EP0553511A1 - Method for the disposal of fluor- and cyanide containing residues - Google Patents

Abstract

For converting water-soluble fluorine compounds into water-insoluble compounds, for decomposing the cyanides and for a low NOx emission, the residues are treated with addition of sulphates of calcium or magnesium in a two-stage fluidised bed at temperatures of 650-900 DEG C, the first stage being run under slightly reducing conditions and the second stage being run under oxidising conditions. <IMAGE>

Description

Some residues contain water-soluble fluorides and cyanides as well as nitrogen compounds. Your landfill is therefore problematic. One such problematic residue is, in particular, the furnace breakout from electrolysis cells of aluminum electrolysis, which consists of the refractory lining and carbon-containing cathode mass, as well as dust arising from aluminum electrolysis. For a safe landfill, it is necessary that the water-soluble compounds are converted into water-insoluble compounds.

From JP-A-50 75 564 it is known to mix waste containing water-soluble fluorine compounds, such as linings of aluminum electrolysis cells with calcium salts, especially calcium chloride and calcium sulfate, and to burn them at 600-900 ° C. in an oxidizing atmosphere. Electric muffle furnaces or rotary tube furnaces are mentioned as firing units. The water-soluble fluorides are converted into water-insoluble calcium fluoride within a treatment time of 30-60 min. In this process, NO _x is generated from the nitrogen compounds contained in the waste and is emitted with the exhaust gas. When using calcium chloride, HCl gas is also generated.

From CEP, March 1986, pages 34 to 38 and "Application of the transpotable circulating bed combuster for the treatment of harzardous waste", Presentation at the 79th Annual Meeting of the Air Pollution Control Association, Minneapolis, Minnesota, June 22-27, 1986 it is known to oxidize the furnace outbreak of aluminum electrolysis cells with the addition of limestone in a circulating fluidized bed Burn conditions at temperatures of 790-870 ° C. The use of limestone leads to the formation of a molten phase and thus to agglomeration and formation of deposits.

The object of the invention is to convert the water-soluble fluorides of the residues as far as possible into water-insoluble compounds, to break down the cyanides as much as possible and at the same time to largely avoid the formation of NO _x from the nitrogen compounds.

• a) die Reststoffe unter Zusatz von Sulfaten von Calcium und/oder Magnesium thermisch behandelt werden,
• b) die thermische Behandlung in einer zweistufigen Wirbelschicht erfolgt,
• c) in der zweistufigen Wirbelschicht eine Temperatur von 650-900°C eingestellt wird,
• d) die erste Stufe der Wirbelschicht unter reduzierenden Bedingungen mit λ < 1 betrieben wird,
• e) die zweite Stufe der Wirbelschicht oxidierend mit einem Sauerstoffgehalt von > 2 Vol.-% betrieben wird, und
• f) das behandelte Material aus der zweiten Stufe abgezogen wird.

This object is achieved according to the invention by a process for the disposal of residues which contain fluorine and cyanide-containing compounds, which is characterized in that

• a) the residues are thermally treated with the addition of sulfates of calcium and / or magnesium,
• b) the thermal treatment takes place in a two-stage fluidized bed,
• c) a temperature of 650-900 ° C. is set in the two-stage fluidized bed,
• d) the first stage of the fluidized bed is operated under reducing conditions with λ <1,
• e) the second stage of the fluidized bed is operated oxidizing with an oxygen content of> 2 vol .-%, and
• f) the treated material is withdrawn from the second stage.

