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09 Feb 2009

METHOD AND ARRANGEMENT FOR TREATING EXHAUST GASES FROM A SUSPENSION SMELTING FURNACE

Background of the invention

The invention relates to a method according to the preamble of claim 1 for treating exhaust gases from a suspension smelting furnace, such as a flash smelting furnace.

The invention also relates to an arrangement according to the preamble of claim 7 for treating exhaust gases from a suspension smelting furnace, such as a flash smelting furnace.

Problem

The sulfur dioxide and flue dust bearing exhaust gases exhausted from the smelting process of a suspension smelting furnace, such as a flash smelting furnace, designed for the processing of sulfidic concentrate, for instance sulfidic copper concentrate or sulfidic nickel concentrate, are conducted through an uptake shaft to a waste heat boiler that comprises a radiation chamber and a convection chamber, and further to an electric filter. The thermal energy contained in the exhaust gases of the waste heat boiler is recovered, and part of the solid substances of the exhaust gases, i.e. flue dust, is separated on the bottom of the waste heat boiler, from where it is removed and returned to the reaction shaft of the suspension smelting furnace. In the waste heat boiler, the flue dust is made to react with sulfur dioxide and oxygen while the dust is still in gaseous state in the boiler, so that the flue dust is sulfatized, and its reactivity with the gas atmosphere is essentially reduced. By means of the energy recovered when cooling the gas, there also is produced vapor that is utilized in other process steps.

In some applications, the waste heat boiler is replaced by direct water cooling, where gases are cooled by a sufficiently large quantity of water, so that also the major part of the flue dust is removed from the gaseous phase along with the cooling water. It has also been suggested that gases could be cooled by water, so that the gas temperature would only drop to the operational temperature required by an electric filter or a bag filter. In that case there would not be created dust sludge, which is the drawback of the above described direct wet cooling process. However, in a waste heat boiler operated in connection with a suspension smelting furnace, this arrangement would not

09 Feb 2009

be feasible, because the flue dusts would proceed as non-sulfatized into the electric filter and react there, rapidly causing disturbance in the operation of the electric filter, owing to a rise in the temperature of the still reacting dust, as well as accretions because of the sticky nature of the dust, resulting in operational interference due to these accretions.

Brief description of the invention

The object of the present invention is to utilize evaporation cooling in the waste heat boiler of a suspension smelting furnace, so that the flue dusts can be recovered in yet dry state and separated from the gas by an electric filter, whereafter they can be fed back into the suspension smelting furnace. Another object of the invention is to realize an arrangement that is economical in investment expenses for treating exhaust gases from a suspension smelting furnace, so that part of the thermal energy contained in the gases is recovered.

The object of the invention is achieved by a method according to the independent claim 1.

Preferred embodiments of the method according to the invention are set forth in the dependent claims 2 - 6.

In a method according to the invention, exhaust gases coming from the uptake shaft of a suspension smelting furnace are treated in two steps before conducting them into an electric filter and further to a sulfuric acid plant.

In the first step, exhaust gases are conducted from the uptake shaft of a suspension smelting furnace to the radiation chamber of a waste heat boiler, where the exhaust gases are precooled, preferably but not necessarily, from the temperature of 1300 ⁰ C down to a temperature of below 650 ⁰ C, and the flue dust contained in the exhaust gases is deactivated by a deactivation gas in the radiation chamber of the waste heat boiler, so that the flue dust contained in the gas is deactivated before being exhausted from the radiation chamber of the waste heat boiler.

