COMPOUNDS. AND ELIMINATION OF POLLUTANTS

The present invention relates to a process for the recovery of water contaminated by organofluorine compounds, comprising steps involving filtration, concentration, reverse osmosis and heat treatment of the concentrated fraction of organofluorine compounds.

Prior art

One of the most serious problems faced by companies that use non-biodegradable organofluorine compounds, in particular perfluoroalkyl compounds, is pollution of groundwater and aquifers by said compounds.

The authorities, public opinion and the legislation are paying increasing attention to the problem, because of the known health risks of some of said compounds.

The problem has been known for some time, and some of said pollutants are suspected of being hazardous to the health; they also present high bioaccumulation and persistence in the environment, and can exercise biological (hormone-like) activity, even at very low concentrations.

However, many of said substances are very important in manufacturing cycles, and are irreplaceable intermediates of various processes. The textile, galvanising, tanning, pharmaceutical and paper industries, as well as manufacturers of organofluorine compounds, therefore need to solve the problem permanently at a sustainable cost, by recovering all or part of the water cleansed of said pollutants.

The constant search for the most sustainable method that could lead to an environmental improvement in industrial wastewater has produced a number of solutions.

For example, WO2017131972 discloses a process of decontamination of water or soil contaminated by perfluoroalkyls and polyfluoroalkyls involving treatment with reagents such as persulphates, oxygen, ozone and phosphates.

The oxidative, destructive approach with said reagents is very expensive and not very effective from the environmental standpoint, since it still produces a saline waste which must be further managed.

Treatment with activated carbons or zeolites is described in Chemosphere Volume 72, Issue 10, August 2008, 1588-1593, and in WO2018102866. Here again, the materials used are too expensive for industrial applications, and technically disadvantageous. Where other organic pollutants are present, the economic impact of the cost of regenerating activated carbons can be very high, and water full of organic substances like that produced by tanneries or textile factories can rapidly exhaust the carbons, requiring expensive regenerations.

Water treatment processes based on ion exchange (EP2102113; EP1314700; EP2431334), electroflocculation (US 9,957,172) and anodic oxidation (WO2010102774; WO201835474) methods have also been disclosed.

The complexity of the solutions described to date does not fully meet the need for a versatile pollutant-removal process which is well suited to wastewater treatment, giving excellent results with a variable input concentration and the presence of other pollutants.

There is also a particularly strongly felt need for a process that can be applied to different types of waste, even in the presence of solid bodies, foreign bodies and other pollutants, and produces water so pure that it can be reintroduced into the manufacturing cycle or discharged without problems to industrial water treatment plants.

Description of the invention

It has now been found that the advantages and purposes described above can be achieved by a process for the recovery of water contaminated by organofluorine compounds which comprises:

a. Separation of solids in suspension in the water, with optional treatment with acids or bases before or after said separation;

b. Concentration of the clarified water from step a) in a multi-effect concentration system;

c. Treatment of the concentrated water from step b) in a reverse osmosis plant to give a concentrated fraction of organo fluorine compounds and a fraction of recovered water;

d. Heat treatment of the concentrated fraction of organofluorine compounds.

The process of the invention is unexpensive and sustainable and produces very small amounts of wastewater, with a consequent environmental improvement.

The process is applicable to water originating from facilities or consortiums operating in various industries that use PFAs, such as the textile industry, the tanning industry, the metallurgical/galvanising industry, the pharmaceutical industry and the chemical industry, and from civil and industrial water purification plants.

Said water can be treated by the process according to the invention either in situ with small-capacity units tailormade to their requirements, or on a centralised site able to treat water originating from various manufacturers.

Detailed description of the invention

Water polluted by PFAS, PFOS, PFOA or other compounds, selected at the starting plants on the basis of the concentration and origin of the pollutants, is sent to a suitable storage plant, for example by road transport (tanker) or through a sewer that collects the water and conveys it to the treatment plant.

Water is then suitably pretreated (step a) to remove foreign bodies, for example by screening or initial filtration. Solids are then removed from solution by centrifugation in a vertical centrifuge, by passing through a two- or three-phase decanter (if an organic substance lighter than water is present), or alternatively by cloth filtration.