The sulfates are used in a stoichiometric amount based on the formation of CaF₂ or MgF₂ from the water-soluble fluorides. In particular, cheap waste gypsum can be used as calcium sulfate, which is disposed of at the same time. The two-stage fluidized bed can consist of a circulating fluidized bed in which the first fluidized bed is formed in the lower part of the fluidized bed reactor with the fluidizing air flowing through the bottom and the second fluidized bed is formed in the overlying part of the fluidized bed reactor by supplying secondary air and possibly tertiary air. The two-stage fluidized bed can also consist of two separate classic fluidized beds, the solid and the gas being drawn off from the first fluidized bed and passed into the second fluidized bed. The temperature in the first fluidized bed is always in the lower range of the temperature range, since endothermic reactions take place there due to the oxygen deficit. The grain size of the residue is less than 2 mm, preferably less than 1 mm when it is introduced into a circulating fluidized bed, and less than 3 mm, preferably less than 2 mm when it is fed into a classic fluidized bed. The task always takes place in the first stage. The residue can be used in solid form or as a slurry. In the processing of furnace breakouts from aluminum electrolysis, the feed material in most cases already contains so much carbon that its combustion can cover the required heat of reaction. If the residue contains too little carbon, the required amount is added in the form of solid, liquid or gaseous fuel. An optimal setting of the reaction temperature is advantageously carried out by appropriate cooling of the solid, which in the case of two classic fluidized beds between the fluidized beds and in the case of a circulating fluidized bed, the solid separated from the discharged suspension is cooled before being returned to the fluidized bed reactor.

The advantages of the invention are that the water-soluble fluorine compounds are largely converted into water-insoluble compounds without agglomeration phenomena and formation of deposits, the cyanides are largely decomposed and the exhaust gas contains only small amounts of NO _x .

A preferred embodiment is that the temperature in the two-stage fluidized bed is set to 750-850 ° C. In this temperature range, good conversion into water-insoluble fluorides and good decomposition of the cyanides with little NO _x evolution is achieved. In addition, the HF emission is low when water is present.

A preferred embodiment consists in that the first stage of the fluidized bed is operated with λ of 0.7 to 0.9. This keeps the NO _x emissions very low.

A preferred embodiment is that the second stage of the fluidized bed is operated with an oxygen content of 6 to 10% by volume. This results in very good decomposition of the cyanides with very good conversion into water-insoluble fluorides and very low NO _x emissions.

A preferred embodiment is that the addition of sulfates of calcium and / or magnesium is 1.2 to 1.5 times the stoichiometric amount, based on the formation of CaF₂ or MgF₂ from the water-soluble Fluoride content in the residues. This results in a good conversion into water-insoluble fluorides with relatively little addition.

A preferred embodiment is that at high SiO₂ content in the residue by adding Al₂O₃-containing substances, the Al₂O₃ content in the mixture is adjusted to at least 15 wt .-%. The addition is required from an SiO₂ content of about 30%. As Al₂O₃-containing substance Al₂O₃, bauxite, clay, dusts, sodium silicate or the like can be used. This avoids the formation of a molten phase at higher SiO₂ contents.

bzw.

$\text{0,01 ≦ Ar ≦ 100 ,}$

wobei

sind.A preferred embodiment consists in that the two-stage treatment takes place in a circulating fluidized bed with introduction of secondary air and possibly tertiary air into the upper part of the fluidized bed reactor. The circulating fluidized bed system consists of the fluidized bed reactor, the return cyclone and the return line. This fluidized bed principle is characterized in that, in contrast to the "classic" fluidized bed, in which a dense phase is separated from the gas space above by a clear density jump, there are distribution states without a defined boundary layer. There is no leap in density between the dense phase and the dust space above it, but within the fluidized bed reactor the solids concentration decreases continuously from bottom to top. A gas-solid suspension is discharged from the upper part of the reactor. When defining the operating conditions using the key figures from Froude and Archimedes, the following areas result:

respectively.

$\text{0.01 ≦ Ar ≦ 100,}$

in which

are.

u

die relative Gasgeschwindigkeit in m/sec.