In the second step, exhaust gases are cooled by water mist in the evaporation cooling unit of the waste heat boiler down to the operation temperature of the electric filter, advantageously but not necessarily down to a temperature below 450 ⁰ C, preferably below 400 ⁰ C, but advantageously so that the exhaust gas temperature is still over 300 ⁰ C, in which case water mist is fed into the exhaust gas, which water mist is vaporized due to the thermal energy contained in the exhaust gas, thus effectively binding energy, and proceeds as vapor, together with the exhaust gas and the particles contained in the

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exhaust gas, to the electric filter. After the second step, the exhaust gas temperature should be more than 300 ⁰ C, because otherwise the exhaust gas temperature remains too near to the acid dewpoint when arriving to the electric filter, which would result in corrosion damages in the electric filter. In a preferred embodiment of a method according to the invention, exhaust gases are conducted from the uptake shaft of a suspension smelting furnace to the radiation chamber of a waste heat boiler, from a copper concentrate smelting process carried out in a suspension smelting furnace. In this embodiment, exhaust gases are in the first step precooled and sulfatized by air, recirculation gas or oxygen-enriched air in the radiation chamber of the waste heat boiler, so that the copper in the flue dust contained in the gas, mainly present as copper oxide, is sulfatized, i.e. the copper oxide in the flue dust reacts with the oxygen contained in the air or oxygen-enriched air and with the sulfur dioxide contained in the exhaust gas, before being exhausted from the radiation chamber of the waste heat boiler, so that when being exhausted from the radiation chamber of the waste heat boiler, the reactive copper of the flue dust is mainly copper sulfate. The recirculation gas is exhaust gas, from which solid particles are removed in an electric filter, and which is fed from the electric filter to the radiation chamber of the waste heat boiler.

In a preferred embodiment of a method according to the invention, exhaust gases are conducted from the uptake shaft of a suspension smelting furnace to the radiation chamber of a waste heat boiler, from a nickel concentrate smelting process carried out in a suspension smelting furnace. In this embodiment, the exhaust gases are deactivated by oxidizing the sulfides possibly remaining in the dust and precooled in the first step in the radiation chamber of the waste heat boiler, so that the nickel in the flue dust contained in the gas, mainly present as nickel oxide, forms nickel ferrite, and the copper possibly contained in the dust as copper oxide reacts further with oxygen and sulfur dioxide, thus being sulfatized before being exhausted from the radiation chamber of the waste heat boiler, so that the nickel contained in the flue dust is, when being exhausted from the radiation chamber of the waste heat boiler, present as solid oxidic nickel ferrite.

The invention also relates to an arrangement according to the independent claim 7 for treating exhaust gases from a suspension smelting furnace.

The preferred embodiments of an arrangement according to the invention are set forth in the dependent claims 8 -12.

In an arrangement according to the invention for treating exhaust gases from a suspension smelting furnace, the waste heat boiler comprises a radiation chamber for receiving exhaust gases from the uptake shaft of the suspension smelting furnace.

09 Feb 2009

The front section of the radiation chamber of the waste heat boiler comprises a deactivation arrangement for feeding deactivation agent to the exhaust gas located in the radiation chamber of the waste heat boiler, for deactivating the flue dust contained in the gas by means of the deactivation agent. In addition, the radiation chamber of the waste heat boiler comprises a heat exchanger arrangement for recovering part of the thermal energy contained in the exhaust gas in the radiation chamber of the waste heat boiler, so that the exhaust gas temperature is dropped, preferably but not necessarily, for example from the temperature of 1300 ⁰ C to a temperature below 650 ⁰ C prior to feeding the exhaust gases from the radiation chamber of the waste heat boiler to the evaporation cooling unit of the waste heat boiler.

The gas cooling system further comprises an evaporation cooling unit, which is arranged to receive precooled exhaust gas from the radiation chamber of the waste heat boiler.

The evaporation cooling unit of the gas cooling system comprises water mist nozzles for cooling the exhaust gases by means of water mist, advantageously but not necessarily down to a temperature below 450 °C, preferably below 400 ⁰ C, prior to feeding the exhaust gases from the evaporation cooling unit of the waste heat boiler to the electric filter. After the evaporation cooling unit of the cooling system, the exhaust gas temperature should preferably be over 300 ⁰ C, or else the exhaust gas temperature remains too near to the acid dewpoint when the exhaust gases are fed to the electric filter from the evaporation cooling unit of the cooling system, and this could result in corrosion damages in the electric filter.

