METHOD AND APPARATUS FOR REDUCING COPPER (I) OXIDE

FIELD OF THE INVENTION

The invention relates to a method and apparatus for reducing solid-state copper (I) oxide into metallic copper. The reduction according to the method is performed at least partially by means of hydrogen. Reduction is carried out at a temperature of 600 - 900 ⁰ C in an apparatus, which comprises a gas- tight furnace that is equipped with an endless belt. In addition, the apparatus includes equipment for cooling, detaching and removing the reduced material from the belt as well as equipment for feeding the reducing gas into the discharge end of the furnace and for removing it from the feed end of the furnace.

BACKGROUND OF THE INVENTION A method and equipment for the reduction of copper oxides CuO and Cu ₂ O into metallic copper powder are described in British patent publication 551 ,249. According to the method, copper oxide is reduced at a temperature of 450 - 55O ⁰ C in a muffle furnace, into which the material is conveyed on a continuous steel belt. It is preferred in the method that free copper is also present with the oxides before commencing reduction. The reducing gas is preferably a mixture of carbon monoxide and hydrogen, but town gas and hydrocarbons in general can also be used. If hydrogen is used alone, it is advantageous that there is an excess of hydrogen so that it can be recirculated. The gas is fed countercurrently in relation to the oxide that is to be reduced. The apparatus comprises two main zones: the heating and reduction zone and the cooling zone. The first stage, i.e. heating and reduction, occurs in reducing conditions. Cooling is performed in non- oxidizing conditions to a temperature where the copper is no longer oxidized to any great extent. The cooling section is equipped with a water jacket. On top of the steel belt there are longitudinal bars, which serve to aid reduction by forming furrows in the oxide layer fed on to the belt and secondly to

achieve a friable layer of reduced copper, which is easy to break in order to form copper powder.

US patent publication 6,007,600 describes a method for fabricating copper hydrometallurgically from a raw material containing copper such as copper sulphide concentrate. In accordance with the method, the raw material is leached countercurrently with a solution of sodium chloride and copper chloride in several stages to form monovalent copper (I) chloride solution. Since both divalent cupric chloride and impurities formed of other metals always remain in the solution, reduction and solution purification of the divalent copper is performed on the solution. The pure cuprous chloride solution is precipitated by means of sodium hydroxide into cuprous oxide (Cu ₂ O) and is then reduced further into elemental copper. The sodium chloride solution formed in connection with the precipitation of cuprous oxide is processed further in chlorine-alkali electrolysis, from where the chlorine gas and/or chloride solution obtained is used for raw material leaching, the sodium hydroxide generated in electrolysis is used for cuprous oxide precipitation and the hydrogen generated is used for cupric (I) oxide reduction.

In connection with the method according to US patent 6,007,600, the reduction of monovalent copper oxide or cuprous oxide into metal is not described in detail; it is only stated that it can be done for instance as solid- state reduction in a shaft or rotary furnace. One suitable reductant is the hydrogen generated in the chlorine-alkali electrolysis that is part of the process. The quality of the copper (I) oxide generated is not described in detail in the patent either, but in practice it is known that it always also contains chlorine, for example in the form of NaCI or CuCI.

In the method according to the above-mentioned GB patent 551 ,249, hydrogen is also used as a reductant of solid copper (I) oxide and reduction is carried out in a belt furnace or muffle-type furnace, inside which runs an

endless belt. However, the method relates to reduction that occurs at a relatively low temperature, so that reduction takes place slowly and is therefore not suitable for current industrial production.

PURPOSE OF THE INVENTION

The purpose of the invention was to develop a method and apparatus for reducing solid-state copper (I) oxide, Cu ₂ O, into metallic copper. Reduction is performed at a temperature of 600 - 900 ⁰ C, so that reduction takes place far more effectively than in the prior art. In copper (I) oxide reduction, it is advantageous to make use, at least partially, of the hydrogen that is formed in chlorine-alkali electrolysis related to the chloride-based leaching process.

