WO2000013780A1

WO2000013780A1 - Method for converting siliceous base particles into granular or monolithic forms

Info

Publication number: WO2000013780A1
Application number: PCT/EP1999/006375
Authority: WO
Inventors: Francesco Corigliano; Letterio Mavilia
Original assignee: Francesco Corigliano; Letterio Mavilia
Priority date: 1998-09-03
Filing date: 1999-08-30
Publication date: 2000-03-16
Also published as: ITMI981955A1; ITMI981955A0; IT1302038B1

Abstract

A method for converting siliceous base particles into granular or monolithic forms, comprising a step for aggregating the particles which provides a mass having a plastic consistency through the interaction of the siliceous base particles with an aqueous solution which comprises an alkaline metal hydroxide and a step for evaporating the water of the mixture, followed by a treatment for baking the mixture at a temperature which is capable of producing permanent bonds among the siliceous base particles.

Description

METHOD FOR CONVERTING SILICEOUS BASE PARTICLES INTO GRANULAR OR MONOLITHIC FORMS Technical Field

The present invention relates to a method for converting siliceous base powders into granular or monolithic forms.

In particular, the present invention relates to a method for agglomerating natural or synthetic siliceous base fines into aggregate forms of the same material. Background Art It is known that granular forms are preferred to fine-powder forms in the field of the treatment of inert materials and particularly in the production of abrasive agents or in the heterogeneous catalysis and treatment of polluting or polluted powders to render them inert.

It is also known that the specialized literature does not include strong bases among inorganic binding agents (gypsum, lime, cement, clay, magnesium oxide and chloride, silica, alkaline silicates). Disclosure of the Invention

The aim of the present invention is to provide aggregates of siliceous base particles which are highly compact and have high resistance to disaggregation.

Within the scope of this aim, an object of the present invention is to provide a method which allows to recover and reuse, for example in the field of abrasives, siliceous base materials meant for discarding and disposal due to their excessively fine dimensions. Another object of the present invention is to provide aggregates of siliceous-vitreous particles by generating around them a film of binding agent which is capable of producing cohesion forces which are sufficient to keep them firmly joined.

Another object of the present invention is to provide a method for producing agglomerates of siliceous-vitreous particles which can be easily reproduced, entails low process costs and uses easily commercially available reagents.

In view of this aim, these objects and others which will become apparent hereinafter, a method is provided, according to the present invention, for converting siliceous base particles into granular or monolithic forms, said method comprising the steps of:

— preliminary aggregation of the siliceous base particles, wherein a mass which has a plastic consistency is provided;

~ evaporation of the water of the mixture, which preferably occurs at a temperature of 70-120°C; and

~ baking treatment of said mixture at a temperature which is capable of producing permanent bonds among the siliceous base particles, said method being characterized in that during said preliminary aggregation step the siliceous base particles are contacted with an alkaline aqueous solution. The alkaline aqueous solution used in the method according to the invention comprises a hydroxide of an alkaline metal, preferably at a concentration between 2 and 19.

According to one embodiment of the present method, said alkaline solution further comprises a suspension of an alkaline-earth metal, preferably in an amount of no more than 40% by weight.

The Applicant has now found that it is possible to provide an effective aggregation of the siliceous base particles by using a hydroxide of an alkaline metal which has been found capable of generating a binding agent, interacting effectively at low temperatures, preferably between 15 and 100°C, with the particles to be agglomerated.

The use of an alkaline metal hydroxide, with preference for sodium hydroxide, in the execution according to the present invention of the aggregation step of the method, leads to the formation of intense cohesion forces among the siliceous base particles. In particular, the highest cohesion forces among the siliceous base particles occur by using a solution of an alkaline metal with a concentration of 2-19 M, more preferably 5-10 M.

According to another embodiment of the method according to the invention, an amount of alkaline solution equal to 15-30% by weight with respect to the weight of the siliceous base particles is added; an amount between 20 and 30% by weight is particularly preferred.

The alkaline solution comprising an alkaline metal hydroxide used in the agglomeration step can be partially replaced by a suspension of an alkaline earth metal hydroxide and can be used at ambient temperature or heated to 100°C.

It has been observed that by adding a calcium hydroxide suspension, preferably at a concentration of no more than 40% by weight, an agglomerate with higher hardness and abrasiveness characteristics is produced. Within the scope of the present invention, the expression "siliceous base particles" includes natural or synthetic powder-like materials containing silica and having a siliceous-aluminous base.

Particles particularly adapted for being subjected to the method according to the invention are in the form of fine powder with a particle size of 10 to 250 μm.

Larger particles can nonetheless be useful in the aggregation mode provided by the present method, in monolithic forms for uses, for example, in the field of building materials.

