EP0421979A1 - Process for diminishing the sulfur emission in sintering processes - Google Patents

Abstract

In a method for reducing pollutant emissions in thermal processes, such as in particular sintering processes, in which a fuel-containing mixture, in particular a coke bed, is ignited, the fuel-containing mixture, in particular the coke, is rolled with Ca (OH) 2 before the fuel-containing mixture is fed in or soaked in a slurry of hydrated lime.

Description

The invention relates to a method for reducing pollutant emissions in thermal processes, such as in particular sintering processes, in which a fuel-containing mixture, in particular a coke bed, is ignited.

EP-A1 39 305 has already disclosed a method for reducing pollutant emissions during sintering, in which an extraneous rust coating is applied as an intermediate layer between the raw sintered layer and the sintered grate. The proposed intermediate layer consists of granular material which should be able to remove pollutants. This granular sorbent material is moistened for significant pollutant emissions, with particularly large effects being achieved with basic slurries or liquids. Lime milk was preferably given as the basic slurry, a correspondingly larger volume of sulfur-containing waste products being formed in addition to the coke ash. In such sintering plants, for example, preliminary products for metal extraction from ores are produced, ore mixtures, concentrates, metallurgical recycling materials being fed onto the sintering plant together with coke breeze. For a layer thickness of approximately 40 cm of the sinter mixture, layer thicknesses for the additional rust covering between 2.5 and 15 cm are proposed in order to achieve an effective desulfurization.

The invention now aims to provide a method of the type mentioned, in which, without increasing the amount of materials to be handled, enriched with pollutants, a reduction in pollutant emissions, in particular an almost complete desulphurization of the exhaust gases can be achieved without expensive flue gas desulfurization. To achieve this object, the invention consists in essential in that the pore-rich fuel fraction, especially the coke, of the mixture is rolled with Ca (OH) ₂ or soaked with a slurry of hydrated lime before the fuel-containing mixture is given up. The measure of rolling the pore-rich fuel fraction, in particular the coke, of the mixture with Ca (OH) ₂ prior to the task of the fuel-containing mixture or of impregnating it with a slurry of hydrated lime was in no way obvious, since the prejudice of the experts in particular existed that such additives to the pore-rich fuel component, in particular coke, have a sensitive influence on the ignition temperature and in particular an increase in the ignition temperature. Surprisingly, it has been found that the ignition temperature of the coke remains almost unchanged when using dry hydrated lime, while the ignition temperature of the coke pretreated in this way could even be reduced when using slurries of hydrated lime. With the lowering of the reaction temperatures, the conditions for an effective desulfurization were significantly improved and more than 90% sulfur binding could be achieved with ease.

The process according to the invention is preferably carried out in such a way that, based on the amount of coke, 5-30% by weight Ca (OH) ₂, preferably 10-25% by weight, are used in dry form and the coke is rolled together with the hydrated lime . In compliance with the specified limit values for the addition of hydrated lime, an almost unchanged ignition temperature of the coke could be maintained in the case of rolling coke with dry lime hydrate, with the treatment of dried coke with a lime hydrate slurry in the stated proportions by weight lowering the ignition temperature by 40-45 ° C was observed. Such a lower ignition temperature had a higher reak activity and led to particularly good sulfur incorporation in the ashes.

In the case of impregnation of the fuel with hydrated lime, a slurry of Ca (OH) ₂ is preferably used in 1 to 3 parts by weight of water per part by weight of Ca (OH) ₂. Surprisingly, slurries with one part by weight of Ca (OH) ₂ and one part by weight of H₂O as well as slurries with one part by weight of Ca (OH) ₂ and three parts by weight of H₂O resulted in a particularly strong reduction in the ignition temperature, whereas a treatment of coke with a slurry from one part by weight of Ca (OH) ₂ to two parts by weight of H₂O still resulted in a lowering of the ignition temperature by about 35 ° C. Despite this reduced decrease in the ignition temperature while maintaining a weight ratio of 1: 2, coke breeze treated with a lime hydrate slurry in a weight ratio of 1: 2 shows particularly high sulfur binding values in the ashes, if at the same time the ashing temperature was kept lower.

