IL110300A - Process for fabrication of artificial lightweight aggregates and aggregates produced by this process - Google Patents

Description

Process for fabrication of artificial lightweight aggregates and lightweight aggregates, produced by this method rrcwy nniN m^O i tmsivnn o>v>n¾Nrri o»niDN>o o^ D'OVUN The Applicant :n\y 2Dn Golan Agregate Ltd. ¾x¾ rmn P.O. Box 12 Katzrin yrap 12.-T.n 12900 12900 *np>» The inventors: :o>N>i2»»n Brodski Monia LulavSt 23/10 23/10 ibn »rn Nazareth Elit 17000 17000 iv y tmn Dr. Wasserman Irene Π3 pnm "ΐ El Khansa St 2A 'N2 OM b> Til Haifa 32245 32245 na'n Dr. Sarkisov George (Uri) (niN) Ά Ν'λ HD'plD T1 Yeeron St 6 6 yiN*v »rn Katzrin 12900 12900 xnp Kashper Janneta no.N'r na¾>p Yeeron St 8 8 TI T 'rn Katzrin 12900 12900 ap Rosenshine Leonid T3wb pwm Bialik St 5 5 p> 2 'm Rishon-Lezion 75242 75242 s -iWNn Process for fabrication of artificial lightweight aggregates and lightweight aggregates, produced by this method.

Field of the invention The present invention relates to the field of manufacturing of artificial aggregates used for making structural lightweight concrete, particularly to fabrication of aggregates by processing of industrial wastes, produced by the combustion of fuel in power stations. h ^ ies ^^^W&lbn Tefers as well to lightweight aggregates, preferably having loose bulk density less than 1 t/m3, which being added to structural concrete enable significant reducing the total deadweight of structure without much loss in strength, with simultaneous improvement of insulating properties and ease handling.

Background of the invention The application of lightweight aggregates in the building industry offers many advantages both technically and economically. A number of methods has been elaborated to obtain lightweight concrete, e.g., blowing air into the concrete while it is setting.

However the most promising method is the use of lightweight aggregates as a filler within the concrete structure, and today there are known several types of aggregates, differing in accordance with their origin.

The following general types of aggregates can be mentioned: - natural aggregates of volcanic origin pumice, lava, pozzolanas, - artificial aggregates, manufactured from industrial by-products, produced by the processing of furnace clinker, cinders, foamed slag, sintered pulverized fuel ash, - factory-made special artificial aggregates, manufactured from expanded clay, shale or slate.

Unfortunately the natural volcanic raw materials are rare and their use as lightweight aggregates is uneconomical.

Expandable clays, shale or slate are suitable raw materials for manufacturing of lightweight aggregates, however these raw materials are not always available or, if available, their chemical and/or mineralogical composition does not allow manufacturing good quality aggregates. On the other hand modification of available local clays by appropriate additives is associated with high costs and does not justify industrial scale production.

Therefore, manufacturing of artificial aggregates by utilization of industrial by-products in combination with available inferior local raw materials, becomes an important technological route.

One of the most common industrial by-products is produced by combustion of coal in a power station.

This by-product is available in the form of bottom ash and fly ash and it will be explained further how lightweight aggregates of high quality can be produced by a simple, economical and reliable process, utilizing this waste material in combination with non-bloating lean clays.

There are known manufacturing processes in which combustion wastes, like fly ash, cinder ash, bottom ash, clinker ash and coal slag, are used in combination with expandable clay or expanded shale for fabrication of lightweight aggregates. See, for example, US4701222.

These processes usually comprise mixing appropriate amounts of fly ash, with clay, pelletizing this mixture and then firing of pellets at appropriate temperature. The fired pellets are then screened and stored in different particle size fractions.

The process disclosed in US4701222 is based on the utilizing of expandable clay or shale, having a high amount of S1O2 and of coal ash with a homogeneous composition.

The technological parameters of this process, like specific surface area of initial composition, mixing conditions, pelletizing conditions, and firing conditions are chosen accordingly, and enable fabrication of pellets with high crushing strength.

In particular, coal ash and clay should be thoroughly processed before mixing so as to have high value of specific surface area, and firing of pellets should be performed at temperature of 100" to 1600°C for 30 to 120 minutes.

The necessity for high specific surface area of ingredients and prolonged firing at relatively high temperatures is associated with high energy demand and makes this process costly.

Besides, these and other parameters employed in the known process cannot be directly employed for manufacturing of artificial aggregates from lean nonbloating clays.