SUMMARY OF THE INVENTION The invention relates to a method for reducing pulverous copper (I) oxide into metallic copper in solid state by means of a reducing gas fed countercurrently in a gas-tight furnace. The furnace is equipped with an endless belt, onto which the oxide to be reduced is fed. The copper (I) oxide to be fed into the furnace is dried and heated in the drying and heating zones, after which reduction is performed at a temperature between 600 and 900 ⁰ C in the reduction zone at least partially by means of hydrogen. The metallic copper that is formed is cooled in the cooling zone to a temperature below 100 ⁰ C, after which the copper sinter is removed from the furnace.

It is typical of the method accordant with the invention that the reduction rate Y of copper (I) oxide complies with the equation Y= 57.54 x X ^{"0 54} , where Y = reduction rate (kg Cu/m ² /h) and X = reduction time (h).

According to one preferred embodiment of the invention, pulverous copper (I) oxide is formed in a chloride-based leaching process of copper raw material, which includes chlorine-alkali electrolysis, and the hydrogen generated in the chlorine-alkali electrolysis is used for the reduction of copper (I) oxide.

The copper (I) oxide formed in chloride-based leaching contains around 1 % chlorine compounds, whereupon an excess of reducing gas is fed to form hydrogen chloride and to eliminate the formation of gaseous copper chloride, CuCI ₂ .

According to one embodiment of the invention, the reducing gas used is hydrogen and ammonia.

It is typical of the method that heat is recovered from metallic copper by routing the reduction gas into the copper cooling zone, so that the reduction gas is heated at the same time. The gas that is to be removed from the furnace drying zone is combusted.

It is also typical of the method that the thermal content of the gas fed from the reduction zone to the drying and heating zone is used for heating and drying the copper (I) oxide.

A solid reducing agent can be used as a reduction aid in the method accordant to the invention. The aid is at least one of the following: wood, wood fibre, sawdust, paper, cellulose, plastic, carbon and graphite. The reduction aid is used as the copper (I) oxide underlayer material on the belt. When organic material is used as the reduction aid, the combustion of the gas to be removed from the drying zone is performed at a temperature between 800° and 1 100 ⁰ C.

It is advantageous for the method according to the invention that the thickness of the copper (I) oxide layer on the belt is 8 - 30 mm. The speed of the belt in the furnace is preferably 0.8 - 2.6 m/min.

It is typical of the method according to the invention that the sintered copper removed from the belt is compacted.

The invention also relates to an apparatus for reducing pulverous copper (I) oxide into metallic copper in solid state by means of a reducing gas. The apparatus comprises a copper (I) oxide feed device, a tubular gas-tight furnace, in which circulates an endless belt, which is made of a nickel-based alloy and/or austenitic heat-resisting and fire-resistant steel. The apparatus also includes equipment for cooling, detaching and removing the reduced material from the belt, and the equipment for feeding the reduction gas and shielding gas at least into the discharge end of the furnace and for removing it from the feed end.

The copper (I) oxide feed device is typically a vibratory feeder. The endless belt is either solid or mesh belt-type.

The gas-tight furnace typically comprises equipment for heating the furnace and cooling its outer surface. The apparatus also includes combustion equipment for the gas that is to be removed.

According to one embodiment of the invention, protective material is arranged on top of the belt.

According to one embodiment of the invention, the apparatus consists of equipment for feeding a solid reduction aid onto the belt. When organic material is used as the reduction aid, the apparatus comprises a post- combustion chamber for burning the gas at a temperature between 800° and 1 100 ⁰ C.

One embodiment of the invention consists of equipment at least for feeding the reduction gas directly into the reduction zone.

According to one embodiment of the invention, the furnace is equipped with a gas-tight inner tube. The inner tube is preferably a copper tube. According

to another embodiment of the invention, the furnace is equipped with a gas- tight outer tube.

