WO2005080616A1

WO2005080616A1 - Process for reducing solids containing copper in a fluidized bed

Info

Publication number: WO2005080616A1
Application number: PCT/EP2005/001120
Authority: WO
Inventors: Peter Sturm; Martin Hirsch; Olli Hyvärinen; Matti Hämäläinen; Maija-Leena Metsärinta
Original assignee: Outokumpu Technology Oy
Priority date: 2004-02-25
Filing date: 2005-02-04
Publication date: 2005-09-01
Also published as: PE20060029A1; DE102004009176A1; DE102004009176B4; AR050762A1

Abstract

The present invention relates to a process for reducing solids containing copper to obtain elemental copper, in which solids containing copper are introduced into a fluidized-bed reactor and are reduced therein with a reducing agent at a temperature of 200 to 1,000 °C. To avoid an agglomeration of the solids in the fluidized-bed reactor also without addition of inert particles, it is proposed in accordance with the invention to adjust the mean suspension density of the fluidized bed in the reactor to less than 1,000 kg/m3.

Description

Process for Reducing Solids Containing Copper in a Fluidized Bed

Technical Field

The present invention relates to a process for reducing solids containing copper, in particular copper(l) oxide, to obtain elemental copper, in which the solids containing copper are introduced into a fluidized-bed reactor and are reduced therein with a reducing agent at a temperature of 200 to 1 ,000°C.

Such processes are used for instance in the hydrometallurgical production of copper. For this purpose, ores containing copper, e.g. sulfidic copper ores, are first of all dissolved in salt solutions and precipitated as copper chloride by means of leaching. Subsequently, the copper chloride is reduced e.g. in a rotary kiln with suitable reducing agents, usually a gas mixture containing hydrogen, at a temperature between 200 and 1,000°C to obtain elemental copper, either directly or upon oxidation with sodium hydroxide solution. However, ores containing copper can be reduced in a rotary kiln only with a comparatively small yield. Another disadvantage of the rotary kilns consists in their small capacity, which is due to the comparatively long reduction times. To overcome these disadvantages, it has already been proposed to use other types of reactor as rotary kilns for reducing solids containing copper.

From US 4,192,676 there is known a process for reducing solids containing copper, in which the same are introduced in solid form into a melting cyclone and under turbulent conditions at a temperature above the melting point of copper are reduced to elemental copper with hydrogen as reducing agent. Therefore, the temperature inside the reactor must be maintained at a minimum of 1,083°C, to ensure that the elemental copper is obtained and maintained in liquid form. Due to the required high reaction temperatures, this process involves an uneconomically high demand for energy. In addition, the high temperatures as well as the liquid copper formed during the reaction place high de- mands on the material of the reactor linings.

Furthermore, there are known processes for reducing solids containing copper in a fluidized-bed reactor, which as compared to the processes based on rotary kilns have a higher yield of copper and a higher capacity. Under the conditions of the fluidized bed, however, the solid particles used tend to undergo agglomeration (sticking), which can lead to a collapse of the fluidized bed. To overcome this disadvantage, US 4,039,324 proposes a process for reducing solids containing copper, in which the same, e.g. copper chloride, are reduced to elemental copper in a fluidized-bed reactor in the presence of chemically inert solid particles. The preferably spherical inert particles, e.g. sand, should physically prevent an agglomeration of the solids during the reduction. A disadvantage of this process, however, is the fact that after the reduction the copper must be separated from the inert particles. To provide for a circulation of the inert particles, the same must additionally be cleaned with hydrochloric acid before being recirculated to the reactor, in order to remove copper agglomerated at the surface.

Description of the Invention

Therefore, it is the object of the present invention to provide a process for reducing solids containing copper, in particular copper(l) oxide, to obtain elemental copper in a fluidized-bed reactor, in which the use of inert particles can be omitted without this leading to an agglomeration of the starting materials, which would disturb the operation of the fluidized bed.

In accordance with the invention, this object is solved by a process as mentioned above, in which the mean suspension density of the fluidized bed inside the reactor is adjusted to less than 1 ,000 kg/m³.

