DE1919750C3 - Process for sintering fly ash - Google Patents

Description

The invention relates to a method for sintering fly ash on fixed or movable grates.

From the pulverized coal combustion of the steam boiler systems of the power plants, fly ash dust suspended in the flue gas is separated in the downstream filters. Depending on the type of coal and the degree of grinding of the coal, this fly ash dust has a bulk density of approximately 0.68 to 0.90 t / m ³ . These fine and mostly light dusts can hardly be stored without danger and nuisance for people, animals and plants, or cause considerable costs if they are stored in natural or artificial cavities, for example as backfill material for excavated tunnels.

Efforts have therefore been made for some time to convert this flight axis into a salable product by sintering, ie by making it into pieces. Due w the high content of silica and alumina in the fly ash itself provides the sinter produced therefrom as Bauzusatzstoff such. B. in the manufacture of concrete or in the manufacture of lightweight building blocks. Depending on the composition of the fly ash, whether rich in silica -r> acid or alumina with a low or high content of fluxes, such as. B. the Aikalioxide, are the Softening points, melting points and pour points at different temperatures. During sintering For fly ash, the softening point is of particular importance, since only the surface layer of the material to be sintered is to be softened in order to be cemented into a larger dust bond.

In general, the softening points of such fly ash from 1200 to 1350 ° C, the melting points from 1300 to 1450 ° C and pour points from 1350 to 1500 ⁰ C.

The contents of these fly ashes of silica, alumina, iron oxide and oxides of the alkaline earth as well as Alkali metals change from fly ash to fly ash, likewise - but in a much lower concentration - the sulfur and phosphorus content in the The table below shows the mean composition of fly ash and its range of variation. These details refer to the anhydrous Substance.

Fly ash analysis Cges. Fe2O3 AI2O3 ΤΊΟ2

5 9 24 2

± 100 ± 60 ± 20 ± 70

CaO SiO2 MgO Na2O K2O P2O5 Sges. fjew .-% 4th 48 2 2 3 0.5 0.4 scattering ± 90 ± 20 ± 75 ± 75 ± 50 ± 100 ± 100

The fly ash predominantly has a dust fineness of <40 μ, namely the proportion of fly ash about 60-80% of these fine particles.

The carbon content of the fly ash is important insofar as it depends on whether the fly ash is self-moving or not. In self-moving fly ash, the carbon content is above a content of approx. 3% by weight. Carbon contents of up to 10 percent by weight are also determined in fly ash. Coal dust must be added to those fly ash that are not self-sustaining in order to make them sinterable. Bituminous and floating coals are preferred. The carbon content of the fly ash is then 3 to 4.5 percent by weight after admixing the coal dust.

The invention is now based on the object of indicating suitable measures with which the properties of the sintered product and the sintering performance can be influenced in the desired manner. In particular, a sintered product with a bulk density of <0.7 t / m ^J , preferably ^{0.5-0.7 μm 1} , M) should be produced.

To solve this problem, the invention proposes that one or more of the following substances be added to the 'sintered material:

a) to increase the sintering rate of oxides tv, alkaline earth metals or the alkali metals or else

other compounds of these metals if they are present as oxides during sintering,

b) to reduce the bulk weight of the sintered

Product that separates water from hydration or micaceous minerals, such as vermiculite and
c) to influence the composition of the grain fractions of the sintered product, the water-soluble silicates or aluminates of the alkali metals or the silicates of the alkaline earth metals which are soluble in other liquids.

However, the total of these additives should not be more than 20% by weight based on the dry substance. According to a further feature of the invention, the moisture of the sinter mixture, which crumbles or may be present in the form of raw pellets or other raw moldings, be less than 18 wt .-%. Under Anti-wetting agents may also be used in certain circumstances will.

It can also be beneficial to add liquids to the substances to be sintered that are themselves lower Have a density than water or a lower density due to the admixture of solid, liquid or gaseous substances Preserve density.

Finally, it is still possible to pre-dry or dry the bulk of the raw moldings before ignition. to preheat.

The method of the invention can find application in both normal and normal sintering under positive or negative pressure.

The path from fly ash to a salable sintered product is generally as follows:

The fly ash is first - depending on its carbon content - mixed with coal dust or not. The sinter mix is then placed on a plate or in a drum with the addition of water pelletized Spherical particles are formed, the diameter of which is generally between 3 and The fines <3 mm that arise during pelleting are sieved off and the next batch of fly ash added again when mixing

The pellet mixture is placed on the sintering belt, the top layer of the pellet bed being ignited by the smoke gases emerging from the ignition hood at a temperature - depending on the operating conditions - between 750 and 950 ° C. As the sintered material continues on the sintering belt, the burning zone moves through the sintered layer as a result of air suction until the burning zone has arrived just above the sintering grate or the grate covering, depending on the mode of operation. Then the sintered cake is cooled and thrown off the belt. The sinter is gently crushed in a crusher and sifted into the various grain classes in a screening system 0.75 t / m ³ , in isolated, very favorable cases even 0.70 t / m ³ . These values are achieved when the resulting sinter cake is mostly loosely baked.

example 1

A fly ash with a carbon content of 1.8% by weight is to be sintered. The fly ash is inherently not sinterable. In order to make them sinterable, coal dust from bituminous coal was mixed with the fly ash so that the mixture had a carbon content of 4.26%. The bulk density of the fly ash was 0.75 t / m ³ .

