Method and apparatus for treating a leaching residue of a sulfur- containing metal concentrate The invention relates to a process and an apparatus for treating a leaching residue generated in a leaching of a sulfur-containing metal concentrate, wherein the leaching residue is fed into a reactor, wherein a fluidizing gas is injected into the reactor to form a fluidized-bed containing at least a portion of the leaching residue, wherein the leaching residue is heated in the presence of inert particles to a temperature between 600 and 900 °C in an oxidizing atmosphere to produce calcined particles and SO ₂ .

Direct leaching of sulfuric zinc concentrate is a well-established applied technology for zinc production. The process generates a considerable amount of ele- mental sulfur that is currently dumped due to lack of proven treatment solutions. With regard to the polymetallic contamination and the poor leachability resistance it is increasingly difficult to obtain authority approval for long-term dumping of this sulfur residue. A typical composition of the residue in dry state shows the following composition: component range preferred range

Total S 40 - 70 wt-% 45 - 65 wt-%

Elementary S 30 - 60 wt-% 35 - 55 wt-%

Pb 1 - 20 wt-% 2 - 15 wt-%

Fe 3 - 20 wt-% 5 - 15 wt-%

SiO ₂ 3 - 20 wt-% 5 - 15 wt-%

Zn 0,1 - 10 wt-% 1 - 5 wt-%

Ag 100 - 1000 g/t 200 - 600 g/t The given composition is also taken as a basis for the current invention. The sulfur is combusted to SO2 and the contained non-combustibles are separated as a solid product, consisting mainly of silica, lead, zinc and iron as well as up to 0,1 wt-% silver. The process allows the utilization of the solid product, namely Fe and Pb calcine in a lead smelter, silver recovery as well as the production of steam and sulphuric acid.

For a further utilization of the metallic compounds, especially silver, it is neces- sary to combust contained sulfur to SO2. Such roasting is e.g. performed in a fluidized-bed like it is for example proposed in WO 201 1 /076995. Therein, sulfur containing leaching residue or part of it is fed to a fluidized-bed treatment in which the residue is burned into sulfuric dioxide and the valuable metals contained in the leaching residue are recovered. To avoid agglomeration, sand is added to the fluidized-bed.

However, later on it is difficult to separate sand particles from the valuable metallic compounds. Therefore, it is the object of this current invention to provide a process and a corresponding apparatus for roasting sulfur-containing leaching residues and, at the same time, minimizing agglomeration as well as providing a complete separation of the calcined material. This problem is solved with a method according to current claim 1 . A leaching residue generated in a leaching of sulfur containing, preferably non-ferrous, metal concentrate is fed into a reactor. Therein, the residue and inert particles are fluidized by a fluidizing gas which is injected from at least one nozzle, preferably from a nozzle grid of the reactor. Thereby, a fluidized-bed is formed, which operates at temperatures between 500 and 900 °C, preferably 600 to 900 °C, most preferably between 650 and 850 °C in an oxidizing atmosphere to produce calcined particles and SO2. After spending a specific residence time, the calcined particles produced out of the leaching residue are withdrawn from the reactor while respective chemical reactions have been carried out.

Most important, the fluidized-bed is designed such that at least 60 wt-%, preferably 80 wt-%, most preferably 90 wt-% (independent from the portion of removed calcined particles) of the inert particles are removed from the fluidized bed while at least 60 wt-% preferably 80 wt-%, most preferably 90 wt-% (inde- pendent from the portion of removed inert particles) of the calcined particles are removed together with a gas stream containing off-gases and the fluidizing gas. This effect can be achieved by different parameters being sensitive values for the respective minimum fluidization velocities of the inert and the calcined particles like particle diameters or densities.

Thereby, the different particles are already separated into the fluidizing-bed reactor, which is why no further particle separation is needed afterwards. By lifting the calcined particles above the fluidized-bed, it is possible to withdraw at least the main part of the calcined particles without any additional inert material.

