WO2007104368A1

WO2007104368A1 - Method for treating the sludge produced by the stone cutting and polishing process

Info

Publication number: WO2007104368A1
Application number: PCT/EP2006/067286
Authority: WO
Inventors: José Leandro MARTÍNEZ CAMÚS
Original assignee: Management & Innovation Sl
Current assignee: Management & Innovation Sl
Priority date: 2006-03-14
Filing date: 2006-10-11
Publication date: 2007-09-20
Anticipated expiration: 2008-09-14
Also published as: ES2281295A1; ES2281295B1

Abstract

Includes the thermal drying of said sludge, using for this purpose thermal energy from cogeneration equipment, and is characterised in that the steam from the drying of said sludge is condensed by also using the thermal energy from said cogeneration equipment. This enables valorization of the sludges from the stone cutting and polishing process, all this with very high energy efficiency and nil environmental impact.

Description

METHOD FOR TREATING THE SLUDGE PRODUCED BY THE STONE CUTTING AND POLISHING PROCESS

The present invention relates to a method for treating the sludge produced by the stone cutting and polishing process that comprises thermal drying of said sludge .

BACKGROUND OF THE INVENTION

Sludges are waste products produced during an industrial process and/or a wastewater treatment process, basically characterised in that their composition presents a very high liquid fraction, often exceeding 80%, which hinders their handling and subsequent treatment.

The extraction and production industries for natural stone such as marble, granite and slate use a large flow of water to carry out the cutting and polishing of the stone. The abundant water used, together with the micronised mineral in suspension generated by the cutting and polishing process, give rise to a waste called sludge.

The sludge obtained from the natural stone cutting and polishing processes is treated physically and chemically to reduce its water content. However, given the nature of the solid material that the aforesaid sludge (micronised mineral) contains, the liquid fraction that is obtained from said physico-chemical treatment still contains a high proportion of mineral in suspension and dissolved salts that hugely hinders its reutilisation in the cutting and polishing process, since it has a highly adverse effect both on the useful life and the maintenance of the machinery used in the treatment facilities.

As regards the dewatered sludge fraction emerging from the aforesaid physico-chemical treatment, this is eliminated by transporting it to a landfill tip, with a very high environmental cost owing to the fact that it is technical-economically impossible separating the micronised mineral from the wastewater fraction that the said dehydrated sludge fraction still contains. Processes for the treatment of sludges are known, such as those for treating the sludges produced by wastewater treatment, which include a stage of drying said sludge with thermal energy in order to reduce its water content by vaporising the water into the atmosphere. Once the sludge is dry, i.e. once its humidity content has been reduced to below 10%, the sludge can be incinerated or processed in order to convert it.

The thermal energy required for carrying out the thermal drying of sludges can be obtained using cogeneration equipment that produces, and at the same time uses, both electricity and heat.

Indeed, sludge-treatment processes are known that make use of the thermal energy coming from the high- temperature (400⁰C to 600⁰C) combustion and post- combustion gases from the cogeneration equipment to carry out the drying of said sludges.

The cogeneration facilities known to date for the drying of sludges nevertheless have the disadvantage of having a very low output, and the products obtained from drying of the sludge need to go through subsequent conversion processes to allow them to be reutilised or taken advantage of. Furthermore, the environmental impact of such cogeneration facilities is usually high, particularly as regards odours and other toxic gaseous emissions, which leads to a high economic cost in impact- correction measures and, frequently, considerable public rejection by the inhabitants of zones close to the cogeneration facilities. DESCRIPTION OF THE INVENTION

The objective of the present invention is to resolve the disadvantages mentioned by developing a method for treating the sludge produced in the stone cutting and polishing process that has high energy efficiency and makes it possible valorization of said sludge.

In accordance with this objective, the method of the present invention includes thermal drying of said sludge, using for the purpose thermal energy provided from cogeneration equipment, and is characterised in that the steam from drying of said sludge is condensed, also by using for this condensation the thermal energy from said cogeneration equipment. Thanks to these characteristics, the method of the present invention has a very high energy efficiency, exceeding 95%, since in addition to the electrical energy produced during cogeneration, use is also made of practically all the thermal energy generated by the equipment that carries out said cogeneration.

Another advantage of the sludge treatment process itself lies in condensation of the steam produced by the sludge drying, enabling a by-product to be obtained which, in most cases, can be reutilised in industry without any need to be submitted to subsequent additional conversion processes .

Surprisingly, the process of the present invention has the advantage of enabling valorization of the sludges produced in the stone polishing and cutting process, all this with very high energy efficiency and nil environmental impact.

Preferably, said steam is condensed by means of absorption refrigeration equipment that uses said thermal energy . Thanks to the absorption refrigeration equipment, some of the thermal energy from the cogeneration equipment can be used directly to vaporise the refrigerant of said equipment and produce cold. Thus, with the thermal energy from the cogeneration equipment itself, the refrigeration energy needed to condense the steam from drying of the sludge is also obtained.