Surprisingly, it could be found in accordance with the present invention that by expansion of the fluidized bed, i.e. by reducing the mean suspension density of the fluidized by, which in this process usually is adjusted to at least 1 ,000 kg/m³, an agglomeration of the solid particles containing copper can reliably be avoided even without addition of inert particles. On the one hand, this is due to the fact that in an expanded fluidized bed the distance of the solid particles from each other is larger, whereby an agglomeration of these particles statistically occurs more rarely. On the other hand, such high gas velocities are adjusted in the fluidized bed reactor that the solid particles are exposed to high shearing forces which in turn act against an agglomeration. In addition, fast reaction times are achieved by the high gas velocities and the related high mass transfer, so that the solid particles containing copper are only briefly exposed to the risk of an agglomeration. Due to the high mass transfer, the retention time of the solid particles can also be reduced considerably, and dead zones inside the reactor, which lead to a deteriorated reaction and mixture, are avoided. In addition, the high gas velocity improves the heat transfer between gas and solids. With the process in accordance with the invention, copper with a purity of more than 95%, in particular more than 98%, can be produced.

Particularly good results are achieved when the mean suspension density of the fluidized bed inside the reactor is adjusted to a maximum of 300 kg/m³, particularly preferably to a maximum of 200 kg/m³, and quite particularly preferably to about 100 kg/m³.

In accordance with the invention, the process of the invention can be performed with any kind of fluidized bed, in particular also with a stationary or circulating fluidized bed, which has a mean suspension density of less than 1 ,000 kg/m³.

In accordance with a particularly preferred embodiment of the present invention, the starting materials containing copper are reduced in an annular fluidized-bed reactor having a mean suspension density of less than 1 ,000 kg/m³, in which a first gas or gas mixture is introduced from below through a gas supply tube into a mixing chamber of the reactor, wherein the gas supply tube is at least partly surrounded by a stationary annular fluidized bed, which is fluidized by supplying fluidizing gas, and the gas velocities of the first gas or gas mixture and of the fluidizing gas for the annular fluidized bed are adjusted such that the Particle-Froude-Numbers in the gas supply tube are between 1 and 100, in the annular fluidized bed between 0.02 and 2 as well as in the mixing chamber between 0.3 and 30. By means of such annular fluidized bed, the advantages of a stationary fluidized bed, such as a sufficiently long solids retention time, and those of a circular fluidized bed, such as a good mass and heat transfer, can be combined by avoiding the disadvantages of both systems. When passing through the upper region of the central tube, the first gas or gas mixture entrains solids from the annular stationary fluidized bed, which is referred to as annular fluidized bed, into the mixing chamber, where due to the high slip velocities between the solids and the first gas an intensively mixed suspension is formed and an optimum mass and heat transfer between the two phases is achieved. By correspondingly adjusting the bed height in the annular fluidized bed as well as the gas velocities of the first gas or gas mixture and of the fluidizing gas, the mean suspension density above the orifice region of the central tube can be adjusted to a value within the range provided in accordance with the invention. The solids retention time in the reactor can be varied within wide limits by choosing the height and cross-sectional area of the annular fluidized bed and can be adjusted to the desired heat treatment. The amount of solids discharged from the reactor with the gas stream is preferably completely or at least partly recirculated to the reac- tor, the recirculation expediently being effected into the stationary fluidized bed. The solids mass flow thus recirculated to the annular fluidized bed normally lies within the same order of magnitude as the solids mass flow supplied to the reactor from outside. Apart from the excellent utilization of energy, another advantage of this embodiment consists in the possibility of quickly, easily and reliably adjusting the energy and mass transfer of the process to the requirements by changing the flow velocities of the first gas or gas mixture and of the fluidizing gas.

In the case of an annular fluidized bed, particularly good results are achieved when the mean suspension density in the mixing chamber is adjusted to less than 50 kg/m³, particularly preferably less than 25 kg/m³, and quite particularly preferably to about 10 kg/m³. The mean suspension density can be adjusted in particular by the degree of banking of the solids in the annular stationary fluidized bed, based on the upper orifice end of the gas supply tube.

