The present invention relates to a method and apparatus for treating process gases by cooling the process gas in the presence of solid particles.

Different high-temperature processes such as melting of ores or metal concentrates and processes with melting, reduction and fuming of metallurgical slags, calcination of cements, high-temperature processes in the chemical industry, etc. generate high-temperature gases containing components that tend to stick to the heat transfer surfaces, thus making the heat recovery from said gases as well as cooling of them difficult. Sticky compounds may also be generated in ordinary gasifying processes. Such compounds that foul heat transfer surfaces are, for example

- compounds that evaporate in the process and conden¬ sate or sublimate by cooling,

molten ^' drops that εolidificate by cooling,

particles that tend to sinter,

fume or aerosol, characterized by a very small par¬ ticle size, usually less than 1 micron, and by a tendency to stick to other fume particles and sur¬ faces met with,

molten or solid compounds resulting from chemical or other reactions.

Depending on the case, a process gas may contain one cr more of the components mentioned above. Their common feature is a tendency to stick to the heat transfer surfaces of the heat exchanger or the boiler when the gas flows through them.

s a result of this, the heat exchanger gradually becomes clogged thus losing its effect, which usually results in running down the process.

The harmful effects of fouling can, in many cases, be reduced by different kinds of blow sweepers or mechanical sweepers such as shakers or blow hammers. The blow sweepers have a disadvantage of consuming high-pressure steam and their sweep gas affecting the composition of the gas to be treated. For reducing gases, for example, air cannot normally be used.

Shakers and blow hammers have proved to be an effective sweeping method under various conditions. Their disadvantage is the restrictions set by them on the boiler structure. Furthermore, shakers are ineffective on superheaters in operation.

Experience has shown that, usually the fouling problem is greatest at a certain temperature range typical to the process where the sintering tendency of "dust" is highest. The reasons affecting such temperature ^" range are explained more in detail in the following.

The following f ctors, among other things, affecting sintering are well known in the field of powder metallurgy and ceramics combustion technology:

- particle size of powder; the finer the particles the lower the temperature in the beginning of sintering,

- when a mixture of compounds reaches a eutectic tem¬ perature in a multi-component system, there will be melt formed in the system, such melt filling the pores between the particles, thereby causing highl effective sintering at a temperature range that can be very narrow indee .

The components evaporated in the process, such as heavy metal and alkalis, tend to condensate or sublimate at a certai temperature characteristic of them. In connection wit cooling, there is formed either melt which condensates on th heat transfer surfaces or on the surfaces of dust particles, thereby making them more sticky, or the evaporated component sublimate direct on the heat transfer surfaces. Phenomena o this kind occur, for example, in the alkali by-pass system in the cement kiln, which is why heat recovery is not usuall succesful in this connection. Corresponding phenomena appear in gasifying processes if the product gas contains alkalis and/or residual tar.

When cooling down close to the solidification temperature, the melt drops in the procesε gas either easily stick to the existing process particles, thereby contributing to the sticking of dust to the heat transfer surfaces, or solidify direct on the heat transfer surfaces and sinter to them.

In fuming operations, metals are intentionally evaporated from molten slag for recovery. For example, Zn, Pb and Sn are separated from the gas phase after evaporation by changing the oxygen potential i.e. by reburning. Especially fine particles or drops are thus formed in the flue gas. The size of par¬ ticles in this kind of fume is typically at the range of 0.1 to 1 micron or even smaller. The fume is characterized by an especially large surface activity and tendency to stick to the heat transfer surfaces, thereby impeding the boiler operation. Therefore, a great deal of fuming operations are still carried cut without heat recovery.

A phenomenon, much like fuming and nowadays well known, appears in the electric reduction processes of ferromixtures .

For example, in the electric furnace reduction of ferrosilicon and silicon, silicon sublimates at a certain temperature zone as silicon monoxide, which oxidates, for example, in the hood of an open or half-closed furnace to silicon dioxide, forming SiO_-fume in the flue gas. With respect to the boiler oper¬ ation, SiO_-fume has proved to be extremely difficult at temperatures exceeding 500 C. In practice, almost all silicon and ferrosilicon are still produced without waste heat re- coverv.

In reduction of ferromixtures and silicon in a closed elec¬ tric furnace, there are formed variable amounts of silicon monoxide and zinc as well as alkali metal vapours depending on the impurities in the feed materials. In cooling of a gas like this, the temperature of which may be even 1000 to 1300 C when coming from the process, the silicon monoxide oxidizes to SiO„-fume, and other vapours mentioned above condensate either direct on the heat transfer surfaces or first as a fume in the flue gas. Thereafter, fumes stick to the heat transfer sur¬ faces, which fairly soon results in decreased efficiency and usually also in clogging of the heat exchanger.

An example of chemical reactions induced by cooling and of fumes resulting from said reactions is the melting of sul¬ phide-based lead concentrates which process generates Pb-PbO-rich flue gas containing SO,, and being of the tempera¬ ture of 1200 to 1300°C. As the gas is cooling in the boiler, evaporated Pb and PbO begin to condensate and, en the other hand , chemical balances change so as to form lead sulphate at the temperature range of about 900 to 50C°C, said lead sulphate separating from the gas phase as fume-like particles. At the same time, a great deal of heat is released due to con¬ densation, and reaction heat is released from the sulph izing reactions. The conditions for sulphatizatioπ are advantageous as the hot gas flow contacts heat transfer surfaces with effective cooling and as at the same time, the heat transfer

surfaces operate as a base onto which the lead sulphite formed separates.

Sintering of particulate material, improved by the sul- phatizing reaction appears in most melting processes of sulphide concentrates, whereby vapors, fumes, melt drops or particles of, for example, lead, copper, zinc, nickel and other metals and oxides are formed, which vapors, fumes, melt drops and particles sulphatize as the gas is cooled. As the melting technology has begun to use more and more concentrated oxygen and pure oxygen, local temperature peaks in the process as well as concentrations of sulphur oxides of gases rise, which results in an increased relative significance of the sulphatizing reaction with consequent fouling problems. Another simultaneous phenomenon has been the exploitation of more and more complex and impure deposits, which has, for example, raised zinc and lead contents in copper concentrates, and considerably increased the share of such components that vaporize and sulphatize intensively in process gas particles as well as fouling problems of heat transfer surfaces.

The problems of sulphatizing can somewhat be helped by bloving extra air into the radiation chamber of the boiler. This contributes to a very complete sulphatization already in the radiation chamber. Technically, this is not, however, advan¬ tageous to the process as the particulate material from boiler is usually returned to the beginning of the process. In this case, returning of sulphate increases the circulation load of sulphur and the energy consumption of the melting process.

The above gives a fairly clear illustration of the reasons for fouling of the heat transfer surfaces. In this connection, however, there is no reason for a more detailed approach.

Sevaral means have been suggested to solve the fouling problems of boilers and heat exchangers. The following gives a more detailed description, by way of examples, of such known arrangements that utilize either fluidized bed technique or characteristics of said technique.

US-patent 2,580,635 discloses a method of condensating sub- limable compounds evaporated in a gas from said gas as fine particles. In the method described, the gas is cooled by fairly rough solids (grain size appr. 0.7 mm) in a vertical chamber where the gas flows upwards and the solid particles downwards. The solid particles, the grain size of which has to be carefully chosen in proportion to the flow velocity of the gas, are cooled in a separate system and then recirculated to the upper side of the system. The use of the method is limited to condensing of condensable vapors into a fine fume.

US-patent 2,583,013 discloses a method of condensating from a gas such sublimable compounds that have evaporated in said gas. In the method solid particles are fed to the gas flow before the heat exchanger, whereby the gas cools in the heat exchanger in the presence of solid particles, and sublimation takes place on the surface of the solid particles suspended in the gas. The solid particles function as nuclei for sublimable material. They decrease fume formation, contribute to scouring of the heat transfer surfaces and improve heat transfer. Impeccable operation calls for a suspension density exceeding 1 166 kkgg//mm aa-t the heat exchanger and a gas flow velocity of 0.9 to 2.1 m/s,

US-patent 2,721,626 discloses a method of coding hot gases that contain solid particles and foul heat transfer surfaces by mixing solid particles into the gas flow, said particles being considerably larger in size (e.g. 10 to 20 mesh) than the solid particles present in the gas prior to the cooler and by leading said gas-solids mixture at a high velocity (3 to 23 m/s) through the cooler, whereby the amount and mesh cf

coarse solids is controlled to cause sufficient abrasion for keeping the heat transfer surfaces clean. After the cooler, original fine particles present in the process gas and the coarse solids added are separated from each other. The use of the method is limited by, for example, the erosion brought by coarse solids, which erosion improves in cleaning but also wears the heat exchanger, thus shortening its life¬ time.

US-patent 3,977,846 discloses a method of separating hydro¬ carbons (tars) from hot gas by condensating hydrocarbons on the surface of particles in a cooled fluidized bed. The method described uses a separate gas as a fluidizing medium and introduces the gas to be treated in a separate duct and through nozzles or openings in said duct to the middle area of the fluidized bed, whereby cooling of the gas and condensation of hydrocarbons take place rapidly so that the hydrocarbons cannot condensate on the reactor walls or on the cooling surfaces, which are disposed in a dense fluidized bed below the gas inlet openings. The method is restricted by the following: as the gas to be treated has to be introduced through nozzles or openings, it is applicable only if the gas does not contain any compounds that would sinter at the inlet temperature. Separate fluidizing medium is also an encum¬ brance. According to experience, disposition of the cooling pipes at the bottom of the fluidized bed foretells con¬ siderable costs and, due to erosion caused by the fluidized bed, also safety risks.

US-patent 4,120,668 discloses a method for cooling gas con¬ taining melt drops and volatilized components either in a cooled fluidized bed or before heat transfer surfaces by means of circulating particles cooled in a circulating fluidized bed reactor. The process gas itself is used as a fluidizing medium, whereby the need for external gas can be avoided. Furthermore, the level of temperature in the fluidized bed or the ratio of gas flow to particle flow in the circulating

fluidized bed reactor have been chosen so that the mixing temperature is below the solidification point of molten and condensating components. In a circulating fluidized bed reactor, particles are introduced into the gas flow through a separate control valve of the fluidized bed reactor, func¬ tioning as an intermediate tank, wherefrom particles, at a high velocity (appr. 10 m/s) flow into the process gas flow, thus mixing with the gas to be cooled.

The method relates especially to cooling of the product gas from a pressurized molten salt gasifier. When the method is applied in conditions, in which a high pressure prevails and in which a low eutectic temperature of the particles calls for a relatively low mixing temperature when compared with the gas inlet temperature, it generally results in large particle flows and high suspension densities, which cause erosion problems, for example in the heat exchanger section.

US patent 4,391,880 discloses methods of separating vol¬ atilized catalysts from product gases and of heat recovery by means of cooling the gas flow by mixing colder, cooled cata¬ lyst particles with it to such an extent that a desired tem¬ perature level can be reached, and by means of separating said particles from the gas flow and by cooling them in a separate fluidized bed cooler before returning the particles to the gas flow. A disadvantage of the system is its being composed of several single processes, between which there are large flows of solid particles.

DE patent publication 3439600 discloses a method of producing and cooling of sulphur-free gas by means of leading the product gases to a fluidized bed. In the method the product gas is led either from above or from the side to a cooled fluidized bed, which is fluidized by an after-cooled and purified product gas. Disposition of heat surfaces in a dense fluidized bed usually results in wearing problems and con-

sequently in safety risks. Leading gas to a dense fluid¬ ized bed and using it for fluidization calls for a system where rather big pressure losses have to be won, which again raises the nominal effect.

Fl patent 64997 discloses a method where the temperature of a gas containg melt drops is, before the heat exchanger, lowered below the eutectic temperature range of melt drops by means of mixing solid particles cooled in the heat exchanger, separ¬ ated from the gas and recirculated, with the gas. In this method, solid particles are simply recycled from the particle separator and instantaneously mixed with the gas in the space above the gas inlet opening.

The method requires a certain minimum process gas flow, on one hand, to prevent particles from flowing out of the system through the gas inlet opening and, on the other hand, to lead the particles with gas flow through the cooler. This is a considerable restriction on the function of the method in practice. Furthermore, one has to consider the poεsiblity of a sudden intermission of the process gas flow which causes the circulating particles in the system to flow down through the gas inlet opening.

Applications where low mixing temperatures are required due to low eutectic temperature of particles and consequently a low mixing temperature, easily result in great densities of

-_ mas? flow, i.e. over 5 kg/Nm ^" ', which brings about greater pressure losses caused by the system as well as erosion problems.

Furthermore, one has to pay attention to the fact that, in those applications in which a low eutectic temperature or some other reason calls for a low mixing temperature and, on the other hand, a high temperature of heat transfer surfaces, the construction is disadvantageous. The length (= height) of the heat transfer surface easily grows to 20...50 . The high

construction together with the high density of the mass flow causes a pronounced meaning of the pressure loss as an obstacle because the pressure loss is proportional to e.g. the height of the heat exchanger.

A well-known and applicable method of cooling process gases is circulating of cooled and purified gas and mixing it with the process gas before the heat exchanger so as to achieve a temperature low enough for eliminating stickiness of particles. Circulating of gas involves three weaknesses:

1. Depending on the inlet, mixing, and outlet temperatures, the amount of gas to be circulated has to be 1.5 tc 4 times that of the process gas. Thus, the amount of gas to be treated in the boiler and the gas purifying equipment will be 2.5 to 5 times that of the process gas, which again results in high investment- and operating costs.

2. In cooling gas by mixing gas with it, the components, such as alkalis, heavy metals etc., evaporated in the process and condensating or sublimating in the cooling system, form a very finely powdered fume. A finely powdered fume is characterized by a lower sintering temperature than a coarser particles of the same material, as earlier stated. Furthermore, the fume is characterized by a tendency to stick to the heat transfer sur¬ faces as also mentioned earlier. Therefore, a mixing temper¬ ature low enough, i.e. a sufficient amount of circulating gas has to be used in order to provide a well-functioning arrange¬ ment. Separating of very finely powdered fumes from large gas flows is technically very difficult. Thus, use of circulating gas means highly expensive arrangements.

3. A large increase in the amount of circulating gas, which for the reasons described above is necessary in practice, considerably lowers the partial pressure of condensating and εublimateing components. Consequently, to effect conden-

sation and sublimation, the temperature must be lower than what is necessary with an undiluted or a little diluted gas. This, on the other hand, increases the need for circulating gas.

Spraying of water or some other evaporating liquid into the gas flow has been used in cooling of process gases and hereby cool the gas before the heat exchanger to a temperature low enough with respect to the stickiness of particles. The method has, for example, the following weaknesses:

if water is used, high consumption of water, considerable increase in aqueous vapour content in the gas flow, great change in the oxygen potential and high fume formation producing particulate material that is very difficult to separate, as earlier stated. Due to the lowered temperature level, there is a great decline in the amount of heat that can be recovered, which is why heat recovery is usually disregarded. Water spraying is mostly used merely as a method ^" of cooling the gas before filtering.

in the chemical industry, it is often possible to spray some liquid which is contained in the gas and then condensed from the gas in the procesε. In other words, it is a process in which the vaporization heat can be utilized. Cooling itself or transfer of heat from the process is effected in a condensator. By spraying the component condensed from the gas, it is easy to regulate the temperature level of the gas in, for example, selective condensation or sub¬ limation without bringing to the gas any foreign components in terms of the process. Like water spraying, this method also involves high fume formation in practice, as to sublimating components. Sublimated fumes can usually be separated only by filtering or by an electric filter.

3 2

The above description gives a rather detailed picture of the phenomena associated both with cooling of high-temperature process gases and consequent fouling problems of heat ex¬ changers, which problems again hamper cooling of gas, economi¬ cally important heat recovery and purifying of gas, the latter being significant both for the process economy as well as in the environmental aspects.

The above also discloses a great number of known methods and their weaknesses.

The purpose of the present invention is to provide a simple and efficient method

- of cooling gases of high-temperature processes, which gases contain evaporated, molten and/or solid components, and

- of recovering heat in a most appropriate manner, for example, as a high-pressure or low-pressure steam etc. or by heating the powdered material to be fed in the procesε or by carrying out a thermal or chemical treatment of the powdered material such as the feed material of the process, by utilizing the heat of the process gas, and

- of purifying gases by minimizing the formation of finely powdered fumes and by adsorbing fumes, melt drops and particles from the gases to be cooled, and of minimizing the occurrence of non-desirable chemical etc. reactions by cooling the gases at a velocity high enough over the desired temperature range, or

- of accomplishing some other desired reaction or phenomenon such a chemical reaction that takes place at a certain temperature, suspension density or within a certain time.

Ail of the above alternatives are not usually possible in one application.

The method according to the invention is characterized in that among the gas to be treated there are, either simultaneously or at short intervals, mixed both solid particles and gas and/or liquid evaporating at a mixing temperature and that at least one of the components, solid particles or gas, to be mixed with the gas is colder than the gas to be treated.

The apparatus according to the invention for treating process gases is characterized in that there is a mixing chamber fitted in the reactor, said chamber having a downwardly tapering conical bottom with at least one inlet opening at the lower end of the conical bottom or in the immediate vicinity thereof, said opening being purposed for the gas to be mixed with the process gas or for the evaporating liquid, and that in the wall ^' or conical bottom of the mixing chamber there iε at least one inlet pipe for the solid particles.

In the method of the invention, mixing of the process gas with gas and solid particles is favourably accomplished so as to cool the process gas from the inlet temperature to the desired mixing temperature at a high velocity, usually at 10 3 - 105 oC/s or even more quickly, m the mixing com¬ partment, whereby cooling to the mixing temperature is effected like extinguishing. Cooling from the inlet temper¬ ature to the mixing temperature iε so quick that there will be no time whatsoever for any undesirable chemical reactions. In the presence of solid particles, condensating and sublimating take place heterogeneously onto the surface of the solids, which prevents formation of furaeε that would be created through homogeneous nucleus formation.

The mixing temperature is preferably chosen so that the components and melt drops to be separated (sublimated/ condenεated) from the gas will solidify and the solid particles that possibly sinter will cool below the temper¬ ature at which the sintering begins and that possible un-

1 £

desirable chemical reactions induced by the change in temper¬ ature will be prevented in the absence of kinetic chances because of the low temperature and that desirable reactions take place at the kinetically favourable temperature range.

The method according to the invention is preferably applied so that the mixture of the process gas cooled to the ixinσ temperature, the cooling gas heated to the mixing temperature, and the flow of solids will be either

a) cooled further to a desired temperature, for example, in a heat exchanger or by mixing cold gas or by spraying liquid, which will evaporate,

and the solids will be separated from the .gas in an appropriate manner, whereafter a suitable amount of them will be returned to the mixing compartment where they wil 1 be mixed with the incoming process gas flow,

and the gas flow will continue to the next stages of the procesε, such as after-cleaning, after- cooling, condensing, etc. , and

after a suitable stage of process, part of the gas flow will possibly be returned (in the form of gas or liquid) to said mixing compartment, where it will be mixed with the incoming gas flow, or

b) solids will be separated from the gas flow at a mixing temperature in an appropriate manner, for example, by a cyclone, filter, or electric filter, and solids will be returned either direct or through a potential intermediary cooler back to the mixing compartment, where they will be mixed with the in¬ coming gas flow,

and the gas flow will continue to the next stages of the process, such as after-cleaning, after-cooling, condensing, etc. After a suitable stage of procees, part of the gas flow will possibly be returend (in the form of gas or liquid) back to said mixing com¬ partment, where it will be mixed with the incoming flow of process gas.

An essential advantage of the method is that, the proportion of the flow of solid particles to the gas flow, both being used for reaching the mixing temperature and at least one of them being colder than the incoming process gas, can be chosen so as to achieve optimal conditions in which

on one hand, fume formation will be minimized, i.e. the flow of solid particles is dense enough to function as a heterogeneous nucleus creator, to the surface of which the sublimating and condensating components will "grow", fume particles adsorb and melt drops stick and solidify,

and on the other hand, the density of formed suεpension will minimize, whereby the harmful phenomena such as great pressure loss, pressure vibrations, wear, slowness of adjustment etc. related to the handling of dense suspension will be minimized or eliminated altogether.

Each characteristic of the present invention will be empha¬ sized depending on the case. For example, when heat recovery iε of primary importance, it is natural to strive ,for the highest possible temperature, within the limits of the operation, in which the force on heat transfer is as big as possible. On the other hand, a sufficient suspension density must, however, be chosen in order to provide efficient ad- sorpotion of solidifying melts, fumes, and condensating components and minimize the cost of gas purification. Further,

the amount end inlet temperature of the circulating gas affects the total amount of gas flowing through the heat surfaces, the density and the flow velocity of suspension and thereby the heat transfer figure and the total amount and cross section/length relation of the heat transfer surface, which may form a highly essential factor both in terms of structure and pressure losses, as earlier stated.

In special cases, use of liquids containing solved salts etc. may be applicable in order to establish a cooling effect. In this case, while the liquid is evaporating in the gas, the compounds solved in the liquid may simultaneously be adsorbed in the circulating particles instead of allowing them to form finely powdered particulate material which is difficult to separate.

Hence, the optimum arrangement is affected by so many factors that it is not possible to give a universal equation for calculating such an arrangment. The optimum arrangement has to be found case by case based on known terms. Essential to the present invention is its flexibility in establishing a well-operating arrangement in quite extensive terms.

Furthermore, the method according to the invention is featured by its adjustability regarding the process gas flow. Namely, use of circulating gas enables, if necessary, staying of solids in the circulation even though the flow of the process gas to be cooled will stop. The risk of solids fallinσ out of the cooler iε hereby eliminated.

The operating manner and the advantages of the method are further described by way of example in the accompanying drawings, in which

Fig. 1 is a schematic illustration of an application of the method according to the inventio , Fiσ. 2 is an illustration of another application of the method of the invention,

Fig. 3 iε an illustration of a third application of the method of the invention, Fig. 4 is an illustration of a fourth application of the method of the invention, Fig. 5 is a vertical sectional view of a detail of an apparatus for applying the method of the invention, Fig. 6 is a sectional view of Fig. 5 taken along line A-A, and Fig. 7 is an illustration of a fifth application of the method of the invention.

Fig. 1 illustrates a system according to the invention, in which system the process gas is cooled and heat is recovered therefrom.

Process gas 1 is fed into a reactor 2 through an inlet opening 4 at the bottom 3 of said reactor. At the bottom part of the reactor is disposed a mixing chamber 5, at the funnel-type bottom 6 of which chamber there is an opening 7 disposed at a distance from the bottom the reactor. The bottom of the reactor and the bottom of the mixing chamber form an air box 8 between themselves, into which box the cooled circulating gas 9 is fed. At the top part of the reactor iε disposed a heat exchanger 10. After the heat exchanger, gas 11 flows to the first cyclone separator 12 in which solid particles will be separated therefrom.

At least part of the solids separated in the first separator is returned to the mixing chamber by means of a return pipe 16. Solids flow down along a slanted surface cf the bottom of the return pipe towards the opening 7 where the process gaε, cooled gas and returned solids will meet. The gas 13 partly purified in the first separator iε led into another cyclone separator 14. Part of the gas 15 purified in the second separator is led into the air box 8 of the reactor. It is also possible to feed new solids to the mixing chamber through a pipe 17.

Example 1

In pressurized systems, the advantages of circulating gas are pronounced. The example below presents a molten salt gasifier mentioned earlier, and let us assume the following:

pressure 10 bar inlet temperature of process gas 1000 °C mixing temperature before heat surfaces 600 °C outlet temperature after heat surfaces 300.°C saturated steam temperature 280 °C average nominal heat of gas 1000 -> 600 C 1.6 J/Nro 3-/ ,oC. average nominal heat of circulating par¬ ticles 0.8 kJ/kg/c,.,

inlet temperature of circulating gas 300 °C average nominal heat of circulating gaε

300 -> 600°C 1.4 kJ/Nm ³ /°C

With relative circulating gas as variable, the following values are obtained:

V Icirculation/ Vtotal particle suspension

Vprocess circulation density kg/Nm ³ kg/Nm kg/m process gaε

0.00 1.00 2.667 2.667 8.339 0.25 1.25 2.22S 1.783 5.77 ^" 0.50 1.50 1.792 1.194 3.735

1.75 1.354 0.774 2.420

1.00 2.00 0.917 0.458 1.433

19

The above table indicates that even as low amount of cir¬ culating gas as 75 % enables decreasing by 50 % the need for cirulating particles, whereby the suspension density will fall by nearly a third. With a 100 % circulating gas, which is often still quite reasonable, it is possible to decrease the particle circulation to a third and the suεpension de εity to a sixth of the original figures without circulating gaε.

3 With as low as 1 to 5 kg/m suspension density, ar average particle density around 10 /m and even higher is achieved, which normally suffices to bring about the above-mentioned desirable phenomena, i.e. to prevent fume formation and to adsorb ultra-fine particles already formed. By regulating the amounts of circulating gas and particulate material, it is easy to establish such case-by-case optimum conditions that, on one hand, the fume formation will be minimized and on the other hand, the suspension density and circulating particles flow with consequent harmful effects will also be minimized.

Example 2

In the example above, the gas inlet temperature 1000° is still quite low. The significance of circulating gas is hereby pronounced, primarily because of pressurizing. For example, in most applications related to melting of sulphide concentrations, the gas inlet temperature ranges from 1200 to 1400°C and an applicable mixing temperature from 500 to 700°C. Considering the high nominal temperature of gas, the signi¬ ficance of circulating gaε car. be seen quite clearly also in depressurizec systems, as shown by the fcllowinσ example:

pressure 1 bar abs inlet temperature of process gaε 1300 ^c c mixing temperature before heat surfaces 600 °c outlet temperature after heat surfaceε 350 °C temperature of saturated steam 280 °C

average nominal heat of gas at the range 1300 -> 700°C 1.9 kJ/Nιrι /

°C average nominal heat of circulating particles 0.9 kJ/kg/ o„ inlet temperature of circulating gas 350 average nominal heat of circulating gaε at the range 350 -> 700°C 1.6 kJ/NirfV

Vcirculation/ total particle suspension V circulation density process process gas

0.00 1.00 3.619 3 . 619 1. 015 0.25 1.25 3.175 2 . 540 0 . 713 0.50 1.50 2.730 1 . 820 0 . 511 0.75 1.75 2.286 1 . 306 0. 366 1.00 2.00 1.841 0 . 921 0 . 258

Hence, by the use of circulating gas, the particle cir¬ culation can be easily decreased to a half and reach a level of suspension densities that is nearly the same as the sum of emission from the process, which in cases like this is generally 0.1 to 0.5 kg/Nm 3. A suspension under 1 kg/Nm3 behaves much like a gas flow, which highly simplifies the implementation of the equipment.

Fig. 2 discloses a circulating gas system where cooling of gas is effected after separation of solid particles, i.e. for purified gas. Here, the method according to the invention is intended for separating evaporated alkalis of cement furnaces from bypass gas, -which operation requires a low mixing tem¬ perature.

Fig. 2 differs from Fig. 1 only in that the circulating gas 9 is led through an intercooler 18 before feeding said, gaε to the air box 8 of the reactor 2. For thiε reason, the same reference numbers denote equivalent parts.

Example 3

The following example studies the effect of circulating gaε on purifying the alkali bypass gas of a cement furnace as well as in heat recovery:

pressure 1 bar abε inlet temperature of process gas 1050 o_ mixing temperature before heat surfaces 350 outlet temperature after heat surfaces 250 temperature of saturated steam 180 average nominal heat of gas at the range 1050 -> 350°C 1.8 kJ/kg/

°C average nominal heat of circulating particles 0.9 kJ/kg/

inlet temperature of circulating gaε after intercooling 150 average nominal heat of circulating gaε et the ranσe 250 -> 350°C 1.5 kJ/Nm ^" / o„

V V circulation/ total particle suεpension Vprocess circulation denεity kg/Nm kg/Nm kg/m ^" process gas

0.00 1.00 14.000 14.000 6.135 0.25 1.25 13.167 10.533 4.616 0.50 1.50 12.333 8.222 3.603 0.75 1.75 11/500 6.571 2.880 1.00 2.00 10.667 5.333 2.337

By the circulating gas, which in this case has simply been subject to intercooling, the suεpension density is easy to reduce to such a level where it is possible to eliminate pressure losses and other problems related to the handling of dense suspension at the same time not losing any essential advantages of the system. In this case, intercooling is a highly simple operation because there are no problems with fuel gases of the cement furnace, nor with acid dew points or water dew points for that matter. On the other hand, inter¬ cooling is effected with a fairly pure gas, which iε why fouling of the intercooler is no problem.

Example 4

The chemical industry provides several examples of processes where chemical compounds can be separated from each other by means of selective condensating and/or sublimating. The fol¬ lowing gives an example of applying the method of the in¬ vention to refining the gas produced in titanium chlorination. Titanium chlorination generates a gaε containing several metal chlorides, from which gas selective condensating or sub¬ limating help separate other chlorides, such as MnCl„, FeCl^ and AlCl^ before condensating the main product, i.e. iCl^ . A small amount of MnCl„ evaporates, because of its high steam pressure, to the chlorination gas, wherefrom it condensates as an impurity to FeCl_, thus deteriorating the quality of ferrite chloride produced.

The following handles selective separation of MnCl.. before sublimating of ferrite chloride.

Melting and boiling temperatures ( C) of chlorides:

Melting Boiling temperature temperature

KnCl ₂ 650 1231

FeCl^ 307 235

TiClg ^• -25 137

In principle, selective sublimation of Mn-chloride is easy because its melting point is over 300 C higher than the boiling point of ferrite chloride. Normally, sublimation of Mn-chloride is effected by spraying a sufficient amount of TiCl.-liquid into a clorination gas of around 1000 C. The TiCl^-liquid hereby evaporates among the gas, thus cooling the gas to about 450 C. Mn-chloride then sublimates as a very fine fume, the separation of which is, in practice, not possible by any simple means such as a cyclone.

Fig. 3 illustrates the use of the method according to the invention for selective separation of Mn-chloride and Fe- chloride from the chlorination gas. Gaε 101, the temperature of which is 1000 C, iε fed to an inlet opening 106 of a mixing chamber 105, to which opening also fluid TiCl. 109 is sprayed, which, while evaporating cools the gas. At the same time the gas meets the flow of solid particles containing MnCl_ par¬ ticles, which flow iε returned from a separator 112 through a return duct 116, whereby sublimation is effected direct onto the surface of the solid particles and formation of fume, which- 'is difficult to separate, is eliminated. Solids con¬ tained in the gas 111 leaving the reactor 102 and having been cooled to 450 C are separated at two stageε in separators 112 and '114. Part of the solids separated in the first separator 112 is discharged through a pipe 119 and all solids from the second separator 114 are discharged through a pipe 120. The above provides a simple means of separating MnCl~ selectively before sublimation of ferrite chloride.

Gas 115 leaving the second separator iε handled ^" in a corre¬ sponding manner in order to separate ferrite chloride 121 in a system 122, where gas is cooled to about 300 C by means of fluid TiCl. 123. TiCl. is separated from the gas 124 leaving the syεtem 122 by means of condensating in an apparatus 125. Part of the separated Ti-chloride is led to the system 122 and another part to the reactor 102.

In several high temperature processes, the process iε fed b a dust-like or pulverous feed material. As an example, let us mention glasε melting, flaεh smelting processes and combustion of cement. In terms of energy economy of the above processes, it would be most reasonable to utilize the heat content of fuel gases in preheating of the feed material. Combustion of cement is a good example of the system which makes use of the heat content of fuel gases in preheating the feed materia in so-called cyclone preheaters where precalcination is also effected. Mostly, however, there is such a disproportion tha the heat content of the fuel gases is too high for the feed to be mixed with the fuel gases either for reasons related t the process technology or to the terms of operation becaus the feed material would become too hot and consequently sinte or start, for example, to react, thus losing propertie demanded of it. In practice, only a certain part of the fue gas heat can be utilized in preheating of the feed material. It should be possible to utilize the rest either in preheatin of blown air or conventionally in, for example, generating o steam. In some cases, part of the fuel gas heat can also b used for preheating of the ^" blown air. In oxygen *blown pro¬ cesses, however, there is no such possibility.

Figure 4 illustrates using of the method of the presen invention for preheating of pulverous feed material. Ho process gas 401 is fed in through an inlet opening 404 of mixing chamber 405 of the reactor 402, whereafter said proces gas meets cooled circulating gaε 409 and a solids flow 41 returned from a separator 412. From a εilo 426 the pulverou feed material is dosed to a mixing chamber. The gaε 435 pu rified in separators 412 and 414 iε cooled in a cooler 427 an part 409 of the cooled gas 428 is led into the reactor. Th temperature of the feed material can, by the help of the cir culating gas _r be easily regulated to an optimal level in term of the operation, the emission from the process can be separ ated and returned to the procesε together with the fee material, and the rest of the waste heat can be utilized in

for example, generating of εteam or heating of blown air or both. Because condensating and fume-forming components as well as molten ones are adsorbed from the σa ε at an early εtage of cooling, heat exchange effected through heat transfer surfaces becomes essentially easier and consequently the equipment more favourable in price.

Example of iron manufacturing:

In the field of iron deoxydation, several different methods have been developed in order to replace, for example, blast furnaces in the manufacture of iron. There is an interesting chance to utilize the heat content and deoxydation potential of the discharge gas of a converter that involves bottom blowing by coal and oxygen, in preheating and preoxydation of the process feed material before actual melting and final deoxydation.

Cooling of gas flow produced by a melting process or part of such flow by water spray, steam, or circulation of gas cooled by water spray is known from several connections. A disadvan¬ tage of these syεtemε is, for example, changing of the gas analysis and oxygen balances or that the heat content of the gas is not possible to recover, as stated earlier. A further disadvantage, both in gas circulation and in water spraying is that, formation of fumes that are difficult to be separated cannot be prevented, as also stated earlier.

Preheating of the feed material iε facilitated by the method of the present invention in the system as shown in figure 4. The method of the present invention provides, in connection of the melting ar.d deoxydation process blown by coal and oxygen, an opportunity, for example, for the following:

- both to regulate the temperature of the discharge gases of the blast furnace blown by coal and oxygen to a desired level

- and to adequately purify the gases

- and further, to utilize the gases in the pre-reduction of the iron concentrate, which is used as a feed material of the

process, before melting and final deoxydation effected in that connection.

In this way, both the heat content and the chemical potential of the process gases can be utilized in the best possible manner and reach the best possible overall energy economy.

Several different process concepts are known in literature which strive for utilizing gases of the coal-oxygen blown melting procesε in pre-reduction of the proceεε feed material. In theεe arrangements, it is recommended to cool, purify and reheat the gaε before the pre-reduction process. The methods are complicated and, above all, they are too expensive to provide for sufficient economy.

By the method according to the invention, it is posεible to simply cool the gases of the blast furnace to a εuitable level of temperature in view of the deoxydation process as well as to purify them from fumes awkward to the deoxydation processes before leading the gases to said procesε by not affecting the analysis of the gas itself. Depending on the deoxydation process used, the gas has to be cooled to a temperature level of 700—1000 C. The method can be realized by, for example, a plant arrangment as shown in fig. 2 w ^τ hich, however, need not have heat transfer surfaces at the top of the reactor 2. A suitable circulating particulate material can be chosen case by case, usually so that it is possible to return it to the process together with the particleεε separated from the process gas. Furthermore, it iε possible to choose the cir¬ culating particles so that they will not, even at high tem¬ peratures, εinter or rhat it iε possible to feed such materialε in the circulating particleε that prevent sintering of such particles. The advantage of the method is that the temperature of the gas is adjustable according to need, the gas iε poεεible to be purified from fumes, and the heat released in cooling can be utilized in generating process steam cr high-pressure steam, not affecting the analysiε of the eras itεelf.

The above discloses application of the method according to the invention to temperature regulating and purifying of the gases of an oxygen-coal blown iron melting reactor before using the gases in the deoxydation process. In this connection, there is also another poεsiblity, which is to some extent analogous with the preheating of the feed material. Here the gases of the melting process are led into an apparatus (fig. 2) according to the invention, where the temperature is regulated suitable for pre-reduction by means of circulating gas and possibly by means of heat surfaces inserted in the circulating fluid bed reactor, and iron concentrate to be pre-reduced is used as circulating particles. The feed of the concentrate and the amount of both the circulating gas and the particles circulated is regulated so as to receive a retention time sufficient for the pre-reduction. Thereafter, the hot pre- reduced material is either fed direct into the blast furnace or it is cooled, posεibly turned to brickets and used, after possible storing, for melting. Feed of hot material direct into the melting process is naturally the best way in terms of energy economy. In practice, there may be ether factors that speak in favour of cooling and storing.

The gas leaving the pre-reduction stage at the same tempera¬ ture as the deoxydated concentrate is still CO-H ₀ -rich gas. This gas is further utilized either in preheating, air pre¬ heating or production of high-pressure steam.

The examples hereinabove present facilities provided by the method of the invention of utilizing the heat content and chemical potential of the process gases in preheating of the procesε feed material and in deoxydation. Furthermore, the description presents the possiblity of preventing certain reactions by cooling the gases past the desired range of temperature at a high cooling velocity.

The method of the invention also enables accomplishment of desirable reactions, as there is the opportunity to adjusting temperature, the solids retention time and the chemical potential of the gas.

The following gives an example of such an opportunity. For example, flash smelting of impure Cu-concentrateε generates a process gaε flow containing particles, said particles con¬ taining, among other things, -precious metals such as Cu, Zn, Pb etc. and in thiε connection less valuable iron. By regu¬ lating the mixing temperature of the gases leaving the process and the temperature of the circulation particles reactor to about 650 to 700°C and by adjusting the oxygen potential of the reactor to an adequate level by feeding oxygen, for example air, into the circulating gas flow, such conditions are provided in the fluidized bed reactor that the precious metals (Cu, Zn, Pb, etc.) contained in the particulate material of the proceεε gaε vrill form water-soluble sulphates whereas iron will remain in the water in the form of insoluble oxide. For example, the constructive arrangement shown in Fig. 4 can with slight modifications be used for this purpose. The process gas 401 leaving the melting furnace and containing SO- and particles will be cooled in a mixing chamber 405 by means of circulating gas 409 and air added thereto (not shown in the figure) to a reaction temperature of 650 to 700°C, which temperature also prevails in a reaction zone 402 also serving as a conveying section and in the circulating particles 412 and 416. From the syεtem is discarded as much particleε as enter the syεtem with gaε and pcεεibly alεo from the make-up silo 426 for further passage to a dissolving stage. By regu¬ lating the amount of particleε being circulated and by ad¬ justing the oxygen level by means of adding air and by choosing the level of temperature, the conditions can be made optimal for each case. In this connection, heat generated by the sulphatizing reaction of the melting procesε can be recovered in the form of, for example, high-pressure steam in the boiler 427.

In preliminary handling of impure Cu-concentrateε , for example to remove As, Sb and Ei in a neutral or mildly deoxvdating atmosphere, partial calcination at a temperature of about 700 C is generally used. In that process, the above-mentioned components evaporize as sulphide in the gas phase and will be separated therefrom at a later stage of treating the gas. Thiε treatment can be effected simultaneously with cooling of the gas in the melting furnace by the method of the invention. Pretreatment of the concentrate, i.e. of. the feed material of the melting furnace may be effected, for example, by an apparatus aε illustrated in Fig. 4, to which apparatus after the ' heat exchanger 427 has been fitted a separator, heat exchanger and one more separator that is not shown in Fig. 4 for further treatment of the gas. The discharge gas 401 from the melting furnace and the feed material of the process are fed through a silo 426 into the mixing chamber 405 whereto circulating gas 409 is also fed so as to εettle the tem¬ perature at a correct level. The particles retention time iε controlled by regulating the amount of particles being cir¬ culated. If necesεary, the deoxydation potential of the εystem allows to be fine-adjusted by feeding, for example, naphta or air to the syεtem either through the circulating gas line or direct to the reactor according to need. The reaction tempera¬ ture preferably exceeds 700 C in order to reach a good result in evaporating impurities, but it iε also determined by the sintering and other properties of the feed material. The concentrate to be handled aε well aε εeparated particles coming from the melting furnace are discharged either as such ir hot tc the feed material of the melting process or they are fed into the process in cold through coding and possible storing. Process gas 415, produced at this stage and con¬ taining both evapcrized impurities in the form of sulhpide (As, Sb, Bi...) and possibly small quantities of elementary sulphur, is led to further treatment.

In further treatment, for example, gaε is oxidized under control by means of additional air (not illustrated in Fig.) before the heat exchanger 427, whereby the above-mentioned impurities will oxidize and cool to a temperature in which, for example Sb_0, and Bi„0^ will sublimate and become separ¬ able from gas. Thereafter, the gas containing Aε_Q, will be further cooled in some suitable manner or by, for example, a method according to the invention in a separate apparatus to a tteemmppeerraattuurree ooff aabboouutt 112200 C, whereby s-O, will sublimate and be separated from the gas.

Thereafter, the gases will be led for further treatment in order to produce, for example, sulphur acid.

In the above-mentioned coupling, the heat needed by the procesε iε received from the gases of the melting furnace, whereas in a separate partial calcination, heat has to be generated by oxidizing part of the sulphide of the con¬ centrate. Thus, the heating value of the concentrate iε εaved for the needs of the melting process itself. At the same time, there has been also gained such an advantage that the number of gas flows to be treated and containing S0 de- creases from two to one and the S0„-content of the gaε flow will rise in comparison with the conventional arrangement.