A chemical treatment can also be performed by adding acidifying and basifying agents to precipitate some organic substances or inorganic salts which can optionally be separated and conveyed directly to the combustion stage (step d) before or after the separation stage.

Examples of said reagents include slaked lime, caustic soda, ammonia, sulphuric acid, hydrochloric acid, nitric acid, ammonium sulphate, acetic acid and phosphoric acid.

The wastewater, clarified in step a, is then reduced in volume (step b) to concentrate the solid or dry part, which thus acquires value in terms of heat value.

A multiple-effect (or step) concentration system is used at this stage.

Multi-effect evaporation is a low-pressure steam process wherein each effect allows steam already used in the previous step to be reused. Such reuse is possible if the uncondensed steam is used at a lower pressure in the next step.

In this way, the concentration operation is cheaper than using direct steam created by a thermal process with steam generators.

The thermal concentration system uses energy to evaporate the water, and then condenses it again.

Multiple-effect concentration is one of the most reliable, soundest and cheapest solutions in the industry, with capacities ranging from 600 to 25,000 rnVday per unit.

Its basic concept is a multi-effect process wherein a water spray is repeatedly evaporated and then condensed, with each effect taking place at a lower temperature and pressure. This highly efficient process multiplies the amount of clean water that can be produced with a given amount of energy, leading to a significant cost reduction.

The use of a multiple-effect evaporator provides the following advantages:

Safe, reliable, continuous operation;

Lower running and maintenance costs;

Environment-friendly solutions to minimise environmental impact;

Rapid installation time;

High availability and very high reliability for continuous operation;

A low-temperature process that minimises evaporation costs;

Customisation to provide cheap, efficient long-term performance: the modular design allows the number of effects in the process to be optimised to suit the specific plant environment and site conditions;

Narrowing and scale formation are prevented by large wetting areas in the heat- transfer pipes;

Reliable wetting operations with no obstructions due to the wide-bore design; Heavy metal ions are prevented from entering the plant by an inline ion trap.

The condensate obtained in step b is then treated in a reverse osmosis plant (step c) to recover the incoming per-/polyfluorinated substances and obtain water with qualities such as to be recoverable in industrial cycles.

The water can then be reused as industrial water in manufacturing cycles in the same industry or the same facility.

The residual fluorinated substances in the concentrated water are conveyed to a final thermal destruction or post-combustion plant (step d).

Said final treatment can be performed in an“in-flight gasification” plant or another type of thermal destruction unit that generates very little slag, fluorinated substances being absent or at any event trapped in“glassy” matrices with practically zero release.

The concentrated suspension of organofluorine compounds is pumped by a volumetric pump to the subsequent thermal destruction unit.

This step can be performed in a reducing environment (pyrolysis/gasification) or an oxidising environment (incineration), the former being preferred due to its greater simplicity in the purification of effluent gases.

The crucial parameter of the fluorinated organic compound destruction process is the temperature which, to guarantee their total breakdown, must exceed l,l00°C for a sufficient time, and in any event longer than 2 seconds (as specified in European Directive 2010/75/EU, art. 50, regarding incineration and co-incineration).

The destruction system in a reducing environment (pyrolysis/gasification) involves the use of plasma torches as a powerful energy source.

The plasma-producing gas can be air, oxygen-enriched air, an inert gas such as nitrogen or argon, etc.

The flow of a concentrated suspension of organofluorine compounds is injected into and nebulised in the plasma stream, which has a temperature exceeding 3000°C. In this way the water immediately evaporates. Moreover, due to the controlled injection of air for the purpose of partial oxidation of the organic substance, while keeping the reaction environment in reducing conditions, the temperature of the gas stream is maintained for a sufficient time at about l250°C, so that all the organic molecules break down into elementary, mainly inorganic molecules (H ₂ , CO, C0 ₂ ), with small fractions of short-chain hydrocarbons (CH ₄ , C ₂ H ₆ , C ₂ H ₄ , C ₂ H ₂ , etc.). Under these conditions, the organic fluorine produces hydrofluoric acid.

The gas is cooled by heat recovery and then purified in a wet scrubber (a Venturi scrubber or the like) operating with an alkaline solution, so that acid gases, primarily hydrofluoric acid, are removed in the form of saline solution. The saline water, after filtration, is disposed of as wastewater.