Preferably, the Particle-Froude-Number in the gas supply tube is between 1.15 and 20, in the annular fluidized bed between 0.115 and 1.15, and in the mixing chamber between 0.37 and 3.7.

The Particle-Froude-Numbers are each defined by the following equation:

with

u = effective velocity of the gas flow in m/s p_s = density of a solid particle in kg/m³ p_f = effective density of the fluidizing gas in kg/m³ dp = mean diameter in m of the particles of the reactor inventory (or the particles formed) during operation of the reactor g = gravitational constant in m/s².

When using this equation it should be considered that d_p does not indicate the grain size (d₅₀) of the material supplied to the reactor, but the mean diameter of the reactor inventory formed during operation of the reactor, which can differ significantly in both directions from the mean diameter of the material used (primary particles). From very fine-grained material with a mean diameter of 3 to 10 μm, particles (secondary parti- cles) with a grain size of 20 to 30 μm are for instance formed during the heat treatment. On the other hand, some materials, e.g. certain ores, are decrepitated during the heat treatment.

In accordance with a development of the invention it is proposed to adjust the bed height of solids in the reactor such that the annular fluidized bed at least partly extends beyond the upper orifice end of the central tube by a few centimeters, and thus solids are constantly introduced into the first gas or gas mixture and entrained by the gas stream to the mixing chamber located above the orifice region of the central tube. The degree of such banking has a substantial influence on the height of the mean suspen- sion density of the fluidized bed inside the reactor.

By means of the process in accordance with the invention, all kinds of solids containing copper, in particular copper oxides, as well as those which beside copper contain other metal oxides, can be reduced effectively. In particular, the process can be used for reducing copper(l) oxide.

In principle, the process of the invention is not limited with respect to the grain size of the solids used. However, it turned out to be advantageous to charge the solids into the reduction reactor in the form of granules with a maximum grain size of 2 mm, which were produced e.g. by microgranulation. Such microgranules are at least partly disintegrated in the fluidized bed of the reduction reactor to obtain particles with a mean grain size of 300 to 400 μm, which promotes the reduction of the solids.

In accordance with a preferred embodiment of the present invention, it is provided that hydrogen-containing gas with a hydrogen content of 80 to 99.9 % and particularly preferably 98 to 99.9 % is supplied to the reactor as reducing agent. Apart from hydrogen, the same preferably contains between 0 and 20 % and particularly preferably less than 10 % inert gas, in particular nitrogen. If an annular fluidized bed is used, the hydrogen-containing gas can be introduced into the reactor through the central tube and/or via the annular fluidized bed, the introduction via the central tube being preferred.

The reaction temperature inside the reduction reactor primarily depends on the kind of copper-containing solids to be reduced and in accordance with the invention lies between 200 and 1 ,000°C. For reducing copper(l) oxide, the reaction temperature preferably is adjusted to a temperature between 300 and 800°C, particularly preferably between 400 and 600°C, and quite particularly preferably of about 500°C. The genera- tion of the gas temperatures necessary for the operation of the reactor can be effected in any way known to the skilled person for this purpose, e.g. by heating the gas with a gas heater indirectly heated with oil or fuel gas. However, due to the exothermic character of the reaction, only little heating is necessary.

In accordance with a development of the invention it is provided that for separating the solids from the reduction gas, a separating stage, e.g. a cyclone or the like, is provided downstream of the reduction reactor, and the separated solids are at least partly recirculated to the fluidized bed of the reduction reactor. When using an annular fluidized bed reactor, e.g. the solids level in the stationary annular fluidized bed of the first reac- tor thus can be controlled or varied specifically, whereas excess solids are withdrawn via a product discharge conduit. ln particular in the case of hydrometallurgically obtained starting materials containing copper, such as copper(ll) chloride or copper(l) oxide produced by leaching ores containing copper, it turned out to be advantageous to dry the starting material before introduction into the reduction reactor, in order to largely remove residual moisture from the solids. For this purpose, all processes for drying solids, which are known to those skilled in the art, are suitable in principle, such as the drying by means of flue gases, which are produced by combustion of oil or fuel gas in a combustion chamber upstream of the drier. For separating the dried solids from the flue gases, a cyclone is preferably provided downstream of the drier as separating stage, from which the solids are intro- duced into the reduction reactor.