Utilization of fly ash or bottom ash and nonbloating clay in the manufacture of a lightweight aggregate is more of an art than a science, since many factors associated with the nature and firing characteristics of clay and fly ash should be thoroughly considered before selecting the proper amount of raw materials and choosing the most suitable technological parameters for carrying out the process on a continuous basis and efficiently from the economic point of view.

It should be emphasized as well that most of the relevant technological parameters are interrelated, and once at least one of them has been improperly chosen, it will result in failure to carry out the whole process.

The other problem which is associated with utilization of inferior raw materials, especially of ashes and clays with inconsistent composition, is the necessity for strict homogeneity in composition of the initial mix so as to ensure continuous production during the further steps.

In order to maintain the required level of homogeneity, the blending and comminution procedure should be properly chosen and carried out.

These and other requirements do not allow direct adoption and/or theoretical optimization of technical parameters taken from known processes. There is still a need for a new, reliable, convenient and economical process, which will ensure manufacturing of good artificial lightweight aggregates from available raw materials, which have inferior and inconsistent quality.

Summary of the invention The object of the present invention is to provide a method for manufacturing artificial lightweight aggregates from the residual of coal combustion in power stations and from lean nonbloating clays, which allows for production of aggregates with properties that are not inferior to the properties of the aggregates produced from bloated clays.

In particular, the main object of the present invention is to provide a new and improved method for manufacturing of artificial lightweight aggregates, which ensures a high level of homogeneity of the initial mix and therefore enables continuous and reliable production, irrespective of the inconsistency in the chemical composition of the raw materials .

The second object of the present invention is to provide a new and simple method for manufacturing of artificial lightweight aggregates , employing relatively a short and low temperature firing procedure which does not demand a high consumption of energy and is , therefore , economical .

The third object of the present invention is to provide new and improved aggregates for use in lightweight structural concrete which has low loose bulk density combined with high crushing strength.

The above and other objects and advantages of the present invention can be achieved in accordance with the following combination of its essential features : a process for fabrication of artificial lightweight aggregates , preferably for use in lightweight concrete , the main component of said aggregates constituting industrial waste resulting from the combustion of fuel in power stations , whereas said method includes the following steps - preparation of a powder mix from raw material , comprising about 60- 90 weight parts of said industrial waste, selected from the group consisting of fly ash, Bo fom asK'~*0r Ifie'ir combination, -~ — — "about O- 40 weight parts of ordinary nonbloating clay, about 2- 15 weight parts of a bloating agent - pelletizing of said powder mix by an appropriate method, resulting in formation of uniform aggregates , - firing said aggregates at appropriate temperature, so as to cause their melting and bloating, characterized in that said raw materials are chosen so as to obtain the following composition of said mix: Si02 50-70 wt.% AI2O3 15-30 wt.% Fe203+Fe0 8-15 wt.% CaO+MgO 3-7 wt.% K20+Na20 2-6 wt.% Said mix is prepared by blending and comminution of said raw materials so as to achieve homogeneous powder mix, containing not less than 90% of fine particles with diameter less than 100 microns, and to said powder mix is added about 0.7-1.5 percent by weight of organic component, capable to provide green strength to said aggregates during pelletizing, and to be burnt during firing.

Conditions for firing of pelletized aggregates are set as follows: - heating up to firing temperature of 1100-1200 degrees C, - maintaining said firing temperature for a period of time required for melting and increase initial volume of aggregates, corresponding to bloating coefficient 2-4,5 - cooling down to room temperature with a temperature gradient, which is significantly less than temperature gradient during heating up, so as to create porous bodies having substantial compressive strength and loose bulk density, suitable for use in structural lightweight concrete.

In accordance with one of the preferred embodiments, preparation of said powder mix is carried out by simultaneous blending and comminution of said raw materials so as to obtain a powder mix, defined by specific surface area in the range of 2000-3000 cm2/g.

Said green aggregates are prepared by pelletizing substantially in a pan granulator, and are dried before firing so as to reduce their moisture content up to 1-8%.

According to another preferred embodiment, preparation of said powder mix includes the following steps: - wet ball milling of 30%- 507. aqueous suspension of said powder mix, - dewatering of said suspension, preferably by means of filter pressing so as to reduce the water content thereof up to 8-20%.

As per still a further preferred embodiment, said bloating component is a magnetic iron oxide and said organic compound is chosen from the group, including any used industrial lubricants, e.g., black mineral oils, grease, motor oil, or the like.

In another preferred embodiment of the present invention the obtained aggregates are characterized in that at least 70% thereof are defined by a size 5-14 mm, by bulk density 250-900 kg/m3 and by water absorption less than 10% (by weight) .