The electric filter is arranged to receive cooled exhaust gas from the evaporation cooling unit of the waste heat boiler. In a preferred embodiment of an arrangement according to the invention, the arrangement is connected to the uptake shaft of a suspension smelting furnace in order to receive exhaust gases from a copper concentrate smelting process. In this preferred embodiment, the deactivation arrangement of the radiation chamber of the waste heat boiler comprises nozzles for feeding deactivation agent in the form of recirculation gas, air or oxygen-enriched air to the exhaust gas located in the radiation chamber of the waste heat boiler, so that the copper in the flue dust contained in the exhaust gas, mainly present as copper oxide, is sulfatized, i.e. so that the copper oxide of the flue dust reacts with the oxygen of air or oxygen-enriched air and with the sulfur dioxide contained in the exhaust gas before being exhausted from the radiation chamber of the waste heat boiler, so that when being exhausted from the radiation chamber of the waste heat boiler, the copper of the flue dust is mainly present as copper sulfate.

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09 Feb 2009

In a preferred embodiment of the arrangement according to the invention, the arrangement is connected to the uptake shaft of a suspension smelting furnace in order to receive exhaust gases from a nickel concentrate smelting process. In this preferred embodiment, the deactivation arrangement in the radiation chamber of the waste heat boiler comprises nozzles for feeding deactivation agent in the form of air or oxygen- enriched air to the exhaust gas flowing in the radiation chamber of the waste heat boiler, so that the nickel contained in the flue dust of the exhaust gas, mainly present as nickel oxide, reacts with the oxygen of air or oxygen-enriched air and with the sulfur dioxide contained in the exhaust gas, before being exhausted from the radiation chamber of the waste heat boiler, so that the nickel of the flue dust is, when being exhausted from the radiation chamber of the waste heat boiler, mainly present as solid oxidic nickel ferrite.

Remarkable advantages are achieved by means of the invention. An expensive investment in the convection chamber of a waste heat boiler can be avoided. However, by using an arrangement according to the invention, a remarkable share, about 70% in comparison with the conventional method, of the thermal energy contained in the exhaust gas is recovered as vapor, yet so that the solid substance, i.e. flue dust of the exhaust gas, can be conducted in dry state to the electric filter, and further preferably fed back to the suspension smelting furnace. An expensive separate treatment of washing water and the sludge contained therein is not needed.

Detailed description of the invention

The invention is described in more detail below, with reference to the appended drawing, which illustrates a preferred embodiment of the invention. From the uptake shaft 2 of a suspension smelting furnace, exhaust gas 3 with a temperature of about 1,300 ⁰ C is conducted from the throat section 4 to the radiation chamber 6 of a waste heat boiler 5.

The radiation chamber of the waste heat boiler 6 is provided with a deactivation arrangement 7, such as a nozzles, that feed deactivation agent 8, such as a sulfatizing gas, for example air or other oxygen bearing gas, to the hot exhaust gas 3 containing flue dust.

09 Feb 2009

The radiation chamber of the waste heat boiler 6 is provided with a heat exchanger arrangement 9 for precooling the exhaust gas 3 in the radiation chamber of the waste heat boiler 6 by recovering part of the thermal energy contained in the exhaust gas 3.

Exhaust gas 3 cooled down to below 650 ⁰ C is conducted through a channel 10 from the radiation chamber of the waste heat boiler 6 to the evaporation cooling unit 1 1 of the waste heat boiler.

Water mist 13 is fed into the top part of the evaporation cooling unit 11 of the waste heat boiler by means of water mist nozzles 12 towards the exhaust gas circulation. As the water is vaporized, the thermal energy contained in the exhaust gas 3 is bound in water, and as a consequence, the exhaust gas temperature is dropped even further.