Sinter coke is primarily used for such sintering purposes, as mentioned above, and in the process according to the invention it is preferred to use sinter coke in a grain size of 0.5-5 mm, preferably 1 to 3 mm.

It has proven to be particularly advantageous if the sintered coke to be used is dried before rolling or before treatment with the hydrated lime slurry, temperatures around 100 ° C. appearing particularly suitable as drying temperatures.

When using a hydrated lime slurry, a high level of sulfur incorporation can be achieved with the least use of foreign material in that the hydrated lime slurry in an amount of 20-50, preferably 25 to 50 wt .-%, based on coke, is used.

As already mentioned above, the sulfur incorporation could be further optimized by controlling the process temperatures and, according to a preferred procedure, the sintering temperatures are kept low.

The invention is explained in more detail below with the aid of examples.

Example 1:

1000 g of sintered coke with a grain size of 1 to 3 mm were dried at 105 ° C. A lime sludge of 125 g Ca (OH) ₂ and 125 g H₂O (1: 1) was added to the pre-dried sintered coke and the coke was thoroughly mixed with the lime sludge. Thereupon it was dried again in a drying cabinet.

Example 2:

1000 g of sintered coke with a grain size of 1 to 3 mm were again pre-dried at 105 ° C in a drying cabinet and a lime sludge consisting of 125 g Ca (OH) ₂ and 250 g H₂O (1: 2) was added. The coke was thoroughly mixed with the lime sludge and stored in the drying cabinet.

Example 3:

1000 g of sintered coke with a grain size of 1 to 3 mm were pre-dried in a drying cabinet at 105 ° C. and mixed intimately with a lime sludge consisting of 125 g Ca (OH) ₂ and 375 g H₂O (1: 3). The mixture was stored in the drying cabinet.

Example 4:

1000 g of pre-dried sintered coke with a grain size of 1 to 3 mm was rolled with 125 g of dry hydrated lime and the non-adhering lime was removed by sieving with a 0.5 mm sieve.

Example 5:

1000 g of pre-dried sintered coke with a grain size of 1 to 3 mm were rolled with 250 g of dry lime hydrate and the non-adhering lime was removed by sieving with a sieve with a mesh size of 0.5 mm.

Carrying out the experiment:

Each of the mixtures of coke and lime sludge or coke and lime hydrate produced in Examples 1 to 5 and a comparative sample which consisted of 1000 g of untreated coke were subjected to the following procedure.

A sample of the coke mixed with lime sludge or hydrated lime was placed in a retort and heated in a tube furnace with air supply at a heating rate of approx. 5 ° C./min. The exhaust gas composition was continuously measured during the heating phase. The ignition point of the sample mixtures was characterized by a significantly faster rise in the sample temperature and the beginning of CO₂ development. The ignition temperatures of the individual samples were 450 ° C. in the case of example 1, 455 ° C. in the case of example 2 and 450 ° C. in the case of example 3. The ignition temperature of the sample of Example 4 was not determined and that of Example 5 was 495 ° C. For comparison, the ignition temperature of the untreated coke with a grain size of 1 to 3 mm is 490 ° C.

In a further test, the sintered coke samples pretreated according to Examples 1 to 5 were subjected to ashing. The ashing was carried out at three different temperatures, namely 900 ° C, 1000 ° C and 1100 ° C. The incorporation of sulfur into the ashes was created here a sulfur analysis before and after ashing. The results of the sulfur incorporation in the ashes are shown in Figure 1, which is a block diagram of the sulfur incorporation in the ashes. In Fig. 1, the percent sulfur incorporation is shown on the ordinate and on the abscissa, arranged side by side, the results of the three ashing tests carried out in each case with untreated coke and with the sintered coke breeze treated with hydrated lime. 1 shows the sulfur incorporation, which was achieved at an ashing temperature of 900 ° C, 2 the sulfur incorporation, which was achieved at ashing at 1000 ° C, and 3, the sulfur incorporation, which was achieved at ashing at 1100 ° C was achieved. From Fig. 1 it can be clearly seen that the untreated coke showed the lowest sulfur incorporation in the ash at all three ashing temperatures. The highest sulfur incorporation value, namely 20.7%, was achieved at the lowest incineration temperature (900 ° C).