The following. The sintering result was obtained:

Sintering output, tato / m ² <3 46.7 Mean Bulk weight of 29 0.79 Grain class 3–10 mm des Sintered product, t / m ³ Grain fineness, mm 3-10 Grain fraction,% by weight 68

ίο This sintering result should now be improved with regard to the sintering performance, the bulk density and / or the composition of the grain fractions by additives according to the invention.
The sintering speed is mainly influenced by alkaline substances such as B. CaO, MgO, CaO · MgO (gebra, inter dolomite) or Na ₂ SiO ₃ increased

B e i s ρ i e I 2

The fly ash is sintered after adding coal dust (as in Example 1) and CaO CaO content in the sinter mixture is 1 percent by weight

The following sintering result was obtained:

Sintering output, tato / m ²
Mean Bulk weight of
Grain class 3-10 mm
of the sintered product, t / m ³
Grain fineness, mm
Grain fraction,% by weight

<3

493
0.75

3-10>
68 4

The increase in the sintering speed is therefore already around 7% with an addition of 1% by weight of CaO.

The bulk density is made up of voluminous substances with a lower bulk density than that of fly ash or reduced by flatulent substances. Under flatulence substances are understood that either only drift like; t. B. mica-like substances or also react like z. B. floating, bituminous coal.

Example 3

Fly ash with a raw vermiculite content of 1% by weight was sintered

The following sintering result was obtained:

Mean Bulk weight of 3 0.69 10 Grain class 3–10 mm 21 17th of the sintered product, t / m ³ Sintering output, tato / m ² 35 Mean Bulk weight of 0.66 Grain class <3mm des Sintered product, t / m ³ Grain fineness, mm 3-10 Grain fraction,% by weight 62

By adding raw vermiculite, the bulk density was already noticeably reduced. With a raw vermiculite content of 5% by weight, a bulk density of the sintered product in the grain class 3-10 mm of approx. 0.56 t / m ^J was even achieved. Instead of raw vermiculite with a water content of approx. 11% by weight, predried vermiculite or mixtures of predried and raw vermiculite can also be used

can be used. But you can also use the voluminous burnt vermiculite, which only has a bulk density of about 0.3 to 0.5 t / m ¹ . Mixtures of raw and burnt vermiculite can also be used.

Finally, it is of great importance to ensure that the proportion of salable sintered product is as high as possible preserved, i.e. to produce as little fine material as possible (<3mm). Suitable additives for this are of all the water-soluble alkali silicates and aluminates. ι ο

Example 4

The fly ash with a content of 3 percent by weight NaiSiOj in the fly ash mixture was sintered. After cooling, breaking and sieving the sintered cast, the following grain sizes were determined:

Grain class, mm 3-10

Grain fraction,% by weight 82

Sintering output, tato / m ² 51.6

With a content of 7.1% by weight Na2SiO ₃ in the fly ash mixture, an even higher grain fraction was obtained.

Grain class, mm 3 - J 0

Grain fraction,% by weight 92

The sintering output here is 54.8 tpd / m ^{2 of} sintering belt.

Claims (1)

Patent claims: l. Process for sintering fly ash, characterized in that one or more of the following substances are added to the material to be sintered: a) to increase the sintering rate, oxides of the alkaline earth metals or the alkali metals or other compounds of these metals if they are present as oxides during sintering, b) to reduce the bulk density of the sintered product or which release hydrate water micaceous minerals such as vermiculite and c) to influence the composition of the Grain fractions of the sintered product are the water-soluble silicates or aluminates of the alkali metals or the silicates of the alkaline earth metals which are soluble in other liquids. Z Process according to Claim 1, characterized in that the total of the additives does not amount to more than 20% by weight based on the dry substance 3. The method according to claim 1, characterized in that the moisture of the sintered material, the crumbly or in the form of raw pellets or other raw materials, less than 18 % By weight 4. The method according to claim 1, characterized in that liquids are added to the sintered material which themselves have a lower density than water or by adding solid, liquid or gaseous substances are given a lower density. 5. The method according to claim 3, characterized in that the bulk of the raw blanks before the ignition is pre-dried or preheated.