As a result of evaporation cooling, the exhaust gases 3 are cooled in the evaporation cooling section of the waste heat boiler down to below 450 ⁰ C, preferably below 400 ⁰ C. After the evaporation cooling unit 1 1 of the cooling system, exhaust gas temperature should preferably be over 300 ⁰ C, or else the exhaust gas temperature remains too near to the acid dewpoint when the exhaust gases are fed to the electric filter 15 from the evaporation cooling unit 1 1 of the cooling system, and this would result in corrosion damages in the electric filter 15.

A mixture of aqueous vapor and sulfatized exhaust gas 3 is conducted in a channel

14 from the evaporation cooling unit of the waste heat boiler to an electric filter 15, where the exhaust gas 3 is separated from the sulfatized solid particles. The separated solid particles 19 are fed, preferably but not necessarily, to the reaction shaft 16 of a suspension smelting furnace.

After the electric filter 15, the sulfur dioxide and aqueous vapor bearing exhaust gas 3 is conducted, in an exhaust channel 17, by means of a blower 18 to a sulfuric acid plant (not illustrated).

Example

Copper is smelted in a suspension smelting furnace. The exhaust gas obtained from the uptake shaft of the suspension smelting furnace is treated, while flowing in the convection chamber of the waste heat boiler, by air blasting, so that there is created a strong turbulence, due to which the flue dust contained in the exhaust gas reacts so that the copper contained therein, mainly present as copper oxide, reacts in its gaseous state with the oxygen of the blasted air oxygen and with the sulfur dioxide contained in the exhaust gas, so that the copper of the flue dust is mainly in the form of copper sulfate when being exhausted from the radiation chamber of the waste heat boiler. In order to

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take place, this reaction requires a certain length of time, and it would not happen, in case the gas were cooled rapidly by water jets/water drops.

The quantity of the exhaust gas entering the convection chamber of the waste heat boiler is 14,700 Nm3/h, and its temperature is 1,360 ⁰ C. Before the electric filter, the exhaust gas temperature is dropped down to the required temperature of 350 °C in two steps: the exhaust gas is first conducted to the radiation chamber of the waste heat boiler, where the thermal energy is recovered, so that when the exhaust gas is exhausted from the radiation chamber, its temperature is 700 ⁰ C. The temperature of the supply water to the radiation chamber boiler is 150 ⁰ C, and it produces vapor at the rate of (60 bar) 7933 kg/h by means of the thermal energy recovered from the exhaust gas.

In the next step, the exhaust gas is conducted, at the temperature of 700 ⁰ C, to an evaporative cooling chamber, where it is sprayed by finely divided water through nozzles at the rate of 3.5 tons per hour, so that the exhaust gas temperature drops down to 350 °C. The essential thing is that the water does not moisten the part of flue dust still contained in the gas, but the flue dust is transferred, along with the gas, to the electric filter, where it is recovered from the exhaust gas, which is thereafter conducted to an acid plant. Because the exhaust gas has reacted with oxygen and sulfur dioxide already in the radiation chamber of the waste heat boiler, its behavior in the electric filter is inert, wherefore undesirable reactions or dust sintering do not take place in the electric filter. The spraying of water can take place through one or several nozzles. If the cooling after the radiation chamber were carried out in a conventional radiation chamber of a waste heat boiler, the obtained total product from the boiler would be 11 ,900 kg/h, i.e. about 33% of this vapor would be lost when applying an evaporative final cooling of the gas. However, normally the vapor produced by the radiation chamber is sufficient to fulfill the own vapor demand of the smelter, and on the other hand, the value of this steam quantity is moneywise compensated by cheaper investments, which is particularly an object of the present invention.

For a man skilled in the art, it is obvious that along with the development of technology, the principal idea of the invention can be realized in many different ways. Hence the invention and its embodiments are not restricted to the above described examples, but they can vary within the scope of the appended claims.