Relatively balanced sulfur binding values at the three ashing temperatures could be achieved with the sinter coke pretreated according to Example 1. The same applies to the sintered coke treated according to Example 3, but overall the percentages of sulfur incorporation here are somewhat lower than those for the sintered coke pretreated according to Example 1.

A particularly high sulfur incorporation value in the ash, namely 96.5%, was achieved by ashing the sintered coke pretreated according to Example 2 at 900 ° C. The values of the sulfur incorporation at the ashing temperatures 1000 and 1100 ° C. were approximately in the range obtained for the ashing of the mixtures according to Example 1 and Example 3.

In the ashing of the sinter coke pretreated according to Example 4, an average value of the sulfur incorporation of 86.8% could only be achieved at an ashing temperature of 900 ° C. In the ashing at higher temperatures, the sulfur incorporation values of around 64% are significantly lower than those values that could otherwise be achieved with sintered coke pretreated with hydrate lime.

Particularly high values for the incorporation of sulfur into the ash could be achieved with the ashing of the mixture of coke breeze and dry hydrated lime prepared according to Example 5. In the ashing at 900 ° C, a sulfur binding of 97.1% was achieved and in that at 1000 ° C a sulfur binding of 93.8%.

Overall, it can be seen that the largest percentage of sulfur incorporation is achieved at the lowest incineration temperature and the relatively lowest percentage of sulfur incorporation can be achieved at the highest incineration temperature. In any case, however, the percentages of sulfur incorporation in the sinter coke pretreated according to the invention are at least four times as high as those which could be achieved in the ashing of untreated coke.

The sulfur emission via sintered exhaust gases depends on the sulfur content of the sintered coke. This sulfur burns primarily to SO₂. A reduction in SO₂ formation is theoretically possible through a sulfidic bond in the sinter itself.

By impregnating the sintered coke with lime milk or rolling it with hydrated lime, an optimal distribution of the sulfide-forming calcium and thus a close contact with the sulfur of the fuel is given. When the coke is burned during the sintering process, not only does SO₂ arise, but also CaS, because reducing conditions are also possible in the coke particle.

The temporary setting of the coke sulfur in the sinter reduces the SO₂ emissions through the sintered exhaust gas and helps to comply with or fall below the prescribed emission values.

Claims (6)

1. A method for reducing pollutant emissions in thermal processes, such as in particular sintering processes in which a fuel-containing mixture, in particular a coke bed, is ignited, characterized in that the pore-rich fuel fraction, in particular the coke, of the mixture before the task of the fuel-containing mixture with Ca. (OH) ₂ is rolled or soaked in a slurry of hydrated lime. 2. The method according to claim 1, characterized in that based on the amount of coke 5-30 wt .-% Ca (OH) ₂, preferably 10-25 wt .-%, are used in dry form and the coke rolls together with the hydrated lime becomes. 3. The method according to claim 1 or 2, characterized in that a slurry of Ca (OH) ₂ is used in 1 to 3 parts by weight of water per part by weight of Ca (OH) ₂. 4. The method according to claim 1, 2 or 3, characterized in that sintered coke in a grain size of 0.5-5 mm, preferably 1 to 3 mm, is used. 5. The method according to any one of claims 1 to 4, characterized in that the sintered coke is dried before treatment with the slurry or before rolling with hydrated lime. 6. The method according to any one of claims 1 to 5, characterized in that the lime hydrate slurry is used in an amount of 20-50, preferably 25 to 50 wt .-%, based on coke.

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