Advantageously, the thermal energy that said refrigeration equipment uses comes from the fluid employed to refrigerate the cogeneration equipment. Thus, the warm fluid from the refrigeration system for the components of the cogeneration equipment can be used as a source of heat for the absorption refrigeration equipment.

Preferably, said thermal drying is instantaneous, and the thermal energy used for the drying comes from the high-temperature combustion gases of the cogeneration equipment .

Instantaneous drying has the advantage of allowing a solid product of very low humidity content to be obtained, directly using for the purpose the hot combustion gases from the cogeneration equipment itself, which gases are injected into the dryer.

Advantageously, the temperature of the combustion gases can be increased by post-combustion of said gases through an additional supply of oxygen. This additional supply of oxygen can be provided either by injecting a mixture of air and oxygen or by injecting pure oxygen.

Post-combustion gases are thus obtained at an approximate temperature of 600⁰C that is very efficient for drying. Also preferably, said instantaneous drying is carried out using a turbodryer. Such an appliance combines turbulence and vacuum effects with high-speed displacement of moist and dispersed solid particles. The turbodryer has the advantage of enabling an intense drying action with minimum heating of the solid. Again preferably, prior to the thermal drying a stage is carried out in which the liquid fraction proportion of the sludge is reduced. Advantageously, said reduction of liquid fraction is carried out by means of a centrifuge that permits efficient separation of a large proportion of solid particles.

According to a preferred embodiment, said liquid fraction is treated in order to reduce its content of solids in suspension. Advantageously, said treatment is carried out by equipment that uses a filtering process.

According to the same preferred embodiment, the effluent proceeding from the filtering of said liquid fraction is treated in order to reduce its content of dissolved solids. Advantageously, said treatment is carried out by means of a thermal evaporation process that uses thermal energy from the cogeneration equipment.

Preferably, in the aforesaid preferred embodiment, the residual fraction of sludge from the treatment of said liquid fraction or effluent is recirculated for drying, thus once again avoiding generating a moist residue of high environmental impact.

Advantageously, said natural stone is marble or granite . Application of the process of the invention to the sludge from the cutting and polishing of the marble enables obtaining, on the one hand water free from materials in suspension and dissolved salts, which can be reutilised in the process without causing the machinery any kind of problem, and on the other hand dry calcium carbonate that can be reutilised as a raw material or additive in another type of industry.

In the present invention, sludge shall be taken to mean an inorganic substance proceeding from the stone cutting and polishing process, whose liquid fraction can be evaporated, totally or partially, by means of the thermal energy produced by cogeneration equipment.

Cogeneration is taken to mean the simultaneous production and use of electricity and heat, by means of cogeneration equipment with motor generators, gas turbines, steam turbines, fuel cells, or any other system or equipment capable of generating electricity and heat from a fuel .

DESCRIPTION OF A PREFERRED EMBODIMENT

Described below is a preferred embodiment of the method of the present invention applied to the sludge produced in the marble cutting and polishing process. This is an inorganic residue that contains water and calcium carbonate as its majority components.

In the embodiment described, the sludge produced by the process is entirely collected into a decanting silo, having been passed first through a spalling basket and lifting pumps. Said silo is equipped with a chemical additives preparation system (flocculants and/or coagulants) that are added to the sludge to improve the decantation efficacy.

At the output from the silo, the sludge has an approximate proportion of 80% water and 20% calcium carbonate. From the silo, the sludges are pumped to a centrifuge in order to reduce their proportion of water.

The liquid fraction of sludge from the centrifuge is subsequently treated by means of filtering equipment that allows the solids in suspension that this fraction still contains, basically calcium carbonate, to be removed. Downstream of the filtering equipment is a second piece of equipment that treats the effluent from the filtering stage by thermal evaporation in order to remove the solids or dissolved salts. This evaporation process employs the thermal energy of the hot water from the refrigeration system of the cogeneration equipment. The effluent from the thermal evaporation process is a high- quality condensed technical water that can be reutilised without the least problem in the cutting and polishing process itself.

In the embodiment described the residual fraction of sludge from the dual filtering and thermal evaporation treatment applied to the liquid fraction coming from the centrifuge is recirculated to the fraction of centrifuged sludge and taken, along with that fraction, to the installation where the instantaneous drying is carried out .

The instantaneous drying installation of the embodiment described comprises a turbodryer and a set of cyclones and sleeve filters.

The drying chamber of the turbodryer is fed with the fraction of sludge to be dried, the high-temperature combustion gases from the cogeneration motors and warm compressed air.

The sludge fraction is introduced into the dryer by means of spraying nozzles, in the case of suspensions and solutions, and by means of an alveolar valve or endless conveyor belt in the case of moist solids. The warm compressed air introduced into the turbodryer drying chamber has the main function of breaking down the sludge, thus increasing its surface area in order to facilitate the drying action with minimum heating of the sludge, which action is basically carried out by the combustion gases that are at an approximate temperature of 400⁰C.

