite), while the percentage by weight of asbestos fibers had significantly decreased, passing from 12% of the untreated sample to 5% of the solid residue at the end of the treatment.

EXAMPLE 3

4 g of previously crushed asbestos cement sample was weighed. These were added to 70 mL of exhausted whey to which 10 mL of 0.5 M oxalic acid was added. The pH was found to be 2.38.

The mixture was stirred at a temperature of 40 ° C for a total time of 48 hours. At the end of this treatment the pH was 6.45.

The resulting solid phase, after filtration, was placed in a hydrothermal reactor together with 70 mL of exhausted whey, at a temperature of 200 ° C, a pressure between 15 and 20 bar for 72 hours.

At the end of this second phase, a complete transformation of the cement matrix was noted, except for a slight percentage of quartz. The percentage by weight of asbestos fibers had significantly decreased, passing from 12% of the untreated sample to 2.5% at the end of the treatment.

EXAMPLE 4.

3 g of sample containing free asbestos fibers were weighed, quantified in a range from 10 to 14% by weight. The sample was added to 50 mL of exhausted whey at pH = 3.98 and was subjected to hydrothermal treatment at a temperature of 150 ° C, a pressure between 6 and 8 bar for 48 hours.

At the end of this treatment the percentage by weight of asbestos fibers had dropped to 2.5%.

EXAMPLE 5.

4 g of sample containing free asbestos fibers were weighed, quantified in a range from 10 to 14% by weight. The sample was added to 60 mL of exhausted whey and 10 mL of 0.5 M lactic acid for a pH value of 3.56. Subsequently, the sample was subjected to hydrothermal treatment at a temperature of 180 ° C, a pressure between 8 and 12 bar for 60 hours.

At the end of this treatment the percentage by weight of asbestos fibers had dropped to 4%.

EXAMPLE 6

4 g of sample containing free asbestos fibers were weighed, quantified in a range from 10 to 14% by weight. The sample was added to 70 mL of exhausted whey and 10 mL of 0.5 M oxalic acid for a pH value of 2.19. Subsequently the sample was subjected to hydrothermal treatment at a temperature of 150 ° C, a pressure between 6 and 8 bar for 72 hours.

At the end of this treatment the percentage by weight of asbestos fibers had dropped to 2.5%.

Claims (14)

CLAIMS 1. Process for treating an asbestos-containing material, which includes: treat the material with whey so as to obtain an acid liquid phase and a solid phase containing asbestos; subject the solid phase containing asbestos to a hydrothermal process at a temperature from 120 ° C to 250 ° C and at a pressure from 5 bar to 20 bar. 2. Process according to claim 1, wherein the whey consists of spent whey. Process according to any one of the preceding claims, in which the hydrothermal process is carried out at a pH value from 1 to 7, preferably from 2 to 6. Process according to any one of the preceding claims, wherein the asbestos-containing material includes asbestos in fibrillar form dispersed in a friable matrix or a cementitious matrix, or also in a compact matrix of the polymeric type. 5. Process according to any one of the preceding claims, in which the asbestos-containing material includes asbestos in fibrillar form dispersed in a compact matrix of the polymeric type and in which said material, before treatment with the whey, is subjected to a pre-treatment heat treatment in order to cause the combustion of the polymer matrix and release the asbestos fibrils. 6. Process according to any one of the preceding claims, in which the asbestos-containing material is previously subjected to a crushing step. 7. Process according to claim 6, in which the crushing step is carried out on the material immersed in a tank containing the whey. Process according to any one of the preceding claims, in which the whey is added with at least one organic acid, in particular lactic acid or oxalic acid, so as to obtain a pH value from 1 to 3. 9. Process according to any one of the preceding claims, in which the whey is used in an amount from 2 to 100 times by weight, preferably from 20 to 40, with respect to the weight of the material to be treated. 10. Process according to any one of the preceding claims, in which the treatment with the whey is carried out under stirring for a time ranging from 12 to 120 hours, preferably from 48 to 72 hours, at a temperature of from 20 to 90 ° C, preferably from 30 to 50 ° C. Process according to any one of the preceding claims, in which the hydrothermal process is carried out at a temperature of from 160 ° C to 200 ° C and at a pressure of from 8 to 15 bar. Process according to any one of the preceding claims, in which the hydrothermal process is carried out for a time ranging from 12 to 168 hours, preferably from 72 to 100 hours, and more preferably not exceeding 120 hours. 13. Process according to any one of the preceding claims, in which, before the hydrothermal process, water or, preferably, whey is added to the solid phase containing asbestos. 14. Process according to any one of the preceding claims, in which, during the hydrothermal treatment, at least one chelating agent is added, selected for example from: oxalic acid, lactic acid, formic acid, or their salts.