To further reduce the operating costs of the process, it is furthermore proposed to circulate the gas containing reducing agent, in that for instance part of the waste gas of the reduction reactor is processed by solids separation, cooling and water separation, refreshed with fresh reducing agent, compressed, heated and recirculated to the reduction reactor.

The invention will subsequently be explained in detail with reference to embodiments and the drawing.

Brief Description of the Drawing

Fig. 1 shows a process diagram of a process in accordance with a first embodiment of the present invention; and

Fig. 2 shows a process diagram of a process in accordance with a second embodiment of the present invention.

Detailed Description of the Preferred Embodiments

In the process as shown in Figure 1 , solids containing copper, e.g. fine-grained, possibly moist copper(l) oxide, preferably with a grain size of less than 2 mm, are charged into a drier 2 via conduit 1 and dried by means of flue gas supplied to the drier 2 via conduit 3, which was previously produced in a combustion chamber 4 by combustion of oil or gas. The copper(l) oxide used as starting material can for instance have been obtained in advance from sulfidic copper ores, in that the ore was dissolved by means of chloride-containing solutions in a three-stage countercurrent system, precipitated as copper chloride upon leach purification, and subsequently converted to copper(l) oxide with sodium hydroxide solution.

From the drier 2, the suspension is supplied by the gas stream into a cyclone 5, in which the dried solids are separated from the flue gas. While the flue gas is withdrawn from the cyclone 5 via a waste gas conduit 7 connected with a gas cleaning device 6, the solids are laterally conveyed into the reduction reactor 9 via a screw conveyor 8. In addition, a gas containing reducing agent, preferably a gas containing hydrogen, is supplied to the reactor 9 from below through conduit 10 with a sufficiently high temperature and a velocity sufficient for forming and maintaining an expanded fluidized bed with a mean suspension density of less than 1 ,000 kg/m³, by means of which gas the solids are reduced to elemental copper. Due to the chosen, comparatively low suspension density, there is no agglomeration of the solid particles during the retention time of the solids in the reactor 9.

The gas stream continuously supplies suspension consisting of elemental copper, rests of non-reduced starting solids, and reduction gas via conduit 11 into a cyclone 12, in which the solids are separated from the reduction gas and recirculated to the reduction reactor 9 via a solids recirculation conduit 13. Product in the form of elemental copper is continuously withdrawn from the reactor 9 via the product discharge conduit 14 and supplied for instance to a melting reactor, in order to be processed there to high- conductivity copper.

The reduction gas separated in the cyclone 12 is circulated, in that it is first of all passed through a heat exchanger 16, then mixed with fresh hydrogen produced for instance in a chlorine-alkali electrolysis and supplied via conduit 17, is liberated from water and ultrapurified in a washer-cooler 18, subsequently compressed, preheated in the heat exchanger 16 and heated by the gas heater 19 to the inlet temperature neces- sary for maintaining the reduction temperature in the reactor 9, and is supplied to the reactor 9 as reduction gas via conduit 10.

The process shown in Figure 2 differs from the one shown in Figure 1 in that an annular fluidized bed reactor is used as reduction reactor 9. The cylindrically formed reactor 9 has a central tube 20 arranged about coaxially with the longitudinal axis of the reactor 9 and extending upwards from the bottom of the reactor 9 substantially vertically, which is surrounded by a chamber 21 of annular cross-section. Both the central tube 20 and the annular chamber 21 can of course also have a cross-section different from the pre- ferred round cross-section, as long as the annular chamber 21 at least partly surrounds the central tube 20.

The annular chamber is divided into an upper and a lower part by a gas distributor 22. While the lower chamber acts as gas distributor chamber 23 for fluidizing gas, the upper part of the chamber includes a stationary fluidized bed or an annular fluidized bed 24 of fluidized solids, e.g. copper(l) oxide, the fluidized bed 24 extending slightly beyond the upper orifice end of the central tube 20.