According to one embodiment of the invention, the sintered copper removing equipment comprises a removing and cutting roller as well as scraping and brushing equipment.

According to one embodiment of the invention, the apparatus includes a device for compacting the removed copper.

The essential features of the invention will be made apparent in the attached claims.

LIST OF DRAWINGS Figure 1 is a principle drawing of the apparatus as a side view,

Figure 2 is a graphical presentation of the passage of an oxide to be reduced through the furnace as a function of time and temperature, and Figure 3 is a graphical presentation of the dependence of reduction effectiveness on layer thickness and the length of the reaction area.

DETAILED DESCRIPTION OF INVENTION

The apparatus accordant with the invention comprises a feed device, a tubular gas-tight furnace in which an endless belt circulates, discharge equipment for removing the reduced material and finishing equipment. In addition, the apparatus includes equipment for feeding gases into the furnace and removing them from the furnace.

According to Figure 1 , the copper (I) oxide feed device 1 is preferably a vibration feeder, into which material is fed from a silo 2. If a solid reduction aid is fed into the furnace, this feed takes place from a separate feeder (not shown in the drawing). The furnace used for reduction is typically a muffle furnace 3, which is made for example from bricks and/or ceramic or steel

tube. The furnace is equipped with an inner and/or outer tube, which has to be gas-tight i.e. in practice resistant to chlorine-containing gas and hydrogen- tight. The inner tube may also be copper tube. The benefit of an outer tube is that any scale that may form will not fall on top of the reduced copper. The furnace comprises equipment for heating the furnace, which is carried out typically from the outer surface, for instance by means of electric resistance or burners. The furnace is equipped with devices for feeding the reduction gas and shielding gas such as nitrogen along the entire length of the furnace. It is advantageous to the apparatus that the gas space remaining above the belt is relatively small, so that it prevents the stratification of the gases according to their specific weight.

An endless solid belt 4 passes through the furnace, a typical drive being driving and pulley wheels 5 and 6. The belt is preferably made of steel, for example austenitic heat-resisting and fire-resistant steel such as EN 1.4854 or EN 1.4835. Nickel-based belt materials are even more durable, but also rather expensive. Both the belt material and the material for the inner surface of the furnace should be selected so that they do not corrode, whereby metal scaling will not fall off among the copper cake to be reduced. The inner surface of the furnace may thus also be of the same material as the surface of the belt. An even layer of oxide is fed onto the belt and the feed device is equipped with an equalizing plate 7 for this purpose. The apparatus is also equipped with an adjusting element to regulate the width of the oxide layer. Control of the feed device is preferably connected to the movement of the belt. The belt is preferably of a uniform (solid), but it may also be formed of components (i.e. a mesh belt).

A protective material that protects the belt in high temperatures may be arranged on top of the belt and under the copper (I) oxide layer. The protective material can be used whether the belt is solid or mesh type. The protective material can act simply as a belt-protecting element or it may also

be used to prevent the belt from causing any contamination to the generated copper product.

The inner section of the furnace is in principle in one piece, but because it can be divided into clear zones for instance by monitoring the temperature and reduction degree, in the description of the method and apparatus we speak of different zones. Thus when regarded in the direction that the belt moves the first zone is the drying and heating zone 8, followed by the actual reduction zone 9 and finally the cooling zone 10.

Equipment for cooling the reduced product is arranged in the cooling zone of the furnace. Cooling may be performed either as direct cooling on the product surface or indirect cooling, whereby cooling is directed to the outer surface of the furnace. A combination of direct and indirect cooling can also be used.

The cooled copper cake is removed from the belt by means of a removing device 1 1. The product discharge equipment typically includes removing, cutting and scraping elements plus belt-cleaning elements. The pressed copper plate finishing equipment is alternatively crushing equipment or pressing equipment for compacting the generated copper cake. The compacted copper product is discharged from finishing as plate-like pieces.