Ar

die Archimedes-Zahl

Fr

die Froude-Zahl

ρ g

die Dichte des Gases in kg/m³

ρ k

die Dichte des Feststoffteilchens in kg/m³

d_k

den Durchmesser des kugelförmigen Teilchens in m

ν

die kinematische Zähigkeit in m²/sec.

g

die Gravitationskonstante in m/sec.²

Die aus dem Wirbelschichtreaktor ausgetragene Suspension wird in den Rückführzyklon der zirkulierenden Wirbelschicht geleitet, dort weitgehend von Feststoff befreit, und der abgeschiedene Feststoff wird derart in den Wirbelschichtreaktor zurückgeleitet, daß innerhalb der zirkulierenden Wirbelschicht der stündliche Feststoffumlauf mindestens das Vierfache des im Wirbelschichtreaktor befindlichen Feststoffgewichtes beträgt. Der abgeschiedene, rückgeführte Feststoff wird in die erste Stufe des Wirbelschichtreaktors zurückgeführt. Die erste Stufe erstreckt sich vom Boden des Wirbelschichtreaktors bis zu etwa 20-40% der gesamten Höhe des Wirbelschichtreaktors. In der zirkulierenden Wirbelschicht können die erforderliche Betriebsparameter sehr genau eingehalten und damit besonders gute Ergebnisse erzielt werden.It means:

u

the relative gas velocity in m / sec.

Ar

the Archimedes number

Fr

the Froude number

ρ g

the density of the gas in kg / m³

ρ k

the density of the solid particle in kg / m³

d _k

the diameter of the spherical particle in m

ν

the kinematic toughness in m² / sec.

G

the gravitational constant in m / sec.²

The suspension discharged from the fluidized bed reactor is passed into the return cyclone of the circulating fluidized bed, largely freed of solids there, and the separated solid is returned to the fluidized bed reactor in such a way that the hourly solids circulation within the circulating fluidized bed is at least four times the solids weight in the fluidized bed reactor. The separated, recycled solid is returned to the first stage of the fluidized bed reactor. The first stage extends from the bottom of the fluidized bed reactor to about 20-40% of the total height of the fluidized bed reactor. The required operating parameters can be adhered to very precisely in the circulating fluidized bed, and particularly good results can thus be achieved.

A preferred embodiment consists in that the recycled solid is cooled in a fluidized-bed cooler before being fed into the fluidized-bed reactor. The vortex cooler is designed as a classic fluidized bed. The heated fluidizing air emerging from the fluidized cooler can be passed into the gas cleaning of the exhaust gas from the circulating fluidized bed or can be conducted as secondary air into the fluidized bed reactor. The cooling in the vortex cooler enables good and simple regulation of the temperature in the circulating fluidized bed.

A preferred embodiment is that the stripped material is post-treated by slurry. So much water is added that dust-free transport or a dust-free landfill is possible. At the same time, there is a further decrease in the soluble fluorine constituents in the slurry. In an aftertreatment of the thermally treated solid, part of the calcium or Magnesium sulfates are not added during the thermal treatment, but only after the treatment.

The invention is explained in more detail with the aid of a flow diagram and examples.

Fig. 1 shows a flow diagram with a circulating fluidized bed.