The present invention in its various embodiments has only been briefly summarized.

For better understanding of the present invention as well as of its advantages, reference will now be made to the following description of its embodiments, taken in combination with accompanying drawings, tables, and examples.

Brief description of the drawings Figs. 1 shows a general flow diagram illustrating the new process according to the present invention.

Fig. 2 illustrates a particular embodiment of the present invention, referring to blending and comminution step.

Detailed description of the preferred embodiments At the outset, the main steps associated with manufacturing of artificial lightweight aggregates according to the present invention as well as equipment required therefore will be described.

The present invention is a process for the manufacturing of artificial lightweight aggregates from raw materials, including a) nonbloating, lean, sinterable clay b) industrial waste, which is a residual, resulting from the combustion of fuel in power stations, preferably fly ash and/or bottom ash c) bloating agent.

Vith reference to Fig. 1 the main steps of this process comprise: - weighing of appropriate amounts of the above-mentioned raw materials, stored separately in an appropriate silo 1,2,3 so as to prepare the charge and pass thereof by the conveyor belt 4 and elevator 5 to an intermediate bin 6. If needed lumps can be disintegrated by breaker 7 and dried within dryer 70 before the charge is fed to a bin, - blending and comminution of the charge components within suitable equipment 8 so as to obtain a homogeneous powder mix, - addition of liquid organic component from container 9 to a powder mix and passing thereof by elevator 10 to intermediate silo 11. - pelletizing of powder mix within granulator 12 so as to convert it into green aggregates, - firing of green aggregates in kiln 13, cooling, screening and storing ready-for- se aggregates, with subsequent classifying in different size fraction within silo 14.

If needed, green aggregates can be dried before firing, e.g., by means of a belt dryer 15 and then transported by elevator 16 to sieves 17 for screening and storing in different size fractions in silo 18.

A heater 19 can also be provided for preheating of dried aggregates before firing.

The technological parameters for carrying out the above mentioned main steps will now be described in more detail.

These parameters are set in such a manner that aggregates with low loose bulk density and sufficient strength are obtained, despite the inferior properties of available raw materials, like inconsistency in their chemical composition and/or insufficient clay bloating ability.

Examples of chemical composition and some relevant properties of raw materials, suitable for carrying out the process in accordance with the present invention are given in non- limiting Table 1 below. As an example of suitable industrial waste, resulting from combustion of coal in power stations and to be utilized in accordance with the present invention one can mention residual, which is usually attributed to fly ash, bottom ash or their combination.

Table 1 As a suitable bloating agent one can use iron oxide powder, for example, available as a by-product of the operation of furnaces, used in iron manufacturing industry. The composition of this powder should contain 96-98% of Fe0+Fe203, in which Fe203 content is at least 707,.

It has been found that as an alternative to iron oxide powder one can advantageously use reject, produced by milling of a flint clay, providing it contains approximately 50% of Fe20V In accordance with the present invention particular amounts of the above raw materials, required for preparation of the charge are as follows: Clay about 10-40 parts by weight Industrial waste about 60-90 parts by weight Bloating agent about 2-15 parts by weight For those compositions, in which iron powder is employed as bloating agent, the amount thereof is 2-57., while in those compositions, which employ reject of flint clay milling, the amount thereof should be 5-15%, preferably 10%.

It has been empirically established that particular amounts of the above raw material components should be chosen in such a manner, that the charge will have the following chemical composition, given in weight percents: Si02 A1203 Fe203+FeO CaO+MgO K20+Na20 The charge is processed within a suitable equipment, so as to blend and comminute the components and to convert the charge into powder mix, defined by homogeneous composition and granulometry, irrespective of inconsistency of available raw materials.

It might be advantageous, if correction of the charge composition before or during blending is effected by addition of the appropriate component directly from the correction silo 60.

The most suitable way for processing of the charge is simultaneous blending and comminution thereof, for example, within a ball mill, which can operate in periodical or continuous mode.

Parameters of ball milling in terms of required capacity of mill, its diameter, charge: media ratio, rotation speed, duration of milling, etc. are chosen in conventional manner so as to obtain powder material, containing at least 90% of fine particles with diameter less than 100 micron, which corresponds to a specific surface area in the range of 2000-3000 cm2/gram.

Despite the fact that simultaneous blending and comminution can be carried out not only by ball milling but also by means of other equipment, suitable for this purpose, e.g., a vibrating mill, nevertheless, it might be particularly advantageous to employ wet ball milling to carry out this step, since it is associated with additional advantages like reduced consumption of energy, uniform and homogeneous dispersion of each of the ingredients within the resulting powder mix and elimination of dust, which might cause environmental pollution problems.