Through conduit 25, e.g. molecular hydrogen is supplied to the reactor 9 as fluidizing gas, which via the gas distributor 22 flows into the upper part of the annular chamber 21 , where it fluidizes the solids to be reduced by forming a stationary fluidized bed 24. The velocity of the gases supplied to the reactor 9 preferably is chosen such that the Particle-Froude-Number in the annular fluidized bed 24 is between 0.02 and 2.

Through the central tube 20, hydrogen-containing reduction gas constantly is supplied to the reactor 9, which upon passing through the central tube 20 flows through a mixing chamber 26 and an upper duct 27 into the cyclone 12. The velocity of the gas supplied to the reactor 9 preferably is adjusted such that the Particle-Froude-Number in the central tube 20 is between 1 and 100. Due to these high gas velocities, the gas flowing through the central tube 20 entrains solids from the stationary annular fluidized bed 24 into the mixing chamber 26 when passing through the upper orifice region. Due to the banking of the fluidized bed in the annular fluidized bed 24 with respect to the upper edge of the central tube 20, the fluidized bed 24 flows over this edge towards the cen- tral tube 20, whereby an intensively mixed suspension is formed, whose mean suspension density is less than 1 ,000 kg/m³ due to the Particle-Froude-Numbers in the central tube 20, the mixing chamber 26 and the annular fluidized bed 24. As a result of the reduction of the flow rate by the expansion of the gas jet and/or by impingement on one of the reactor walls, the entrained solids quickly lose speed and fall back again into the annular fluidized bed 24. The non-precipitated amount of solids is discharged from the reactor 9 together with the gas stream via the duct 27. In the cyclone 12, precipitated solids are recirculated to the reactor 9 via conduit 13, whereas the still hot exhaust gas is reprocessed as described in Figure 1 and recirculated to the reactor 9 via the central tube 20.

The invention will subsequently be described with reference to an example demonstrating the invention, but not restricting the same.

Example (Reduction of copper(l) oxide)

In a plant corresponding to Figure 1 , 8,340 kg/h of filter cake containing copper(l) oxide with a residual moisture of 12.5 % were supplied to the drier 2 via the solids conduit 1 , and flue gas with a temperature of 1 ,000°C, which was generated in the combustion chamber 4, was supplied via conduit 3. The suspension of dried solids and flue gases was continuously introduced into the cyclone 5, in which both components of the suspension were separated from each other.

While the flue gas was withdrawn via conduit 7, the dried solids were conveyed into the reduction reactor 9 with a mass flow rate of 7,300 kg/h via the screw conveyor 8. Via conduit 10, 10,200 Nm³/h of reduction gas, which was previously heated in the gas heater 19 to the inlet temperature necessary for maintaining the reaction temperature of 500°C, was supplied to the reactor 9 and blown through the reactor 9 with a velocity maintaining an expanded fluidized bed with a mean suspension density of about 100 kg/m³. Solids entrained by the reduction gas were separated from the suspension in the cyclone 12 and recirculated to the reactor 9 via conduit 13. Spent reduction gas was cooled to about 200°C in the heat exchanger 16, mixed with 1,200 N/m³ of hydrogen supplied via conduit 17, and purified in the washer-cooler 18 and at the same time cooled to about 35°C. Upon compression of the gas by about 600 mbar, the same was preheated to 300°C in the heat exchanger 16, heated further to 350°C in the gas heater 19, and supplied to the reactor 9.

Finally, 6,490 kg/h of elemental copper as product with a temperature of 500°C were withdrawn from the reduction reactor 9 via conduit 14 and molten in a downstream electric arc furnace.

List of Reference Numerals

I solids conduit 2 drier

3 flue gas supply conduit

4 combustion chamber

5 cyclone after drying

6 gas cleaning device 7 waste gas conduit

8 screw conveyor

9 reduction reactor

10 reductant supply conduit

I I conduit 12 cyclone downstream of the reduction reactor

13 solids recirculation conduit

14 product discharge conduit

15 gas conduit

16 heat exchanger 17 supply conduit for fresh reducing agent

18 washer-cooler

19 gas heater

20 gas supply tube (central tube)

21 annular chamber 22 gas distributor

23 gas distributor chamber

24 annular fluidized bed

25 fluidizing gas supply conduit

26 mixing chamber 27 duct

Claims

Claims:

1. A process for reducing solids containing copper to obtain elemental copper, in which solids containing copper are introduced into a fluidized-bed reactor (9) and reduced there with a reducing agent at a temperature of 200 to 1 ,000°C, characterized in that the mean suspension density of the fluidized bed in the reactor (9) is adjusted to less than 1 ,000 kg/m³.