The system of the circulating fluidized bed consists of the fluidized bed reactor (1), the return cyclone (2) and the return line (3). Furnace breakout in the appropriate grain size is charged into the mixer (5) via line (4). Calcium sulfate is fed via line (6) and material containing Al₂O₃ into the mixer (5) via line (7). The mixture is charged via line (8) into the lower part of the fluidized bed reactor (8). Fluidization air is introduced via line (9) through the bottom of the reactor and forms the first fluidized bed in the lower part of the fluidized bed reactor. Secondary air is introduced via line (10) and tertiary air via line (11) into the upper part of the fluidized bed reactor, where it forms the second fluidized bed. The gas-solid suspension is passed from the fluidized bed reactor (1) via line (12) into the return cyclone (2). The separated solid is returned to the lower part of the fluidized bed reactor via the return line (3). A partial flow of the separated solids or the entire solid can be introduced via line (13) into the vortex cooler (14), which is designed as a classic fluidized bed. Fluidizing air is conducted into the vortex cooler (14) via line (15). The cooled solid is via line (16) in the lower part of the Fluidized bed reactor (1) returned. The gas phase is passed from the separating cyclone (2) via line (17) into a second cyclone (18). The solid separated there is returned via line (19) to the lower part of the fluidized bed reactor (1). The gas phase is passed from the cyclone (18) via line (20) into the Venturi scrubber (21). Dusty, heated fluidizing air from the vortex cooler (14) is also conducted via line (22) into the venturi scrubber (21). The gas from the venturi scrubber (21) is passed via line (23) into the wet dedusting device (24). The cleaned gas is discharged via line (25). The sludge obtained in the wet cleaning process is fed into the settling tank (27) via line (26). The overflow is fed into the Venturi scrubber (21) via line (28) and the underflow is introduced into the stirring tank (30) via line (29), into which calcium sulfate is introduced via line (31). The suspension is fed via line (32) into the mixer (33) into which the solid separated from the vortex cooler (14) is passed via line (34). The separated solid is aftertreated in the stirring tank (30) and in the mixer (33). The solid is discharged to the landfill via line (35).

The table below shows the results of treatment of furnace breakout from aluminum electrolysis. In experiments Nos. 7 and 8, an oven cutout with the specified composition without additives was used as the feed. In experiments 1 to 6, a mixture of this furnace outbreak and Al₂O₃ was used. In experiments 9 to 12, another furnace outbreak with the specified composition was used without the addition of substances containing Al₂O₃. Calcium sulfate or calcium carbonate was added in all cases in a molar ratio of Ca: F of 0.6. The experiments 1 to 3 and 8 show the results of comparative experiments without the addition of calcium sulfate. Experiments 5 and 6 show the results with the addition of calcium carbonate instead of calcium sulfate. Experiments 7 and 8 show the result of using furnace excavation material with a very high SiO₂ content. In the column "Solubility" the solubility is given according to the DEV-S₄ leaching. In experiment 12, 5% CaSO₄ was added in the thermal treatment and 2.7% CaSO₄ in the aftertreatment.

Claims (9)

Process for the disposal of residues containing fluorine and cyanide-containing compounds, characterized in that a) the residues are thermally treated with the addition of sulfates of calcium and / or magnesium, b) the thermal treatment takes place in a two-stage fluidized bed, c) a temperature of 650-900 ° C. is set in the two-stage fluidized bed, d) the first stage of the fluidized bed is operated under slightly reducing conditions with λ <1, e) the second stage of the fluidized bed is operated oxidizing with an oxygen content of> 2 vol .-%, and f) the treated material is withdrawn from the second stage. A method according to claim 1, characterized in that the temperature in the two-stage fluidized bed is set to 750-850 ° C. A method according to claim 1 or 2, characterized in that the first stage of the fluidized bed is operated with λ from 0.7 to 0.9. Process according to claims 1 to 3, characterized in that the second stage of the fluidized bed is operated with an oxygen content of 6 to 10% by volume. Process according to claims 1 to 4, characterized in that the addition of sulfates of calcium and / or magnesium is 1.2 to 1.5 times the stoichiometric amount, based on the formation of CaF₂ or MgF₂ from the water-soluble fluoride content in the residues. Process according to claims 1 to 5, characterized in that at high SiO₂ content in the residue by adding Al₂O₃-containing substances the Al₂O₃ content in the mixture is adjusted to at least 15% by weight. Process according to claims 1 to 5, characterized in that the two-stage treatment takes place in a circulating fluidized bed with introduction of secondary air and possibly tertiary air into the upper part of the fluidized bed reactor. A method according to claim 7, characterized in that the recycled solid is cooled in a fluidized-bed cooler before being fed into the fluidized-bed reactor. Process according to claims 1 to 8, characterized in that the material removed is aftertreated by slurry.

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