FR2501669A1 - Slaking fly ash contg. free lime - using superheated water under suitable conditions of temp. and pressure - Google Patents

Abstract

In a process of slaking fly ash contg. free lime, the ash is wetted using superheated water under conditions of pressure and/or flow ensuring sufficient wetting to effect slaking before vaoorisation. The ash may be heated before contact with the superheated water so as to obtain the temp. necessary for the slaking reaction. The resulting fly ash is used in the building industry and/or in public works for mfr. of pastes, mortars, concretes, etc.. The amt. of water necessary for complete slaking of the amt. of free lime at any moment can be established, esp. for fly ash obtd. from the combustion of lignite, without excessive cost and without destroying some of the hydraulic elements present.

Description

 The present invention relates to improvements made to the extinguishing processes of fly ash containing free lime for the use of these extinguished ash in the manufacture of hydraulic binders.

 It is known that the major difficulty in extinguishing the ashes consists in hydrating the free lime thereof without destroying the inked fraction.

 Although this problem is identical to that of quenching hydraulic lime, it is understandable that it is very different: the fineness of the ash (approximately 2000 Blaine) gives aluminates and silicates more aptitude to hydrate than in the case of hydraulic lime which occurs before slaking in the form of blocks of all sizes, therefore with very small contact surfaces.

 It has been recommended to grind the ashes before extinction to break the outer layer which coats the anhydrous lime; if this process has many advantages, in particular an improvement in the rapidity of the reaction, it also increases the risks of hydration of the hydraulic fraction if the hydration conditions are favorable.

 It therefore appears that the rational extinction of the ashes is a most delicate problem since it is a question of subjecting to the action of water products which are attacked by it and which lose, on contact, their properties.

Numerous experiments have shown that 10 - Above 1200C, silicates and aluminates are no longer
attacked neither by water nor by steam.

20 - Free lime from ash has no affinity for vapor if
the ash was previously brought to a sufficient temperature
to avoid condensation.

It follows that the conditions for the rational extinction of the lignite ash will be to carry out this operation respecting the two imperatives below 10 - Ash temperature close to 1200C so as not to attack the
clinkerized fraction.

2 "- Hydration of quicklime from liquid water, not
steam.

 All the current ash extinction processes have actually sought to achieve these conditions, although the means used are very different.

We will briefly recall the principle of the main procedures commonly used:
1 - Extinguishing in oven or silo:
After having been previously humidified in a mixer,
the ashes are introduced into the upper part of an insulating silo (or oven) and collected at the lower part. The temperature rises as a result of the hydration of CaO and the excess water is vaporized; a part escapes by the "chimney" but one cannot avoid that a certain quantity of vapor condenses on the already cooled parts, below 1000C; it follows a partial hydration of the aluminates and silicates, hydration aggravated if the humidification -is carried out cold.

20 - Extinguishing in a water injection grinder
This process is characterized by prior grinding of the ash and by its extinction in the presence of water vapor at atmospheric pressure, the extinction phase never preceding the grinding phase.

 Although the grinding of the ash is favorable to the speed of the reaction, the time for the ash to pass through the grinder is relatively short to obtain complete extinction.

 On the other hand, the vaporization due to the heat given off by the grinding bodies and by the exothermic reaction of the hydration of the lime, does not make it possible to rigorously fix the quantity of water necessary for quenching.

 The device, continuously discharging, undergoes the variations inherent in the raw product - initial temperature of entry into the mill, chemical composition, fineness -; this results in a fairly large variation in the rate of ash extinction.

To overcome these drawbacks, the operator injects an excess quantity of water which results in a drop in temperature.
turns below 100 C with the aforementioned drawbacks.

3 - Rotating fire extinguisher supplied with hot ashes
The ashes brought beforehand to a temperature of around
130oC are introduced into a heat-insulated rotary oven with hot water injection.

 The exothermic reaction should, in principle, ensure that the temperature remains above 1000C, but there is, in fact, a drop in temperature after the initial reaction zone; there is a risk of clogging in the device due to the condensation of excess water vapor; this condensation can only be avoided by careful insulation of the appliance or by adding calories.

 Vaporization has the same drawback - difficulty in precise dosing of water - as in the water injection mill; on the other hand, heating the ashes makes it possible to maintain more regular extinguishing conditions. It is also obvious that the process is much less expensive than the water injection mill.

However, these methods are not satisfactory due to the fact - either that part of the hydraulic elements are destroyed - or that the quenching of free lime is not complete - or that the cost price of quenching makes the little process
profitable, due to the difficulty of reheating the ashes in
this particular case.

 However, the present invention overcomes the above drawbacks and aims at an improvement made to these extinguishing processes making it possible to fix at any time the quantity of water necessary for the total extinction of the quantity of free lime present in the fly ash. , in particular those resulting from the combustion of lignite.