With reference to Fig. 2 a wet ball mill 80 is shown, in which the charge is fed from bin 6 and is wet milled in the form of an aqueous suspension having water content in the amount of 30-50 weight percents.

After completing the milling step, the obtained suspension can be homogenized, for example by means of a chain mixer 81, and then dewatered in a vacuum filter 82, so as to reduce its water content up to 12-187. before adding the liquid organic component from container 9.

It has been empirically established that it might be especially advantageous from an economic point of view, if as such organic additive is used an industrial organic waste, chosen from a group, including used industrial lubricants like black oil, grease, used motor oil, etc.

The organic component has a dual function: - it serves as a temporary binder, providing for green strength to aggregates, which are formed during pelletizing and - as a promoter for bloating, since it contributes to total porosity after being burnt out during the firing of green aggregates.

In order to fulfill these functions most efficiently, the amount of organic component in a powder mix should be 0.7-1.5 weight percents. It is recommended to add this component thereto as an aqueous solution or emulsion, having organic component: water ratio = 1:1.

Pelletizing of the resulting powder mix is carried out preferably within a pan granulator so as to obtain approximately spherical green aggregates. Parameters of the pelletizing procedure like pan diameter, rotation speed, inclination of the pan, and amount of organic component be chosen in a conventional manner so as to achieved the required capacity and required granulometrical composition of green aggregates. It might also be recommended to introduce an organic component in certain places of the pan during the pelletizing steps, instead of adding to it the powder mix.

In practice for obtaining fired aggregates with a 4.75-9.5 mm diameter, the appropriate parameters of pelletizing procedure should be set so as to obtain green pellets having a 3.5-6.0 mm diameter. These parameters are as follows: Pan inclination angle 42 degrees Pan rotation speed 11.5 rev/min The input feed 10 t/hr If more coarse 9.5-14 mm diameter fired aggregates are required, the appropriate parameters of pelletizing procedure should be set so as to obtain 6.90-8.0 mm diameter green pellets.

These parameters are: Pan inclination angle 37 degrees Pan rotation speed 10.0 rev/min The input feed 9.0 t/hr It should be pointed out that the pelletizing procedure can be carried out not only in a pan granulator, but also in any other suitable state-of-the-art equipment, e.g., an intensive mixer, fluidized bed granulator, etc.

The aggregates are usually dried after pelletizing at 90- 180°C within a belt drier 15 so as to reduce their moisture content up to 1-87. and then are screened up to required size fractions, to be fired within kiln 13. It might also be advantageous to preheat the aggregates before firing at approximately 300°C within heater 19. A rotary kiln 13, preferably operating in a direct mode is used for firing dried aggregates and converting them into porous bodies. Hot waste gases, generated during firing can be advantageously utilized for heating the drier 15 and heater 19, thus making the whole process more economical.

The firing schedule, which is most suitable for manufacturing of lightweight aggregates in accordance with the present invention includes: - fast heating with gradient 500-900°C/min, preferably 800°C/min, up to firing temperature of 1100-1200°C. - firing at this temperature for a period of time sufficient to melt and bloat aggregates, resulting in an increase of their initial volume, and - slow cooling with gradient 40-100 °C/min, preferably 50 *C/min up to room temperature so as to achieve porous strong aggregates having loose bulk density not more than 1000 g/cm3 combined with compressive strength, and thus enabling their use as an additive in constructional concrete structures.

The total time for completing the firing procedure does not exceed half an hour, if there is no preheating, or 35 min including thereof.

It has been found that by virtue of the above mentioned firing schedule, it is possible to perform the whole firing step very quickly and at the same time cause a melting of aggregates, accompanied by the formation of a viscous liquid capable of increasing its volume during bloating and retaining its increased volume during cooling.

Furthermore, despite the absence of inherent bloating ability in the raw materials, it is possible to achieve a significant increase in the initial volume of aggregates, corresponding to bloating coefficient in a range of 2,1-4,2 despite the relatively short temperature interval of 20-60 °C, in which the bloating effect takes place.

It can be readily appreciated that by virtue of the short firing procedure the whole process becomes more economical.

After firing the aggregates are screened in different particle size fractions and stored before delivery.

Particular non- limiting examples for carrying out of the manufacturing method in accordance with the present invention will now be shown.

Examples 1,2,3 below summarize starting composition charge.