Fig. 7 illustrates a system for treating process gases 501 in a reactor 502, whereto process gas is introduced through aπ inlet opening 504 disposed in the upper part of said reactor. Circulating gaε which has been cooled and cir¬ culating particles are also led to the upper part cf he reactor. Proceεε gaε, circulating gas and circulating par¬ ticles swiftly mix in the upper part of the reactor at the mixing space 505. The gas/particle εuspenεion that has reached the mixing temperature then flows downstream down¬ wards in the reactor. The embodiment illustrated in Fig. 7 includes a heat exchanger 510 diεpoεed in the bottom part of the reactor, in which heat exchanger heat iε recovered

from the procesε gaε. Below the reactor is arranged a particle separator 512 where solid particles are separated from the gas/particle suεpension. Various known techniques are applicable to particle separation, which can be effected at either one or several stages. In the application illus¬ trated in Fig. 7, gas that has been partly purified is led through a secondary gas cooler 527 to another particle separator 514.

Part of the purified gas 515 is recycled by means of a pump 529 to the upper part of the reactor in the form of cir¬ culating gaε 509. Prior to leading to the reactor, the cir¬ culating gas is mixed with solid particles 516 and 520 separated from either one or both particle separators 512 and 514. Particleε may also be returned separated from the gaε to the upper part of the reactor, by uεing various known means of transport such as pneumatics, hoists or elevators, screws etc.

Aε shown in Fig. 7, heat can be recovered from the process gaε in the reactor 502 in the heat exchangerε 510. In some appli¬ cations it may, however, be advantageous tc first pre-purify the gas, for example, in the purifier 512 and thereafter lead it through the heat exchanger 527. Especially with great amounts of particleε, it may be beneficial to remove wearing material from the gas prior to the heat exchanger. On the other hand, if desired, it is possible to effect heat recovery in the reactor only and leave out the other heat exchanger 527.

With the system illustrated in Fig. 7, it is possible to achieve all the above advantages of the method of the invention, i.e. rapid cooling fast suspension formation, selective condensing etc.

Constructionally, the system differs from earlier appli¬ cations in that, in previous ones it is mostly necessary to force the solid particles into the gas flow whereas the construction according to the invention brings about spontaneous mixing at the inlet opening.

Further advantages of the system illustrated in Fig. 7 are as follows:

- Circulation of particleε is easy because the particles flow through the system independently of the process gas flow. The function of the system needs no minimum process gas flow.

No actual mixing chamber is needed.

- The pressure loss iε minimal because throttling at the inlet opening need not be remarkable.

The above description presents several advantages of the method of the present invention. The above examples indicate that a combination of circulating particles and circulating gas optimal can be optimized for each case. This may, however, result in too low an average particle denεity prevailing in the mixing section aε to fume formation and adsorption. This aεpect has to be considered in the arrangement. The examples above handled the density of the suspension circulated through the reactor, which density could by means of circulating gas be adjusted to a level of 0.5 kg/m and even below that. In this case, the average amount of particles depending on the grain size, iε 10 / cr even less, whereby the average distance between the particleε will become as high as 10 mm. Prerequisite for prevention of fume formation and adsorption of ultra fine particles is usually a higher particle density,

7 8 3 e.g. 10 - 10 /m , at mixing whereby the distance between particleε iε 5...1 mm. There are several ways of arranging this kind of equipment. A simple and preferred way of imple¬ mentation is to build the mixing section of the reactor in such a way that either the bigger or the smaller part of the particles therein is in internal circulation in the mixinσ

section and that only part of the particles is led to the flow circulating through the upper part of the reactor which functions as a conveyor. This is simply realized, for example, so that the effective cross section of the mixing section is bigger than that of the conveying section. In this case, the average flow velocity of the mixing section iε correspondingly smaller than that of the conveying section whereby the sus¬ pension density prevailing in the mixing section will be higher.

Furthermore, the geometry of the mixing section is so arranged that, there is formed an internal circulation which iε forced to return to the mixing point. In this way, the suspension density of the mixing section and especially of the mixing point allows to be adjusted within a large range. Generally, a

7 8 3 suspension density of 10 to 10 particles/m iε εufficiert at the mixing point, whereby the suspension contains, depending on the particle size and nominal weight etc. , 10 to 100 kg/m" of solid particles. Thus, it is not a question of an actual conventional fluidized bed, where the suspension density is

3 3 hhuunnddrreeddss ooff kkiillooggrraammss//mm ^'' aanndd pressure losses correspondingly at a considerably higher level,

Fig. 5 and 6 illustrate a preferred embodiment of the appar¬ atus according to the invention. Said figures present the lower part of the reactor 2 illustrated in Fig. 1, which lower section comprises the mixing chamber 5, which again comprises a conical bottom 6 tapering downwards, at the loweεt point of which there iε a gaε inlet 7. Tc the air box 8 , formed between the reactor bottom 3 and the mixing chamber bottom, iε tangentially connected an inlet pipe 29 for cooling gaε 9. To the reactor bottom iε centrally fitted an inlet 4 for procesε gaε 1. Return pipeε 16 for εeparated solid particles, connected to the mixing chamber, lead the returned particles in a downwardly circulating movement towards the gas inlet opening 7. The croεε surface of the mixing section is bigger than that of the reactor thereabove, which reactor functions .as a conveyor.

It is obvious that the mixing section illustrated in Fig. 5 and 6 can also be arranged in another way. Therefore, the conical bottom of the mixing chamber may be provided with openings through which at least part of the gas to be cooled is led. Part of the solid particles may be fed into the mixing chamber through the gas inlet pipe 29.

The operating principle of the mixing section as shown in Fig. 5 and 6 brings out the characteristics of the method of the present invention, such as

a) the suspension density and temperature of the mixing section and especially of the mixing point are adjustable within a large range to a level appro¬ priate for each case,

b) the density and solids flow of the suspension entering the conveyor and the particle separator can be minimized to an optimal level case by case, whereby problems related to a high suspension density, such as wear, system for treating solid particles, pressure losses, etc. will be minimized.

The invention is not limited to the applications and arrange¬ ments presented hereinabove, but various modifications, appli¬ cations and constructions are possible within the inventive scope of the claims.

Although the cooling gas in the application examples is circulating gas, it is obvious that some other appropriate gas, such as air, can be used as a cooling gas. At the mixing temperature at the i ixing point, the evaporating liquid can be, for example, water. The method can also be used for the evaporation of liquids and in that connection for the recovery of solids, as stated above.