As noted in the description of the invention, the temperature of the combustion gases from the cogeneration motors can be increased to approximately 600⁰C by means of an additional supply of oxygen and by carrying out post- combustion of the aforesaid gases. Said additional supply can be carried out by means of atmospheric air, an oxygen and air mixture or pure oxygen.

The turbodryer has a system of recirculation and control of aspiration of particles that permits the moistest particles and/or agglomerates to be sent back to the drying chamber. The dry and disintegrated particles of mineral material are taken along with the water vapour produced during the drying to the set of cyclones and sleeve filters, where they are separated from vapour.

The water vapour from the drying of the sludge is condensed in the condenser of the absorption refrigeration equipment that uses the thermal energy of the hot water (9O⁰C to 95⁰C) from the refrigeration of the cogeneration equipment motors. The result is a high-quality technical water that can be reutilised in the cutting and polishing process without causing any problem to the equipment, while at the same time permitting a major saving of the increasingly scarce resource that is water. As regards the dry micronised mineral from the dryer, which is calcium carbonate in the case of the embodiment described, this is graded in a classifier and, optionally, ground in order to obtain a uniform granulometry suitable for the requirements of the markets in which it has a high value as a raw material or as an additive (lime markets, cement industries, paper, paints, plastics, adhesives, sealants, pharmaceuticals and cosmetics, manufacturing of carpets, rubbers, construction pre-manufactures, agricultural use, and so forth) . Surprisingly, the method of the present invention enables valorization of the inorganic sludge produced in the stone cutting and polishing process, by means of a cogeneration appliance or system that provides the entire electrical and thermal/refrigeration energy requirements of the process and ensures total recovery of the components of said sludge.

The method of the present invention has the advantage of possessing very high energy efficiency, since practically all the energy of the fuel (natural gas, propane, butane, diesel, fuel oil, biodiesel, etc.) used in the cogeneration equipment is made use of in the form of electrical energy and thermal energy. The electrical energy generated sufficiently meets the process needs, while the thermal energy generated is basically used to carry out thermal drying of the sludge and to meet the needs of the absorption refrigeration equipment that condenses the steam produced by the process.

Again surprisingly, the method of the present invention has the advantage of enabling valorization of the components of the sludge produced in the stone cutting and polishing process.

The table inserted below shows by way of example the balance of materials of the method of the present invention applied to a marble cutting and polishing facility that produces some 12,000 Kg/h of sludge containing 80% water and 20% calcium carbonate.

As the aforesaid table shows, the method of the present invention enables recovery of all the components of the sludge, water and calcium carbonate. The conditions under which said components are recovered are optimum for ensuring that use is made of them.

Table 1: Balance of materials of the method of the present invention applied to a marble cutting and polishing facility .

CENTRIFUGING, THERMAL DRYING AND SLUDGE CONDENSATION

FILTERING AND EVAPORATION OF THE LIQUID FRACTION FROM THE

CENTRIFUGE

Claims

C L A I M S

1. Method for treating the sludge produced by the stone cutting and polishing process, which includes the thermal drying of said sludge, using for the purpose thermal energy from cogeneration equipment, characterised in that the steam from the drying of said sludge is condensed, also using for this process the thermal energy from said cogeneration equipment.

2. Method according to claim 1, characterised in that said steam is condensed by means of absorption refrigeration equipment that uses said thermal energy.

3. Method according to claim 2, characterised in that the thermal energy used by said refrigeration equipment comes from the fluid used to refrigerate said cogeneration equipment.

4. Method according to any of the preceding claims, characterised in that said thermal drying is instantaneous, and the thermal energy used for said drying comes from the high-temperature combustion gases from said cogeneration equipment.

5. Method according to claim 4, characterised in that said instantaneous drying is carried out by means of a turbodryer.

6. Method according to claim 1, characterised in that prior to the thermal drying, a stage is carried out in which the proportion of liquid fraction of said sludge is reduced.

7. Method according to claim 6, characterised in that said reduction of liquid fraction is carried out by means of a centrifuge.

8. Method according to claim 6, characterised in that said liquid fraction is treated to reduce its content of solids in suspension.

9. Method according to claim 8, characterised in that the effluent from the treatment of said liquid fraction is treated to reduce its content of dissolved solids .

10. Method according to claim 9, characterised in that said treatment to reduce the content of dissolved solids is carried out by means of a thermal evaporation process that uses thermal energy from the cogeneration equipment .

11. Method according to either of claims 8 and 9, characterised in that the residual fraction of sludge from the treatment of said liquid fraction or effluent is recirculated to be dried.

12. Method according to claim 1, characterised in that said sludges are residues produced during the marble cutting and polishing process.

13. Method according to claim 1, characterised in that said sludges are residues produced during the granite cutting and polishing process.