Example 1 Raw material Composition in weight parts Industrial waste 60-90 Bottom ash nonbloating clay 10-40 Iron powder 2.0-5.0 Organic component 0.7-1.5 Example 2 Raw material Composition in weight parts Industrial waste 60-90 Fly ash nonbloating clay 10-40 Iron powder 2.0-5.0 Organic component 0.7-1.5 Example 3 The above listed compositions were used for fabrication of lightweight aggregates from the raw materials listed in Table 1.

Parameters of the process were set as explained above and properties of obtained aggregates are summarized in non limiting Table 2.

In this table bloating coefficient means increase in volume of a green aggregate after firing at corresponding firing temperature, measured as a ratio between volume of fired aggregate and respective green pellet.

Table 2 1 Water absorption has been tested for different fractions of fired aggregates, in accordance with ATSM C127-1984.

Results of this test are presented in Table 3 below.

As can be seen fired aggregates, having properties listed in 5 Table 2 can be produced in a wide range of size fractions sufficient to fulfill any customer requirements.

Table 3 The aggregates, manufactured in accordance with the present invention were used in structural concrete compositions. Properties of these compositions were tested and compared with properties of similar composition, employing conventional artificial aggregates and having the same size and loose bulk density.

Results of comparative testing are summarized in Table 4 below, from which one can clearly see that quality of structural concrete, containing aggregates, manufactured in accordance with the present invention is similar to that employing conventional aggregates, manufactured from shale or clay.

Table 4 1 As can be seen, the present invention provides for a simple, economical and reliable process for manufacturing good quality artificial lightweight aggregates, by utilizing inferior quality raw materials.

Vhilst in the embodiments described above the present invention has been described in connection with the manufacturing process employing particular raw materials, it should be appreciated that it should not be limited to the above-described embodiments and that changes and modifications can be made by one ordinarily skilled in the art, without deviation of the scope of the invention, as will be defined below in the appended claims.

It should also be appreciated that features disclosed in the foregoing description, in the following claims and/or in the accompanying drawings, and/or examples and/or tables may, both separately and in any combination thereof, be material for realizing the present invention in diverse forms thereof.

Claims (5)

-25- 110300/2 Claims:

1. A process for fabrication of artificial lightweight aggregates, preferably for use in lightweight concrete, the main component of said aggregates constituting industrial waste resulting from the combustion of fuel in power stations, whereas said method includes the following steps - preparation of a powder mix from raw materials, comprising about 60-90 weight parts of industrial waste, selected from the group consisting of fly ash, bottom ash or their combination, about 10-40 weight parts of ordinary nonbloating clay, about 2-15 weight parts of a bloating agent - pelletizing of said powder mix by an appropriate method, resulting in formation of uniform green aggregates, - firing said green aggregates at appropriate temperature, so as to cause their melting and bloating, characterized in that said raw materials are chosen so as to obtain the following composition of said mix: Si02 50-70 wt.% A1203 15-30 wt.% Fe203+FeO 8-15 wt.% CaO+MgO 3-7 wt.% K20+Na20 2-6 wt.% said raw materials are blended and comminuted so as to achieve -26- 110300/2 homogeneous powder mix, containing not less than 90% of fine particles having diameter less than 100 microns, and to said powder mix is added about 0.7-1.5 percent by weight of organic component, capable to provide green strength to said aggregates during pelletizing, and to be burnt out during firing, wherein conditions for firing of green aggregates are set as follows: heating up to firing temperature of 1100- 1200 degrees C, maintaining said firing temperature for a period of time required for melting and accompanied by an increase in initial volume of aggregates, defined by a bloating coefficient of 2-4.5 cooling down to room temperature with a temperature gradient, which is significantly less than temperature gradient during heating up to firing temperature.

2. A method as defined in Claim 1, characterized in that said powder mix is prepared by simultaneous blending and comminution of said raw materials so as to obtain a specific surface area in the range of 2000-3000 cm2/g, and said green aggregates are prepared by pelletizinng substantially in a pan granulator with subsequent drying before firing so as to reduce their moistuure content up to 1-8%.

3. A method as defined in Claim 2, characterized in that preparation of said powder mix includes the following steps: wet ball milling of aqueous suspension of said powder mix dewatering of said suspension, preferably by means of filter pressing so as to reduce the water content thereof up to 8-20%.

4. A method as defined in Claim 3, characterized in that said bloating component is a magnetic iron oxide and said organic compound is chosen from the group, including any used industrial lubricants, e.g., black mineral oils, grease, motor oil, or the like.

5. Artificial lightweight aggregates, manufactured according to a method defined in Claim 1, characterized in that at least 70% of said aggregates are defined by a size 5-14 mm, by bulk density 250-900 kg/m3 and by water absorption less than 10%. For the Applicant: