WO1996019416A1

WO1996019416A1 - Process and device for the thermal treatment of mineral granules

Info

Publication number: WO1996019416A1
Application number: PCT/EP1995/005045
Authority: WO
Inventors: Wolfgang Mueller
Original assignee: Sandoz Ltd.; Sandoz-Patent-Gmbh; Sandoz-Erfindungen Verwaltungsgesellschaft Mbh
Priority date: 1994-12-21
Filing date: 1995-12-20
Publication date: 1996-06-27
Also published as: DE4445654A1; AU4346596A

Abstract

The invention concerns a process for the thermal treatment of mineral granules in a continuous furnace. The time spent by the granules in the furnace is selected to ensure optimal ceramification and the granules are spread out on an inert base to prevent clumping. The ceramified granules can be used as light sand in construction materials.

Description

METHOD AND DEVICE FOR THE THERMAL TREATMENT OF MINERAL GRANULES

There are already processes for processing gravel sludge or fly ash in such a way that they are built into a ceramic structure or glazed. However, the known methods work in such a way that the substances to be rendered inert are subjected to a longer heat treatment, primarily in a rotary kiln, and thus have the known disadvantages of all ceramics or foam glasses treated in such an oven (open porosity, damaged surfaces, water absorption).

EP 134.584 describes a method and device which is suitable for the thermal treatment of granules from sludges. However, this method is based on countercurrent heating with hot gas, which on the one hand leads to turbulence and thus to an increased risk of the molten particles sticking, and on the other hand to the production of large amounts of exhaust gases.

It has now been found that by using a continuous furnace, these disadvantages can be avoided and a targeted thermal treatment of mineral granules is possible.

The invention thus relates to a process for the thermal treatment of mineral granules, characterized in that the granules are heated in a continuous furnace and are cooled again after leaving the furnace. An essential aspect of this method is that the granulate lies in a single-layer particle layer on a base which is to avoid caking or is coated with a suitable agent for this purpose.

The method according to the invention can be explained as follows: The material to be treated, which is added to the furnace in the form of granules with a grain size of less than 20 mm, passes through it in a specifically selected throughput time. The lead time is determined by the following factors: furnace length; Treadmill speed; Granular shape and size.

At an oven temperature that is only slightly above the melting temperature of the granulate particles, the product is slowly and evenly melted and vitrified. This gives it the desired closed surface and fine-pored internal structure. The product properties can be influenced favorably by means of a controlled temperature profile along the direction of movement of the particles. The particles should not touch to prevent sticking together. For energy reasons, the furnace should be designed so that the substrate for the particles is kept at the highest possible temperature level.

The furnace is heated using electrical resistance heating or a fossil fuel flame or a combination of both.

The continuous furnace can be linear or circular. The product is placed on a base which is moved by means of moving rollers (roller furnace) or on transport trolleys (bogie hearth furnace) or on a turntable. In order to keep heat losses from cooling the underlay or transport trolley low, two ovens with the opposite transport direction should be arranged in parallel next to or on top of each other in a longitudinal design.

The furnace described in this way is used in a temperature range of approx. 1000 ° C to approx. 1400 ° C, whereby the particles need different residence times depending on the size of the ceramic. The dwell time is between 30 seconds and 10 minutes and can vary with the throughput speed, the length of the furnace or the length of the heated zone or by using multiple feed points along the heated zone or the furnace temperature.

The atmosphere in the interior of the furnace (heat treatment room) can, depending on the lining material and the material to be treated and the desired product properties, be operated oxidizing to reducing. A certain air exchange (process gas) inside the furnace keeps the furnace atmosphere clear, that is, unclouded (important for radiation heat transfer).

The advantage of the furnace lies in the targeted control of the glazing process through a well-controlled supply of heat within a defined period of time.

In a special embodiment of the method according to the invention, the furnace is provided with a special feed device which is particularly suitable for the introduction of granules. This device brings the prepared granulate into the oven. The device must be cooled because of the oven radiation. be insulated. It is important to ensure that the steel temperature does not exceed 1100 ° C in order not to damage the steel parts and to prevent caking of the feed material. On the other hand, the added particles must not cool down again here.

The feed device itself and the cooling devices are to be controlled and monitored by means of suitable measuring and regulating elements.

Distribution of materials in the furnace

Since the heat is supplied in the furnace by radiation from the inner wall of the furnace and / or by convection, it is important that the material to be treated does not cover itself and thus hinders the flow of heat. For this reason, the distribution of the material in a single-layer layer is of great importance.

If desired, the continuous furnace can also be combined with a preheater, which heats the granulate to a desired temperature and allows optimal treatment in the actual furnace. The preheater has the task of heating the granules to the highest possible temperatures. The drying of the granules can be integrated into it. The decarbonation (expulsion of CO ₂ ) takes place at 700-1000 "C. This must take place completely. For this purpose, the material must be kept at this temperature for 10 to 40 minutes. The material is then heated further, with the sticking together or sticking together The dwell time and the heating-up profile must be like this should be chosen so that the additives (blowing agents) added to the granulate are not damaged.

This process is preferably carried out in a gas or electricity-heated rotary drum or a belt or tunnel oven, to which the moist or dried raw granulate is metered. A fluid bed furnace that ensures sufficient energy supply can also be used. In any case, the material must be kept in constant motion, as it could tend to stick together even at temperatures below 1000'C. The granules must be moved gently, as otherwise undesired abrasion or even destruction of the granules will occur.

The method according to the invention can be carried out in various embodiments. The preferred embodiment is the production of light sand based on the method of EP 134.584, in which mineral starting materials are granulated together with a blowing agent, dried and ceramized in a continuous furnace. If desired, waste materials can also be added to the mineral raw materials, which can be safely disposed of in this way, either without (in order to obtain the most compact product possible for landfill) or with blowing agents (for the production of light sand that is used as a concrete aggregate or mortar, Roads / railway substructure, etc. can be used).

By using blowing agents, the final product can be adjusted to the desired weight properties in terms of density and bulk density. This is particularly important if the end granulate is to be recycled at prices in line with the market. If recycling is out of the question, there is no point in using blowing agents, since the end product should be as compact as possible to save landfill space.

In the preparation process of the raw granulate, it is possible to mix in special reaction aids that not only enable physical inertization in the final ceramic granulate, but also chemical inertization in addition to the physical one. Depending on the waste material and base material, this effect is already available from the base materials even without reaction aids. In addition, certain reactions can be supported in a continuous furnace with a controlled furnace atmosphere (eg increased O ₂ content or the like). The raw materials for the method according to the invention are divided into ceramic-compatible base material, waste material, reaction auxiliaries, blowing agents and binders.

Base material: a ceramic-capable mineral sludge is normally used as the base material, such as that found in gravel pits as washing sludge, as river and lake deposits, etc. Fly ash from hard coal or lignite power plants or waste incineration plants can also be used as the basic material. The base material should be available in the finest possible form (grain size below 60 μm) in order to have a sufficient reaction surface. The chemical and mineralogical composition of the base material has to be examined more closely, especially with regard to the content of glass-forming components and the chemical reactivity with the waste material. The constancy of the composition of the basic material must also be monitored over the delivery period. The compatibility of the raw material and the additives with the furnace lining and the underlay material must be examined.

Waste: The material to be rendered inert is called waste. These are primarily powders and sludges containing heavy metals, but they can also be other chemical compounds to be disposed of, which are converted by heat treatment or rendered inert by ceramization. The exact composition and the expected reactions must be known. Here, too, there is a demand for the finest possible material, which may have to be ground beforehand, if not finely enough (grinding fineness 10-20 μm). Depending on the concentration, the waste materials are to be mixed with the basic material. A content of pure waste material compounds of 10-30% by weight or more of the base material can be assumed in the finished material. In contrast to the basic material, the waste material must be stored closed (silos) and metered into the basic material using suitable dosing methods and mixed homogeneously with it. Any dust leakage into the environment must be prevented by suction.

Reaction aids: If necessary, additional components can be added to the base material in order to achieve a certain chemical reaction with the waste material or to improve the ceramization process. Depending on the basic and Waste material various reaction aids in question. For cost reasons, the use of reaction aids is restricted as much as possible. Here too there is a demand for delicacy.

Blowing agent: If the end product is to be recycled at the hardware store, the end product can be inflated with a blowing agent and thus provided with a lower density and bulk density. These are primarily economic and application-related reasons that lead to this requirement. Naturally, the mechanical strength properties of this lightweight aggregate will be worse than that of a compact unit. The respective advantages and disadvantages must be determined precisely and weighed up against each other. Suitable blowing agents are inorganic or organic compounds which split off gases at the ceramizing temperature or react to gases and thus lead to a porous ceramic structure. Here again there is a demand for delicacy. In general, between 0.05 and 5 percent by weight of blowing agent is added to the raw material. With higher blowing agent concentrations, there is a greater increase in volume. However, the particles obtained have only a low strength.

Binder: In the case of poor granulation and binding ability of the base material / waste material mixture (e.g. possible when using hard coal fly ash as base material), the raw granulate strength must be improved with a binder. Chemical binders can be used as binders, but physical binders are advantageous. Such is e.g. the admixture of clay-like substances (similar to the mud-like base materials) that lead to capillary force bonds.

Regarding the raw materials, it should be noted that three requirements must be met for the production of light sand:

appropriate clay components or other binders are intended to ensure that they can be granulated; due to the specific energy requirement in the furnace, they should have the lowest possible carbonate content; and

their chemical composition should be such that melting and subsequent glazing take place.

Apart from the basic material, all of the above materials are stored closed in silos or tanks. They are measured using a suitable metering device and fed to a mixer or a mixing granulator. There the substances are mixed intimately and then granulated in the same machine or on a downstream machine. Depending on the moisture content of the materials, additional water is added or water is carried away (dried) while warm air is supplied, or a partial stream of the raw material is dried and mixed with wet material in order to obtain the optimum granulation moisture.

Mixing the components:

An absolutely homogeneous mixture is absolutely necessary and must be ensured by the choice of the mixing process. If the raw material and additives are exclusively dry powders, they can be mixed dry. If the raw product is moist, such as gravel sludge, the route of re-mashing and mixing in the suspension should be filtered, which is then filtered on a suitable filtration device (e.g. filter press) for residual moisture of 15-20%. Ideally, the aggregates are mixed in at the source of the pebble sludge in the washing water cleaning system of the gravel works.

At the same time as the above materials, all dust from the system (from filters, cyclones, sieves, etc.) is returned and mixed with the other material. For the granulation process, it is advantageous if a certain amount of dust can be added (faster granulation). The raw granulate formed in this way should be approximately in the grain size of 1-20 mm in diameter and advantageously be spherical in shape. The granulation process can be carried out in one or two stages, the latter in order to build up larger pellets by continuously adding dry raw mixture and liquid.

After the granulation, which is carried out in batch operation or continuously, the raw granulate is fed to the dryer via a suitable buffer device and a coarse sieve, from which the process is carried out continuously. In the coarse sieve, all larger tubers, adhesions etc. that are not finely granulated are sieved out and can be fed back into the process via the base material.

In the dryer, either a drum dryer, a floating bed dryer or a belt dryer, the raw granulate is largely dried with hot exhaust air from the system. The expelled water is fed into the environment with the drying air via the filter system. If it is necessary, the dryer can also be heated indirectly in the case of a drum dryer, the expelled moisture condenses and cleaned of any pollutants.

After the dryer has been discharged, the dry raw granulate can be passed over a small roller crusher in which the desired maximum grain size is limited, i.e. Any oversize is crushed in the crusher and does not burden the process. However, an optimal granulation process advantageously produces a narrow range of granules of the desired size class. After the crusher, the granulate is fed to the fine sieve with a pneumatic lift, a crusher or another suitable conveying device, where the undersize grain (dust) is screened out, which like all other process dusts in the mixer is fed back to the raw material. In the case of a directly heated preheater, the dust is discharged with the flue gas, so that there is no screening.

After the sieve, the remaining Gutkom is dosed into the preheater and preheated there as described above. Depending on the waste material to be treated, the temperature in the preheater must be reduced to avoid polluting the hot air.

If necessary, the preheating stage can also be dispensed with. This extends the dwell time in the continuous furnace accordingly. The heat to be supplied for preheating and possibly decarbonation depends on the product, waste material and permissible preheating temperature. This will usually be between 400-1100 "C. After the preheater, the granulate is dosed into the continuous furnace.

The preheated granulate is metered into the continuous furnace and passes through it. During the passage, the granulate absorbs heat due to the very intense radiation or convection heat of the furnace and reaches the ceramic temperature of the base material. Depending on its composition, this is in the range of 1150-1350 ^* 0. Depending on the temperature of the preheating, the maximum size of the granules that can be ceramized in this way is in the range of approximately 1-20 mm in diameter or larger. The continuous furnace can be heated electrically or with fossil fuels.

The heating in the granulate takes place from the outside in, i.e. the surface of the grain very quickly reaches and exceeds the ceramizing temperature, it reaches a pasty to liquid state. As the throughput time progresses, this state moves inwards, so that until the grain leaves the oven, the grain reaches this state inside. Due to the surface tensions, the ceramized grain becomes spherical or ellipsoidal. The single-layer distribution of the granules on the base prevents the grains from touching each other. At the end of the kiln process, the grain has an evenly ceramized structure, in which any waste materials are evenly distributed.

Depending on the base material and the type of waste, these are not only built into the ceramic structure in a solid and insoluble manner, but chemical connections are also formed between the base material (possibly with reaction aids) and the waste. In addition to physical inerting (integration into the ceramic structure), these can also chemically inert the waste. Due to the high surface temperature of the grain, a liquid film forms over the grain, which at least partially prevents evaporation of the waste materials outside the grain.

In contrast to all processes taking place in the rotary kiln, the grains in the continuous furnace have no mutual contact. Because of this and because of the short hot action time, the danger of escaping gases from the grain and the resulting open porosity, as well as the damage to the surface of the grain by rolling on each other, is only very limited and the environmental resistance of the end product is significantly improved.

If blowing agents are also added to the material mixture, these in turn form gases at the above temperatures, which lead to a fine-pored expansion of the grain to 1 to 10 times the volume. Due to the above, very fast running processes, it is very important that all materials to be reacted have the greatest possible fineness (large reaction surface = fast reaction). These reactions can be promoted by the furnace usable atmosphere, which can be partially adapted in the system. Of course, certain reactions with the furnace atmosphere (burns, etc.) will take place on the surface of the grain and also due to gases emerging from the grain, which make it necessary to continuously replace the furnace atmosphere. This furnace atmosphere can be suctioned off at the front or rear of the furnace and a corresponding replacement can be supplied.

In the production of porous ceramic beads (use in light mortar or concrete), a fine-pored structure with a uniform bulk density is of great importance for the strength. Since small particles require less heat in the furnace, there is a risk that they will be "overheated" and thereby form hollow spheres that are less stable, or that they will already collapse into solid particles. If there are high demands on the product, it is therefore advisable to fractionate the granulate by sieving and to only give one particle size class (e.g. 1, 0 - 1.5 mm or 1.5 - 2 mm etc.) to the furnace so that the Process conditions such as the temperature, the temperature profile over the length of the furnace and the dwell time can be optimally adapted to the particle size. It is also conceivable to give up the different fractions at different points in the furnace or to operate several furnaces with different dwell times.

The hot liquid product can be cooled in a continuous stage in the continuous furnace below the solidification temperature or in a floating bed cooler with cold air. By cooling, the surface of the grain is cooled within a fraction of a second, that this solidifies. The cold air largely cools the grain in a floating bed within a few seconds and the heat content is transferred to the cooling air. This in turn is used as hot and drying air in the preheater and dryer.

In contrast to conventional, large-sized ceramics, the ceramic structure of the product according to the invention can be cooled very rapidly, since on the one hand the round shape of the grain gives off a uniform amount of heat and on the other hand the heat is given off very quickly due to the small size of the grain.

The cooled, ceramized granulate is discharged from the cooler and blown into a silo. At this stage, the whole process is complete and the waste material is inertly integrated into the ceramic grain. This can then be installed or deposited.

As already indicated, the ceramized granulate can be used as light sand in building materials such as concrete, mortar, etc. and serves as a replacement for conventional, heavier materials. In particular, a light mortar or lightweight concrete for the renovation of buildings or a structural concrete can be produced from it.

Claims

PATENT CLAIMS

1. A process for the thermal treatment of mineral granules by heating in an oven and cooling again after leaving the oven, characterized in that the granules are heated in a continuous layer in a single-layer particle layer on a base during a specifically selected cycle time.

2. The method according to claim 1, characterized in that the treatment is carried out at a temperature of about 1000 "C to 1400 ° C.

3. The method according to claim 1 or 2, characterized in that the granules are added to the furnace in such a way that a uniform supply of heat is ensured and the particles are prevented from sticking together.

4. The method according to any one of claims 1 to 3, characterized in that the transport base of the continuous furnace is equipped with a protective layer which prevents the product from caking on the transport base.

5. The method according to any one of claims 1 to 4, characterized in that the raw materials consist of a ceramic-compatible base material and, if desired, waste materials, reaction aids, blowing agents and / or binders.

6. The method according to claim 5, characterized in that the base material, if appropriate with prior admixture of binders, reaction auxiliaries and / or blowing agents, is previously granulated.

7. The method according to any one of claims 1 to 6, characterized in that the granules are brought to a temperature of up to 1000 ° C in a preheater before they are placed in the continuous furnace. Ceramicized granules, which were produced by the method according to any one of claims 1 to 7.

Use of a ceramized granulate according to claim 8 as light sand in building materials, for example in light mortar or light concrete for the renovation of buildings or in structural concrete.