By way of example, materials that can be used in the method according to the invention are the ashes from coal-fired electric power stations or from incinerators of municipal solid waste (MSW) or solid residues of productive industrial activities such as mining, extraction, metallurgy, ceramics, etcetera. In particular, it is possible to subject to the method according to the invention natural materials such as pumices, tufas, opals, diatomaceous earths, obsidians, perlites, geyserites, tripolis, clays, lagoon sludges, as well as artificial materials such as coal ash, MSW incinerator ash, ash from cogeneration plants supplied with wood waste, common glass powder, metallurgical slag, silica gels, filterpressed sludges from conditioning plants, mining powders, etcetera. The siliceous base powder aggregative step is preferably performed by using rotary or press machines.

Agglomeration by rotation advantageously provides for the use of devices of the type with a rotary container, such as an inclined disk, inclined cone and rotating drum. Agglomeration under pressure provides, for example, for the use of devices such as piston presses, hydrostatic presses, roller presses and extrusion presses.

According to a preferred embodiment, the siliceous mass having a plastic consistency is given a preset shape before being subjected to the drying step.

The intermediate drying step, which occurs at a temperature lower than the subsequent baking temperature, leads to the substantially complete evaporation of the water trapped during the agglomeration step. Permanent consolidation of the cohesion bonds between the individual siliceous base particles, or with particles of other materials if inert materials of various kinds are used, is performed during the baking step.

The baking times and temperatures are adjusted according to the chemical, physical and mechanical characteristics that the finished product must have. In general, preference is given to temperatures between 200 and 800°C and more preferably between 300 and 700°C, with times advantageously between 1/2 and 3 hours, making sure that the temperature increases and the increases in the baking times synergistically contribute to increasing the hardness, wear-resistance and compression-resistance of the agglomerate. The following examples are provided merely by way of illustration of the present invention and must not be construed as limiting the scope of said invention, such scope being as defined by the appended claims.

EXAMPLE 1 Formation of granules with soft-abrasive properties from pumice powder. 200 g of fine pumice powder (humidity content about 1%, fraction passing through a screen with 75μm holes ≥ 80%) originating from the grinding of waste fragments and meant for discarding were placed in a laboratory mixer with a capacity of 1 liter. 50 g of an aqueous solution at 17.5% by weight of sodium hydroxide was added, and the mixture was agitated for 10 minutes at about 100 rpm in order to uniformly distribute all the liquid in the solid. The mixture was then transferred into a rotary granulator with a capacity of 2 liters, where, due to the centrifugal force applied to it by rotation at about 600 rpm, after 20 minutes, a granulate constituted by spheroidal particles with a variable diameter in the range of 0.2 to 2.0 mm was produced. At this point the granules are already able to maintain the shape acquired by rotation and to be free from intergranular adhesion.

The granulate was finally subjected to a thermal treatment aimed at consolidating the shape of the individual granules and at developing strong and durable cohesion forces among the particles that constitute the individual spheroids. For this purpose, the granulate was conveyed into a rotary electric oven with a countercurrent flow of air which is heated to about 600°C in the central part of the oven (transit time in this region > 1/2 hour) and preheats the granulate and cools the baked granulate in the end regions.

The resulting granular material has abrasive properties which are extremely similar to those of natural pumice granulated by grinding from waste fragments. However, for an equal abrasive power, artificial granules more or less hard with respect to the natural ones can be obtained by increasing or respectively decreasing the baking times and temperatures in the range of 20 to 120 minutes and of 400 to 800°C. In any case, about 200 g of granular pumice aggregates with a bulk density in the range between 1.4 and 2.0 g/cm³ and a porosity between 25 and 40% leave the electric oven. EXAMPLE 2

Formations of building materials from pumice mine waste. 250 g of pumice, waste from the extraction process of a mine (run-of- mine fraction, produced by separating the pumice fragments larger than 2 mm and constituted by pumice which is highly impure for mineral particles having a crystalline structure and a dark gray color and a humidity content between 18% and 22%), were introduced in a laboratory mixer with a capacity of 1 liter. 4 g of powdered calcium hydroxide and 8 g of sodium hydroxide solution at 50% by weight were added thereto. After activating agitation at about 100 rpm, the result after approximately 15 minutes was a uniform mass with such a plasticity as to be able to durably acquire the chosen shape and dimensions. For this purpose, the mass was transferred into a small laboratory extruder and drawn in the form of parallelepipeds measuring 8 x 4 x 2 cm. Said parallelepipeds were dried in an air-circulation stove for 4 hours at 70°C and then baked at 700°C for 2 hours in an electrically heated laboratory oven. Products with a total weight of about 206 g, with a specific gravity between 1.3 and 1.5 cm³/g and with a compression resistance of 130 to 180 kg/cm² were obtained. These mechanical and physical characteristics are not very different from those of the more common aggregates used for building work. EXAMPLE 3