The gases fed into the furnace are typically reduction gas and inert shielding gas. The inert shielding gas is preferably nitrogen. The reduction gas used at least partially is hydrogen. When the reduction of copper (I) oxide is the finishing treatment for a copper raw material leaching process, in which copper (I) oxide is formed and which includes chlorine-alkali electrolysis, the hydrogen used in reduction is obtained as a product of the electrolysis. Other reduction gases in addition to hydrogen that can be used include ammonia.

The reducing gas is fed into the furnace from the opposite end to the oxide to be reduced i.e. into the furnace discharge end. Some of the reduction gas can also be fed directly into the reduction stage, as shown in Figure 1. For the sake of the furnace temperature balance, it is preferable to feed the reduction gas into the cooling section of the furnace. The removal of the gases from the furnace is done typically by means of a hood 12 from the feed end of the furnace. It is also possible to implement gas removal from under the belt. The apparatus is equipped with combustion equipment 13 for the removed gas, since hydrogen-containing gas has to be burnt for safety reasons. If, in addition to hydrogen, reduction materials containing organic substances are used for reduction, harmful substances may be formed in the furnace in addition to hydrogen, and they are burnt in a post-combustion chamber, the temperature of which is regulated to between 800 - 1 100 ⁰ C by means of a flame.

The copper (I) oxide in the method accordant with the invention, which is preferably a copper (I) oxide from a chloride-based leaching of a copper sulphide raw material i.e. from the HydroCopper ^® process, is routed to a silo, from where it is fed via a vibratory feeder onto the furnace belt. A copper layer is formed advantageously on the surface of the belt from the copper being reduced, and said layer acts as an autogenic lining and protects the belt from corrosion. Where necessary, for example when the belt is a mesh belt, some underlayer material can be fed onto the belt first so that the oxide remains on the belt. An underlayer material can also be used on top of a solid belt. The underlayer material can be fed on top of the protective material mentioned above or preferably it acts as a protective material itself. One purpose of the underlayer material is to facilitate the removal of the copper cake that is generated. The underlayer material can also simultaneously act partly as a reducing substance. Suitable reducing underlayer materials include wood, wood fibre, sawdust, paper, cellulose, plastic, carbon and graphite.

The copper (I) oxide coming from a chloride-based copper raw material leaching process contains around 1 % chlorine in various forms such as NaCI, CuCI, and CuCI ₂ . The oxide powder generated in the leaching process is fine-grained, around 30 - 60 micrometres. The moisture content of the oxide is around 8 -10% H ₂ O, so the first stage of the reduction treatment is the drying of the material. The oxide is fed into the furnace on the belt belonging to the furnace structure, where the first zone is drying and preheating. In the drying and preheating stage, the water contained in the oxide evaporates and the oxide heats up to a temperature where reduction begins. In addition to the external heating of the furnace, the thermal content of the hot furnace gases fed there from the reduction zone is also exploited in drying and heating. The gases act as the shielding gas for the zone and simultaneously the gases also cool down.

An excess of reducing gas in relation to the amount needed for the process is fed into the reduction zone. The reduction gas content has to be kept high and it is regulated so that all the oxide fed into the furnace is reduced into metal. In addition, the amount of reduction gas is adjusted so that the chlorine separated from the oxide immediately forms hydrogen chloride upon evaporation i.e. hydrochloric acid and does not evaporate as copper chloride vapour. Copper chloride (CuCI ₂ ) easily reacts with the iron in furnace structures to form iron chloride, and thus corrodes the structure and forms black rust among the metallic copper.

As shown in Figure 2, the reduction of copper (I) oxide begins at 300 ⁰ C, but the reaction is still fairly slow. When effective and fast reduction is desired, it is advantageous that the furnace temperature be raised to the region of 600 - 900 ⁰ C. The speed of the belt in the furnace is dimensioned so that the chosen oxide layer is reduced throughout its depth during the time that the belt is in the reduction zone of the furnace. The reduction reaction is exothermic.