In addition, the specific during operation is beneficial for a further reason. Should all particles lump together, the agglomerates will obtain a higher effective particle size, enhance higher weight and, therefore, will sink down in the fluidized-bed. In the lower part of the fluidized bed, the inert particles, due to their higher concentration, prevent already lumped calcined particles from sintering as it is well-known from the processes being state of the art.

Furthermore, the lifting of the calcined particles above the fluidized bed results in a more uniform reaction of the oxygen contained within the fluidization gas and the sulfur within the residue. This occurs since the oxygen concentration is maximum at the inner circuit, while the sulfur concentration follows the exact opposite trend, i.e. it is minimum or zero at the inner circuit, is higher in the first zone and highest in the second zone due to the concentration of the residue particles. As a result of the more uniform sulfur oxidation, hotspot formation which is a main cause of sintering is avoided. In this context, the inert substances, preferably located next to the bottom in a particular high concentration, operates like a form of an isolating layer, while the term "inert" is used to describe a substance that is not commonly reactive during the partial roasting . It is preferred that the claimed process features two zones being arranged above each other with respect to the reactors' height. In this option, at least 60 wt-%, preferably 80 wt-%, most preferably 90 wt-% (independent from the portion of calcined particles) of the inert particles are found in a first zone of the fluidized-bed, while at least 60 wt-%, preferably 80 wt-%, most preferably 90 wt- % (independent from the portion of inert particles)of the residue particles or calcined residue particles are found in a second zone above the first zone. The formation of these two zone occurs during steady state reactor operation and is particularly apparent during fluidizing gas ramp down (in case of controlled shut downs or planned trips).

Fine particles found in the first zone are removed from the fluidized-bed while fine particles from the second zone are removed together with the gas stream containing off-gases and the fluidizing gas. The existence of these different zones can be adjusted by different parameters or different densities of the resi- due particles and/or the inert particles as particle diameters and densities are sensitive values for the respective minimum fluidization velocities. Moreover, the formation of these two zones becomes more apparent by ramping down the supply of the fluidization gas to the reactor, which results into a "safe" in terms of sintering potential short or long reactor shut down as explained in the para- graphs below. Additionally, such mode of operation protects the reactor from sintering also during planned or unexpected operational stops. Gradual ramp down of the fluidization gas supply results to a more distinct formation of the two fluidization zones since the reactive necessity comes close to or falls below the minimum fluidization velocity of the inert particles (during fluidization gas ramp down) while still being above the corresponding minimum fluidization velocity of the residue/calcined particles. Further gas ramp down to the point where inert particles are no longer fluidized with a subsequent abrupt stop of the fluidization gas supply results to a shut-down where the first zone contains a maximized inert amount (higher than during steady state operation) so no sintering can occur during the period where the reactor is not in operation. Moreover, the reactor sintering potential during start-up is also minimized in the vicinity of some nozzles to the first zone which exhibits a maximized inert content. Any sintering processes taking place in the second zones are reversed during start-up due to the momentum and resulting movement of the first zone.

Another option of the current invention is that at least 60 wt-%, preferably 80 wt- %, most preferably 90 wt-% (independent from the calcined particles) of the inert particles and at least 60 wt-%, preferably 80 wt-%, most preferably 90 wt-% (independent from the portion of inert particles) of the leaching residue and/or the calcined particles are found in a common mixing zone. Preferably, they are homogeneous mixed. Thereby, a dilution of the residue is achieved, whereby agglomeration is prevented.

Preferably, the diameter of at least 70 wt-%, preferably at least 80 wt-%, of the calcined particles is below 60 μιτι or the diameter of at least 70 wt-% preferably 80 wt-% of the inert particles is between 0,05 to 3 mm, preferably 0,1 to 2 mm. Thereby, the separate withdrawing is possible. Furthermore, the latter values are typical size for calcined particles S1O2 particles in the form of sand, which is why no further pretreatment is necessary.

It is especially cost effective to use S1O2 particles as inert particles since sand is cheap, regularly available and easy to handle. Further, S1O2 is also contained in a typical leaching residue as mentioned above.

Moreover, a preferred design of the claimed process uses 0,01 to 1 t inert material, preferably sand, per ton of dried leaching residue. Thereby, it is possible to achieve that the injected leaching residue does not reach the reactor bottom, but is combusted in the upper part of the bed and transported thereof as calcined particles by the off-gas stream.

In addition, the average residence time for the material fed as leaching residue and removed as calcined particles is between 20 to 200 min, preferably between 30 to 180 min. Thereby a complete turnover can be achieved.

The average residence time for the inert particles is in the range of several hours, preferably 2 to 10 hours, most preferably between 3 and 7 hours which is why it is possible to keep this solid stream small .

Preferably, the inlet velocity of the fluidizing gas is between 0,2 to 2 m/s, preferably 0,5 to 1 ,5 m/s. Due to this parameter, it is possible that after drying and/or reducing the sulfur content of the calcined particles these particles were lifted above the second zone in a so called free-board zone. Therefrom, the particles can be withdrawn together with the fluidizing gas stream and separated, e.g. with a cyclone. Fluidizing velocity in the sense of the invention is the velocity of the gas phase, generated in the furnace at operating conditions related to the empty furnace. The oxidizing atmosphere is preferably adjusted such that λ as the oxygen-fuel equivalence ratio (defined as the ratio between the oxygen mass stream entering the fluid bed furnace divided by the minimum oxygen needed to achieve complete stochiometric combustion of the introduced sulfur residue) is between 1 ,1 and 1 ,8, preferably 1 ,1 and 1 ,5, most preferably 1 ,3 and 1 ,5 to ensure a complete turnover. Therefore, it is proposed to use air or oxygen enriched air as fluidizing gas, since air is a cheap source for the oxygen needed for complete sulfur combustion. However, it is possible to use nitrogen or any other inert gas as fluidizing gas, whereby a gas with oxygen content is separately introduced.

The sulfur residue is mostly available as filter cake. To feed the material ho- mogenously into the reactor, it is proposed to mix the material with water, preferably by means of an intensive agitator in order to disintegrate lumps or agglomerates. In this scenery, the solid content is adjusted to 30 to 65 wt-%, de- pending on the content of the non-combustibles in the residue.

Further, it is also possible to mix the leaching residue with any inert material, preferably the inert material used in the fluidized-bed, whereby most preferably also material already used in the fluidized-bed is admixed to the residue. There- by, it is possible to form granules and feed the residue in form of a more homogenized particles into the reactor. As a result, it is easier to adjust the fluidizing parameters like fluidization velocity so that most of the calcined particles can be withdrawn together with the off-gas stream. Further, this option has a benefit of a higher steam production out of the off-gases since no water is added to the feed material. Concluding, the overall energy balance is improved.

It is also preferred to separate the particles removed from the fluidized-bed into inert and calcined particles, so valuable metallic compounds being removed together with the inert particles can be regained. In this context, it is also preferred to use a mill in this separation stage to liberate conglomerates of calcine and inert material to optimize the process and reach a better separation. Furthermore, it is also preferred to separate the particles out of the second zone from the off-gases, so the particles can be cooled and the energy consumption of the process is improved by recycling at least parts of the particles' energy. The off-gases are fed to a post combustion chamber, a boiler, a hot gas cleaning as well as a wet gas cleaning stage.

As already addressed, the energy balance of the process can be optimized by recycling energy gained in the cooling of inert particles as well as the cooling of calcined particles. Thereby, the inert and/or the calcined particles are cooled by preheating a gas stream and the preheated gas stream is recycled to a process stage before the reactor, preferably a mixer for the granules and/or to the reactor itself. Most preferably, the cooling of the inert and/or calcined particles can be used to preheat the fluidized gas and/or the oxygen source.

Moreover, the invention is directed to an apparatus with the features of claim 14. Such an apparatus for treating a leaching residue generated in a leaching of sulfur-containing, preferably non-ferrous metal concentrate comprises a mixing tank to form a slurry out of the residues. Alternatively the leaching residue is treated together with inert material/sand in a high intensity mixer to generate granules. Further, it comprises a reactor in which during operation a fluidized- bed is formed. The reactor features at least one feeding conduit for feeding the residue into the reactor, with at least one conduit for feeding for treating a leaching residue generated in a leaching of a sulphur-containing non-ferrous metal concentrate into the reactor, a supply conduit for feeding a fluidized gas into the reactor, at least one offtake line for withdrawing calcined particles produced out of the leaching residue together with the off-gas from the reactor and with an outlet line for withdrawing inert particles from the fluidized bed, wherein the outlet line is positioned such that at least 60 wt-% of the inert particles are withdrawn through this outlet line. With this arrangement, agglomerations in the reactor are avoided and, simultaneously, calcined particles are withdrawn with- out being substantially mixed with the inert particles, preferably at least 10 wt-% of inert particles are entrained in the off-gas together with the calcined particles. Further, an apparatus according to the invention is equipped with at least one feeding device to transport a slurry or granules into the reactor. In a preferred embodiment of the invention, a fluidizing gas is supplied to a fluidizing-bed reactor to a so called nozzle grid, a plate containing nozzles with 1 to 300 holes per m ² furnace area. The nozzles may be of several types including the following: (i) not extending from the nozzle grid and having one orifice in the upward direction, (ii) extending above the nozzle shaft having one or more than one orifices at angles between 0 and 180° and (iii) equipped nozzles same as a latter with an added characteristic of a cap to further protect blocking of the orifices.

Preferably, the apparatus also features a first cooler connected to the outlet for the inert particles, and a second cooler connected to a cyclone, wherein the calcined particles are separated from the off-gas for a separate handling of both sorts of particles

Further developments, advantages and possible applications of the invention can also be taken from the following description of the drawings. All features described and/or illustrated form the subject matter of the invention per se or in any combination, independent of their inclusion in the claims or their back reference. Fig. 1 shows schematically a process according to the invention using a slurry and

Fig. 2 shows schematically a process according to the invention using granules.

Fig. 1 shows a slurry tank 10, in which the sulfur-containing residue formed in a direct leaching process is filled in via conduit 1 1 . The sulfur residue is mostly available as filter cake. The material is mixed with water through conduit 12 by means of an intensive agitator in order to disintegrate lumps and agglomerates. The solid content is adjusted to 30 to 65 wt-%, depending on the content of non- combustibles in the sulfur residue.

The slurry is injected into the fluidizing-bed reactor 20 by means of conduit 13. The fluidizing-bed reactor 20 contains a bed of fluidized sand. The sand serves two purposes, namely at first providing a stable bed of fluidized solids into which sulfur residue can be injected and where all reactions shall take place and second preventing sintering of the non-combustibles by separation of individual PbSO ₄ /PbO grains.

The fluidized sand bed in the fluidized-bed reactor 20 holds the non-combustible compounds from the injected sulfur residue mainly in the upper part of the bed, while the bottom is largely depleted of non-combustibles. The combustion/roasting process is conducted in such a way that the injected slurry is dis- tributed homogenously across the fluidized sand bed. Most of the injected material does not reach the furnace bottom but is combusted in the upper part of the bed. The fluidizing velocity is adjusted such that the very fine non-combustibles (x ₈ o < <40 μιτι) are entrained in the process gas for the most part and leave the furnace with the off-gas via offtake conduit 21 . A smaller part of the non-combustibles form agglomerates and are discharged with some portion of the sand through an outlet conduit 22, like e.g. an overflow weir, as it is common practice in roasting of zinc concentrates and pyrite. Another part of the non-combustible forms relatively coarse agglomerates (> 1 mm) segregating to the bottom of the fluidized-bed. This material is discharged in intervals of several hours through a not shown bottom discharge.

In the fluidized bed reactor, the different separation from inert and calcined particles by different withdrawing positions is achieved by proper adjustment of the sand ratio (0,01 to 1 t per dried sulfur residue), sand particle size (0,1 to 2 mm) and fluidization velocity (0,5 to 1 ,5 m/sec) in combination with the utilization of the outlet during continuous operation as well as a fine granulometry of the non-combustibles (xso < <40 μιτι) which is inherent to the process.

The stream of calcined particles and the off-gas is fed through offtake conduit 21 into a cyclone 30, wherein the gas stream is separated from the calcined particles. The separated particles are fed via conduit 32 into a cooler 50a, like a cooling drum. As a heat transfer medium, gas is fed into the cooler via line 51 . The cooled cyclone discharge is fed via line 52 into line 57.

The off-gas stream containing mainly non-combustibles together with small sand particles is further fed via conduit 31 into a post-combustion 33 to oxidize sulfur fumes with additional air or any oxygen containing gas. Dust generated therein is fed via conduit 36 into collecting conduit 57.

The off-gas is further directed via conduit 34 into a waste heat boiler 40 for heat recovery by steam production. Solids separated in the waste heat boiler are also combined via line 48 with all product streams in conduit 57. In special cases, e.g. when the capacity of the envisaged unit is too small, the waste heat boiler 40 may be replaced through an evaporative cooler.

Moving downstream the gas path, solids are fed into a standard hot gas clean- ing system 42 via conduit 41 . The off-gas in the hot gas cleaning 44 is further used for sulfuric acid production after cleaning in a standard wet gas cleaning system 45.

Particles separated in the hot gas cleaning system 43 may be also mixed through conduit 47 and 48 with the total product stream in conduit 57. However, in the case that the solid stream (or any other solid stream) contains still a significant amount of sulfide sulfur then the stream is recirculated, preferably to the slurry tank via conduit 43. Also, even it is not shown it is possible to recycle the solid stream into the reactor 20. Further, also not shown recycling conduits from the post combustion 33 and/or the boiler 40 are possible.

Inert particles withdrawn from the fluidized-bed of the fluidized-bed reactor 20 are fed into the cooler 50b where they are also cooled with air or any other gas. This hot gas stream can be used as fluidizing gas and fed via line 24 into the fluidizing bed reactor via line 23. It is possible, that both coolers are designed as cooling sections and use the same heat transfer medium. Also, even if it is not shown, it is possible to have separate lines for the heat transfer medium in both cooling sections. The cooled sand is fed into an optional mill and separation unit 55 via conduit 54, wherein the metallurgical particles are separated from sand. The metallurgical particles are collected in conduit 57, wherein also all other product lines will feed in. The sand or any other inert particles can be recycled into the fluidized-bed reactor 20 directly via conduit 56.

Fig. 2 shows a nearly identical process. The only difference is that not a slurry, but granules are fed into the fluidized-bed reactor 20. This option has a benefit of a higher steam production since no water is added to the feed material. The sticky sulfur residue filter cake needs to be disintegrated in order to permit a controlled feeding into the furnace 20. This is achieved by means of mixing the sulfur residue with sand in a mixer 14, preferably a high shear mixer. Therein the sulfur residue is fed in via conduit 1 1 as well as additional sand is fed in via conduit 15. It was found that an addition of 1 ,5 to 4 t of sand per ton of dried sulfur residue is required to achieve a free flowing feed mixture. The grain size of the sand is preferably 0,1 to 1 mm. The intensive mixing generates granules with a suitable grain size for the fluidized-bed roasting (the bulk of the solids being between 300 and 600 μιτι, while solids between 0,1 and 3 mm will still be present).