Formation of granules with abrasive properties from waste glass powder. 200 g of mixed-color waste glass powder (obtained from glass broken up by grinding to less than 100 μm) were placed in a laboratory mixer with a capacity of 1 liter. 52 g of an 18% aqueous solution of sodium hydroxide, preheated to 80-90°C, were added to the powder and the entire system was agitated for 10 minutes at about 100 rpm in order to uniformly distribute all the liquid in the solid. The mixture was then transferred into a rotary granulator with a capacity of 2 liters, where, due to the centrifugal force applied to it by rotation at about 500 rpm, after about 30 minutes, a granulate constituted by spheroidal particles with a variable diameter in the range of 0.2-1.2 mm was produced.

The resulting granulate was then baked in the manner described in example 1.

The resulting material had semi-soft to hard abrasive properties as a function of the temperature (400-800°C) and time (20-120 minutes) used for baking.

EXAMPLE 4

Formations of materials for building work from coal ashes and/or ashes from municipal solid waste incinerators. 250 g of fly ash of coal or bottom ash of coal or ash from a MSW incinerator or a mixture thereof were placed in a laboratory mixer with a capacity of 1 liter. 46 g of a 22% aqueous solution of sodium hydroxide (40% of which can be advantageously replaced with a limewater suspension at the same concentration) were added to the ash (or mixture of ashes). After activating agitation at about 100 rpm, a uniform mass with a plasticity which allowed it to be durably given the chosen shape and dimensions was obtained after 25 minutes. For this purpose, the mass was transferred into a small laboratory extruder and drawn in the form of parallelepipeds measuring 8 x 4 x 2 cm. Said parallelepipeds were dried in an air-circulation stove for 4 hours at 70°C and then baked at 650°C for 2 hours in an electrically heated laboratory oven. Products with a total weight of approximately 250 g, with a specific gravity between 1.0 and 1.4 g/cm³ and with a compression resistance of 90 to 120 kg/cm³ were obtained. These physical and mechanical characteristics are not very different from those of the most common aggregates used for building work. The disclosures in Italian Patent Application No. MI98A001955 from which this application claims priority are incorporated herein by reference.

Claims

1. A method for converting siliceous base particles into granular or monolithic forms, said method comprising the steps of:

ΓÇö aggregation of the siliceous base particles, wherein a mass which has a plastic consistency is provided;

ΓÇö evaporation of the water of the mixture; and

ΓÇö baking treatment of said mixture at a temperature which is capable of producing permanent bonds among the siliceous base particles, said method being characterized in that during said aggregation step the siliceous base particles are contacted with an alkaline aqueous solution.

2. The method according to claim 1, characterized in that said alkaline aqueous solution comprises an alkaline metal hydroxide.

3. The method according to claim 2, characterized in that said alkaline metal hydroxide has a concentration between 2 and 19 M.

4. The method according to claim 2 or 3, characterized in that said alkaline solution further comprises a suspension of an alkaline-earth metal hydroxide.

5. The method according to claim 4, characterized in that said suspension of an alkaline-earth metal hydroxide is present in an amount of no more than 40% by weight of said aqueous solution.

6. The method according to any one of claims 1 to 5, characterized in that said aggregation step occurs at a temperature between 15 and 100┬░C.

7. The method according to any one of claims 1 to 6, characterized in that said alkaline aqueous solution is added in an amount between 15 and 35% by weight with respect to the weight of the siliceous base particles.

8. The method according to any one of claims 1 to 7, characterized in that said baking treatment occurs at a temperature between 200 and 800┬░C.

9. The method according to any one of claims 1 to 8, characterized in that said baking treatment lasts between 1/2 and 3 hours.

10. The method according to any one of claims 1 to 9, characterized in that said siliceous mass with plastic consistency is given a preset shape before being subjected to the evaporation step.

11. The method according to any one of claims 1 to 10, characterized in that said siliceous base particles have dimensions between 10 and 250 ╬╝m.

12. The method according to any one of claims 1 to 11, characterized in that said aggregation step occurs in an apparatus provided with a rotary container in order to produce granular forms.

13. The method according to any one of claims 1 to 11, characterized in that said aggregation step occurs in a press in order to obtain monolithic forms.

14. The method according to any one of claims 1 to 13, characterized in that said evaporation step occurs at a temperature between 70 and 120┬░C.

15. An article obtainable from the method according to any one of the preceding claims 1 to 14.