2. The process as claimed in claim 1 , characterized in that the mean suspension density of the fluidized bed in the reactor (9) is adjusted to a maximum of 300 kg/m³, particularly preferably to a maximum of 200 kg/m³, and quite particularly preferably to about 100 kg/m³.

3. The process as claimed in claim 1 or 2, characterized in that a first gas or gas mixture is introduced from below through a gas supply tube (20) into a mixing chamber (26) of the reactor (9), the gas supply tube (20) being at least partly surrounded by a stationary annular fluidized bed (24) which is fluidized by supplying fluidizing gas, and that the gas velocities of the first gas or gas mixture and of the fluidizing gas for the annular fluidized bed (24) are adjusted such that the Particle-Froude-Numbers in the gas supply tube (20) are between 1 and 100, in the annular fluidized bed (24) between 0.02 and 2, as well as in the mixing chamber (26) between 0.3 and 30.

4. The process as claimed in claim 3, characterized in that the Particle-Froude- Number in the gas supply tube (20) is between 1.15 and 20.

5. The process as claimed in claim 3 or 4, characterized in that the Particle- Froude-Number in the annular fluidized bed (24) is between 0.115 and 1.15.

6. The process as claimed in any of claims 3 to 5, characterized in that the Particle-Froude-Number in the mixing chamber (26) is between 0.37 and 3.7.

7. The process as claimed in any of claims 3 to 6, characterized in that the mean suspension density in the mixing chamber (26) is adjusted to less than 50 kg/m³, particularly preferably less than 25 kg/m³, and quite particularly preferably to about 10 kg/m³.

8. The process as claimed in any of claims 3 to 7, characterized in that the bed height of solids in the reactor (9) is adjusted such that the annular fluidized bed (24) at least partly extends beyond the upper orifice end of the gas supply tube (20) and that solids are constantly introduced into the first gas or gas mixture and entrained by the gas stream to the mixing chamber (26) located above the orifice region of the gas supply tube (20).

9. The process as claimed in any of claims 3 to 8, characterized in that the reducing agent is introduced into the reduction reactor (9) via the gas supply tube (20).

10. The process as claimed in any of the preceding claims, characterized in that as starting material to be reduced copper oxide is used, particularly preferably copper(l) oxide.

11. The process as claimed in any of the preceding claims, characterized in that solids containing copper are introduced into the reduction reactor (9) in the form of e.g. granules produced by microgranulation with a maximum grain size of 2 mm.

12. The process as claimed in any of the preceding claims, characterized in that hydrogen-containing gas is supplied to the reactor (9) as reducing agent.

13. The process as claimed in claim 12, characterized in that hydrogen-containing gas with a hydrogen content of 80 to 99.9 % and particularly preferably 98 to 99.9 % is introduced into the reactor (9).

14. The process as claimed in claim 12 or 13, characterized in that the hydrogen- containing gas contains between 0 and 20 %, in particular less than 10 %, nitrogen.

15. The process as claimed in any of claims 12 to 14, characterized in that the reaction temperature in the reduction reactor (9) is adjusted to a temperature between 300 and 800°C, particularly preferably between 400 and 600°C, and quite particularly preferably of about 500°C.

16. The process as claimed in any of the preceding claims, characterized in that downstream of the reduction reactor (9) a separating stage (12) is provided for separating the solids from the waste gas, and that the separated solids are at least partly recirculated to the fluidized bed of the reactor (9).

17. The process as claimed in any of the preceding claims, characterized in that the solids containing copper are dried before being charged into the reduction reactor (9).

18. The process as claimed in any of the preceding claims, characterized in that upon reprocessing by solids separation, cooling and water separation, at least part of the waste gas of the reactor (9) is heated and recirculated to the reduction reactor (9).