A final dedusting system, for example with a wet electrostatic filter, enables a combustible gas usabld for energy recovery to be obtained. In particular, the water vapour necessary for the operation of the other sections of the integrated process, schematically illustrated in Figure 1, can be produced.

Conversely, the destruction system in an oxidising environment (incinerator) uses combustion of an external fuel (usually natural gas) to maintain the desired temperature conditions in the combustion chamber (T>l l00°C).

Fuel combustion takes place in an industrial burner; the aqueous suspension is nebulised in the combustion chamber.

Under the effect of the high temperature, complete dissociation of the fluorinated organic molecules takes place, with the formation of hydrofluoric acid. Other acid gases (such as hydrochloric acid and hydrosulphuric acid), heavy metal salts and powders may be present in effluent gases, depending on the aqueous suspension fed into the incinerator.

The effluent gases are then dedusted with a suitable system (such as ceramic filters) and undergo heat recovery in a suitable regenerator.

In particular, the water vapour necessary for the operation of the other sections of the integrated process can be produced.

Before expulsion into the atmosphere, the effluent gases are purified in a wet scrubber (a Venturi scrubber or the like) operating with an alkaline solution to remove acid gases. The saline water, after filtration, is disposed of as wastewater, as schematically illustrated in Figure 2.

The systems described above can also be suitably combined so that other solid waste and other liquid waste can then be treated simultaneously. Solid waste containing heavy metals such as chromium is preferably added at the reducing stage, producing a gaseous waste that can be conveyed to the subsequent oxidative stage, whereas liquid waste can be suitably dispersed at the oxidative stage to produce energy, which is suitably recovered in a “cogeneration” system that produces electricity and steam, as schematically illustrated in Figure 3.

The plants used in these stages can be small, and therefore suitable for small local facilities wishing to solve their own wastewater problem, or large, possibly also using internal heat recovery systems, electricity and steam cogeneration systems, and integrated water treatment and recirculation systems.

Description of figures

Figure 1 illustrates the balance of a process according to the invention with a destruction system in a reducing environment.

Figure 2 shows the balance of a process according to the invention with a destruction system in an oxidising environment.

Figure 3 shows the balance of a process according to the invention with a destruction system in a reducing environment followed by a destruction system in an oxidising environment.

Figure 4 shows the balance of a process according to the invention applied to the tanning industry.

The invention is illustrated in greater detail in the examples below.

Example 1:

27,000 litres of water with a high content of perfluorinated polymers originating from industrial effluent (about 10,000 ng/litre of PFAS and COD of 5,000 mg/litre) are centrifuged with a pilot decanter at a flow rate of 1000 litres/h to separate the solids in suspension. The result is about 400 kg of sludge, with dry matter of about 30% w/w.

The solid-free water is concentrated in a pilot concentration unit such as a rising- film plate evaporator at the flow rate of 300 litres/hour of evaporated water.

540 kg of concentrate and about 26,000 kg of condensate are obtained.

The condensate is concentrated in a reverse osmosis plant at about 6 litres/minute until a further 310 litres of concentrate have been obtained. On exit from the osmosis plant the water has total PFAS concentrations below 1 ng/litre.

540 kg of first concentrate and 310 kg of reverse osmosis concentrate are combined and pumped into a pilot thermal oxidation unit at a temperature ranging between 1250 and l300°C to completely break down the perfluorinated substances and any other organic compounds that may be present.

The outgoing gas is analysed and treated with an absorber supplied with a dilute solution of sodium hydroxide before emission into the atmosphere.

Example 2:

20,000 litres of water with a high content of perfluorinated polymers originating from industrial effluent (about 14,000 ng/litre of PFAS and COD of 4,800 mg/litre) are centrifuged with a pilot decanter at a flow rate of 1000 litres/h to separate the solids in suspension.

The result is about 400 kg of sludge, with dry matter of about 30% w/w.

Said sludge is mixed with other solid waste:

400 kg of dry purification sludge

400 kg of non-hazardous waste from the leather industry (coloured trimmings and buffing dust).