 The process recommended in accordance with the invention is essentially characterized in that it consists in bringing the fly ash to be treated into contact with superheated water at a temperature preferably higher than 1300C.

According to an advantageous characteristic of this process, the ashes to be treated are, before their contact with the superheated water, at a temperature equal to or higher than 900C or are brought to this temperature before this contacting.

 The invention also relates to a device for implementing this method, this device essentially comprising an autoclave equipped with means making it possible to maintain the pressure, before injection of water, a value greater than the pressure of introduction of the water. overheated.

 According to an advantageous embodiment, the pressurization of the autoclave is ensured by a fluid, preferably hot, such as air or water vapor.

 The device for implementing the method of the invention further comprises means for recovering the heat released by the extinguishing reaction, these means possibly being used for reheating the ashes to be treated or as an addition of calories for heating. superheated process water.

 Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given with reference to the appended drawing illustrating, diagrammatically, an apparatus which can be used for implementing the method according to the invention.

 Preferably, measures will be taken so as to limit as much as possible the heat loss from the ashes between the dust collectors forming part, as is known per se, for example of a thermal power station and the extinguishing station in order to supply the appliances of this station in fly ash as hot as possible.

This result can be obtained, for example, by insulating said dust collectors, by limiting the transport distances of the ashes, by carrying out this transport in a heated medium, thus avoiding any intermediate long-term storage.

 By proceeding in this way, it is then possible to carry out an extinction as complete as possible without prior reheating and with all the guarantees of obtaining a good quality product.

Of course, the process of the invention also applies to ashes, the initial temperature of which is arbitrary, for example the temperature of the ashes stored or transported over a long distance. In this case, it is envisaged to carry out a preliminary reheating using any known means such as for example that which will be discussed below during the description of the apparatus illustrated in the appended figure.

 The essential characteristic of the process of the invention is therefore to ensure the complete extinction of the free lime contained in the fly ash from the combustion of lignite, by wetting in the liquid phase, by means of superheated water, whatever the temperature of the ambient medium but on the condition that the pressure of this ambient medium is such that at no time this superheated water is vaporizable. As a result, this wetting has a double role, namely that of reheating the ashes if they are not at the desired temperature and that of supplying the necessary quantity of extinguishing water.

 By doing so, a rapid and homogeneous wetting occurs, which can be favored by any stirring system, either by means of agitators working in a static enclosure, or by means of rotary printing apparatus. an action of mixing the medium to be treated.

 With regard to the possible reheating of the ash to be treated, this reheating can be obtained by recovering the quantity of heat supplied by the exothermic reaction for quenching quicklime: CaO + H20 pCa (OH) 2 + 15, 56 cal / g. Thus the extinction of 1000 kg of crude ash having a free CaO content of 15 X releases more than 41,000 calories. If we consider that 14,000 calories are necessary to heat a tonne of ash from 500C to 12O0C, we understand that the calories released by the reaction can be used for other purposes than reheating the ashes.

This recovery of calories can be carried out - either by means of exchanger tubes located inside
the appliance (as we will see below) - either by the use of the released vapor which can
either be injected directly into another device to
reheating of ashes if necessary,
either be treated with a view to reheating the extinguishing water,
circulating water in double jackets and tubes
exchangers, air for heating transport circuits, etc.

In addition to the economic interest, there is a technical interest; indeed, these heat exchanges being extinguished can allow a limitation of the pressure rise in the device.

An apparatus for implementing the process of the invention will be designed to withstand pressures of the order of 6 to 10 bars, this pressure being a function of the ash temperature, the free lime content, the temperature superheated water, fill factor, etc.

Both for reheating the device and to avoid heat loss, the device will be equipped with a heat-insulated double jacket allowing the circulation of a fluid at temperature> 1300C.

The exchanger tubes will be fixed inside the device, these tubes can be integrated into the stirring system; these exchangers can be supplied either with superheated water - heating phase of the enclosure - or with cold water - calorie recovery phase.

In addition to the water inlets, an injection of air - preferably hot - or even steam, will be provided to pressurize the enclosure, in order to avoid vaporization of the superheated water.

To facilitate heat exchange between ash and steam, or superheated water, between ash and inner tubes, as well as for wetting the ash with extinguishing water, we will adapt a homogenization system preferably by mechanical agitation - device rotary or static device with agitator - or possibly by fluidification.

The loading will be done by means of a dosing hopper, the rapid emptying being preferably obtained pneumatically.

The appended figure shows the diagram of an apparatus comprising the means which have just been listed. This device comprises - a rotary autoclave (1) - a raceway (2) - a double casing (3) - a fixed journal (4) - the exchanger tubes (5) - a supply of superheated water (6) - an air or steam inlet (7) - a supply and return circuit for the exchanger tubes (8 and 9) - a servomotor for controlling the autoclave obturator (10) - a supply circuit and return of the servo motor (a safety valve (12) - a steam release valve (13)
As an indication, the operating process may be as follows depending on the temperature of the ash.