Prior to feeding into the fluidized-bed reactor 20, the granules are passed via conduit 16 into an optional drier 17 in order to increase their stability. Removed water is withdrawn via conduit 18. The dryer 1 7 may utilize preheated air which is passed by directly or indirectly to another process stage and/or preheated water or other liquid through a heat source inherent or external to the process. Moreover, the dryer 17 may also be electrically heated or designed as a fluidized-bed. The necessity of the drying stage depends on the characteristics of the sulfur residue. It is well possible that not at all sulfur residues will require drying prior to roasting.

The combustion/roasting in the fluidized-bed reactor 20 differs from the process in Fig. 1 in so far that the entire bed volume of the fluidized-bed reactor 20 is used for the combustion/roasting process. Sintering is tackled by optimal sepa- ration of sticky particles (lead sulfates, -oxides) in a sand matrix. Combustion takes place at 650 to 850 °C with a λ factor of 1 ,1 to 1 ,5 as in the process according to Fig. 1 . In balance of the fluidized-bed reactor 20 exhibits a heat deficit, a carbonaceous fuel may be combusted in the fluidized-bed reactor 20 to maintain the desired operation temperature. This is also possible for process according to Fig. 1 . However, the process shall be self-sustaining to be attractive.

The fluidization velocity is in the typical range of stationary roasting, namely 0,5 to 1 ,5 m/sec. The major calcined portion is discharged via the outflow weir. Small sand and calcine particles are entrained in the roaster off-gas. The fluid iz- ing bed reactor 20 has a bottom discharge for internal discharging or eventual coarse agglomerates. Calcine discharged from the fluidized-bed reactor 20 is cooled in a cooling drum 50 (a,b). Separation of sand and silver containing valuable components is achieved by treatment in an attrition stage and subsequent classifying by screening or air classification.

The off-gas contains mainly non-combustibles together with some small portion of sand. It has to be noted that the solid particle contains most of the silver as well as lead sulfates/oxides, zinc sulfates/oxides, iron mainly as hematite and silica and shall be sold and further treated in a lead smelter.

In an additional process stage clean sulfur can be separated from the sulfur residue in a vacuum distillation stage that is operated with steam being generat- ed in the waste heat boiler at 250 to 300 °C. The non-combustible fraction is thus enriched up to 60 wt-% and exists as very fine (xso < < 40 μιτι) suspended solids in a liquid sulfur phase. This sulfur phase is atomized to fine particles (xso < 80 μιτι) and can be used for combustion as described for both figures. Evaporated sulfur is condensed in a bath of liquid sulfur at a temperature short below the evaporation temperature of sulfur. Evaporated impurieties as mercury are thus kept in gas phase and can be separated from the sulfur. The off gas is further cleaned in state of the art gas cleaning stages. The condensed sulfur is pure elemental sulfur and can be sold as product. The sand (or any other inert particles) removed in the mill and separation unit are at least partly recycled to the mixer for forming granules via a conduit 61 . However, it is also possible to recycle at least parts of these particles into fluid- ized bed reactor 20. Pre-heated air (or any other gas) from at least one cooler 50a, 50b are fed at least partly into the dryer 17 via conduits 62, 63, where they are used for drying and/or pre-heating the granules.

Further, at least parts of the pre-heated gas from at least one cooler 50a, 50b in the fluidized bed reactor 20 via conduits 62, 64 , where it can be used as flu- idzing gas and/or oxygen source.

List of references

10 mixer

1 1 - 13 conduit

14 mixer

15, 16 conduit

17 dryer

18, 19 conduit

20 fluidized-bed reactor

21 offtake conduit

22 outlet conduit

23, 24 conduit

30 cyclone

31 , 32 conduit

33 post combustion stage

34 - 36 conduit

40 boiler

41 conduit

42 hot gas cleaning

43, 44 conduit

45 wet gas cleaning

46 - 48 conduit

50a, 50b cooler

51 - 54 conduit

55 mill and separation unit

56, 57 conduit

61 - 64 conduit