The resulting paste is further dried in an airstream at 60°C for 24 hours to obtain a product with about 90% dry matter. 900 kg of dry sludge is obtained.

The solid-free water is concentrated in a pilot concentration unit such as a rising- film plate evaporator at the flow rate of 300 litres/hour of evaporated water. 440 kg of concentrate and about 19,100 kg of condensate are obtained.

The condensate is concentrated in a reverse osmosis plant at about 6 litres/minute until a further 480 litres of concentrate have been obtained. On exit from the osmosis plant the water has total PFAS concentrations below 1 ng/litre.

440 kg of first concentrate and 480 kg of reverse osmosis concentrate are combined to give a final concentrate rich in fluorinated polymers.

900 kg of dry sludge is fed into a pilot gasification plant based on the shaft furnace, using air as gasification gas. 920 kg of concentrate containing fluorinated compounds is injected into the crude syngas. After mixing, the crude syngas is mixed with a stream of air plasma to adjust the temperature of the gaseous mass to a value ranging between 1250 and l300°C, to completely break down the perfluorinated substances and any organic substance that may be present.

The outgoing gas is analysed and treated with an absorber supplied with a dilute solution of sodium hydroxide, and then burned in a torch before emission into the atmosphere.

Example 3: Industrial facility:

About 2400 tons/day of water with a high content of perfluorinated polymers originating from industrial effluent (about 7,000 ng/litre of PFAS and COD of 3,000 mg/litre) are centrifuged with 4 industrial decanters to separate the solids in suspension.

The result is about 2 tons/hour of sludge, with dry matter of about 30% w/w.

Said sludge is mixed with other solid waste:

2 tons/h of dry purification sludge

2 kg of non-hazardous waste from the leather industry (coloured trimmings and buffing dust).

The resulting mixture is further dried in an industrial flash dryer, obtaining about 3 tons/hour of powdery material with about 90% dry matter.

The solid-free water is concentrated in a multiple-effect concentration unit such as a rising- film plate evaporator at the flow rate of 100 tons/hour of evaporated water. 2.5 tons/hour of concentrate and about 95 tons/hour of condensate are obtained.

The condensate is concentrated in a reverse osmosis plant at about 2,000 litres/minute until a further 3 tons/hour of concentrate have been obtained. On exit from the osmosis plant the water has total PFAS concentrations below 1 ng/litre.

The 2.5 tons/hour of first concentrate and 3 tons/hour of reverse osmosis concentrate are combined to give a final concentrate rich in fluorinated polymers.

The 3 tons/hour of dry sludge, after suitable compacting treatment to increase the mechanical properties of the material, are fed into an industrial gasification unit based on the shaft furnace, using air as gasification gas.

The contaminated process gases originating from the drying and evaporation stages are also mixed into the gasification air.

The molten inorganic mass (about 1 ton/hour) is extracted from the bottom of the shaft furnace at a temperature of about l500°C, and rapidly cooled in a stream of water. Particles with an inert glassy matrix incorporating all the metal elements present in the mixture introduced in ionic form, and small flakes of metal alloy, mainly consisting of iron and chromium, solidify in said stream. After separation of the two types of material on the basis of their density, the inert glassy matrix can be sent for recovery as inert filler material, while the metal alloy is recoverable in the metallurgical industry.

5.5 tons/hour of concentrates containing the fluorinated compounds are injected into the crude syngas exiting from the top of the shaft furnace. After mixing, the crude syngas is mixed with a stream of air plasma to adjust the temperature of the gaseous mass to a value ranging between 1250 and l300°C, to completely break down the perfluorinated substances and any organic substance that may be present.

The crude syngas, after energy recovery of sensible heat, undergoes a process of dedusting and purification to remove gaseous components hazardous to the environment (such as acid gases and heavy metals). After purification, the syngas is used as combustible gas (about 10,000 Nm ³ /h is produced with a net heat value of about 1.4 kWh/Nm ³ ) for energy recovery in endothermic engines or steam generators, for the production of electricity, steam and hot water. Electricity and steam can be used in the operation of the industrial waste treatment plant, while any hot water can be used in district heating/air-conditioning plants.

The combustion gases, analysed with a continuous system after the optional step of reduction of nitrogen oxides, are expelled into the atmosphere.