Ierocas: Hot ashes
This is the case for ashes from, for example, groups from a thermal power plant whose average temperature in the dust collectors is of the order of 150 ° C .; which would supply the extinguishing station with ashes at 120/130 "C.

In this case, operations could be limited to - warming up the device - loading - Water injection.

The injection of cold or hot water causing a drop in temperature and a slight delay in triggering the reaction, the extinction conditions are clearly improved by using superheated water, in accordance with the invention. The duration of the extinction phase is only a few minutes.

2nd ~ case ~ Ash ~ hot 90 l110 C for example)
This is the case with the ashes of a group whose ash temperature varies from 950C to 1200C in the dust collectors and which will arrive at the extinguishing station between 90 "C and 1100C depending on the precautions taken for their transport. the most interesting case.

For this type of ash, the operating process is as follows - reheating of the ashes in the buffer hopper (double jacket,
exchanger tubes, steam, etc.) - temperature rise of the jacket (T) 1300C) of the device - circulation of superheated water in the exchanger tubes - loading of the device into crude ash - pressurization of the pregnant looks hot, preferably, or
steam admitted by 7, in order to avoid vaporization of water
superheated - Injection of superheated water: the ash temperature reached
quickly 1100C to 1200C and the reaction starts immediately.

- extinction phase: a few minutes - recovery of calories by exchanger tubes - exhaust and recovery of steam by 13 - emptying of ash extinct 3rd case: Cold ashes
This is the case with stored ashes, their temperature is very variable depending on the duration of silotage and supplies.

If the techniques described above do not allow the ashes to be reheated to a temperature above 100 "C, provision should be made to preheat the ashes with steam - either in an independent device specially designed for this purpose.
treatment, and in this case the extinction process will be identical
to the previous ones if the autoclave is supplied with hot ashes; - either directly in the autoclave and in this case pressurization and
the reheating of the ashes can be done by steam beforehand.
nary of extinction operations.

 One of the extinguishing techniques described above will be adopted depending on the temperature reached by the ashes after reheating.

 By proceeding in this way, we realize that, compared to the previously known methods, the invention provides a clear improvement for extinction by guaranteeing not only the achievement of extinction without risk of hydration of the clinkerized part but by allowing self-regulation of the extinction rate, whatever the variations in the free lime content of the ash; indeed, the extinction conditions are such that the water is fixed in priority and very quickly by the ash for transformation of CaO into Ca (OH) 2 and that the excess water is vaporized, therefore without risk of hydration of the clinkerized part.

 By implementing the invention, the lignite ash as well. extinct acquires characteristics, the main ones being the absence of swelling, great finesse, exceptional maneuverability, a low shrinkage rate as well as both hydraulic and pozzolanic qualities. It can therefore be used, given these characteristics, either as a binder or as a constituent of a binder.

Indeed, it can be compared to both - hydraulic lime; - an artificial cement, type of masonry binder; - with pozzolanic cement; - to a constituent meeting the definition of standard P 15301 paragraphs 3.3.5 (lignite fly ash) and 3.3.6. (active fillers).

These different applications thus allow a significant energy saving and recovery of by-products partly considered until now as "waste".

 It follows that the uses of the ash extinguished by the method according to the invention can be very diverse as will be obvious to those skilled in the art.

 It goes without saying that the present invention has been described for purely explanatory and in no way limitative and that any useful modifications may be made without departing from the scope of the invention, as defined by the claims below.

Claims (7)

 1. Improvement in the process for extinguishing fly ash containing free lime, characterized in that the wetting of these ash is carried out in the liquid phase, using superheated water under pressure and / or flow ensuring wetting necessary for extinction before vaporization.  2. Improvement according to claim 1, characterized in that the ashes are optionally reheated before being brought into contact with an overheated strip in order to obtain the temperature conditions necessary for the extinction reaction.  3. Apparatus for implementing the method according to claim 1 or 2, characterized in that it comprises an autoclave (13 provided with means (7) for obtaining a pressure higher than the inlet pressure of the water superheated (6), means (3-5) for the possible reheating of the ashes and / or the enclosure and means of agitation and stirring of the medium (2-4).  4. Apparatus according to revendi cation 3, characterized in that the pressure of the autoclave is brought to the desired value by possibly hot flair, steam or superheated water.  5. Apparatus according to claim 3 or 4, characterized in that the stirring or stirring of the medium is provided either by mechanical stirrers, or by the rotation of the autoclave itself, or by fluidization.  6. Extinct ash obtained according to any one of claims 1 to 5.  7. Use of the ashes resulting from the process according to claim 1 in the building industry and public works for the preparation of pasta, mortars, concretes and the like.