Our research has shown that the reduction in the reduction zone occurs in accordance with the following equation:

Y= 57.54 x X ^{"0 54} (1 ) where Y = reduction rate (kg Cu/m ² /h) and X = reduction time (h)

It has also been shown that the preferred thickness of the oxide layer on the belt is in the region of 8 - 30 mm, when the belt speed is in the region of 1 m/min. The times presented in Figure 2 are indicative and used an oxide layer that was 20 mm thick. The belt speed in the furnace, depending on the situation, is preferably 0.8 - 2.6 m/min.

In addition to gaseous reductants, if necessary solid reductants may also be used. The solid reductant used may be wood, wood fibre, sawdust and materials prepared from wood fibre (paper, cellulose), plastic, carbon and graphite, and is used as the underlayer material for copper (I) oxide especially when the belt is a mesh type.

When copper (I) oxide powder is reduced into metallic copper at the temperature according to the invention, at the same time it loses its pulverous nature and is sintered together into a porous cake on top of the belt. Sintering is one of the benefits obtained with the method, since a sintered plate has a denser surface than porous powder and thus the plate does not oxidize as easily as a porous powder. The reduced copper is cooled carefully to a temperature below 100 ⁰ C in a shielding gas, so that its surface does not oxidize. As shielding gas, the reduction gas fed into the furnace works well in addition to an inert gas. The reduction gas may also partially be fed directly into the reduction stage, but in the gas cooling stage however an inert shielding gas such as nitrogen is always used to prevent copper oxidation. As stated above, cooling may be performed either as direct or indirect cooling or a combination of both.

The cooled copper sinter is removed from the belt with removing equipment.

The equipment comprises various rolling mills like a removing roll and a cutting roll and, in addition, belt scraping and brushing devices. The copper cake must be removed so that it can next be fed into a melting furnace to fabricate the desired copper product. Suitable cake processing methods are cake crushing or cake compacting i.e. compression into a plate for example by means of a rolling mill. Compression in particular can minimize copper oxidation. The compacted copper plates are fed into the melting furnace for further processing.

The gas removed from the drying end of the furnace contains water and hydrogen chloride in addition to hydrogen. The gas is combusted either before or after the scrubber, where the hydrogen chloride is separated from the gas.

EXAMPLES Example 1

Copper (I) oxide powder with a particle size of around 30 - 60 micrometres was fed into the furnace. The moisture content of the oxide was 10 % H ₂ O and the chlorine content 1 %. The amount of other impurities was less than 100 ppm. The average density of the powder was 1800 kg/m ³ , varying between 1500 - 2000 kg/m ³ and the copper content 88.8 wt %.

The copper (I) oxide powder was fed on the belt into the furnace in a 20mm- thick layer and conveyed through the furnace at a rate of 1 m/min, whereby the reduction of the oxide into metallic copper occurred in the time presented in Figure 2. After the reduction zone the product was cooled to a temperature below 100 ⁰ C. After cooling, the copper content of the product removed from the furnace was found to be over 99.99 wt % and the oxygen content below 300 ppm. The product fulfils the requirements of LME Grade A (LME = London Metal Exchange).

Example 2

The reduction rate of copper (I) oxide in an industrial-scale furnace is 70 - 100 kg Cu/m ² /h and the temperature in the reduction zone in the region of 650 - 800 ⁰ C. The belt width is 1.5 m and copper (I) oxide powder is fed onto the belt in a 1.4 m-wide and an average 16 mm-thick layer so that the thickness of the layer may vary between 10 and 20 mm. The average density of the powder was 1800 kg/m ³ , varying between 1500 and 2000 kg/m ³ , and the copper content 88.8 wt%.

The belt speed can be adjusted between 0.8 and 2.6 m/min and, depending on this and on the reduction temperature, the reduction time varies between 12 and 20 min (excluding the cooling stage). After reduction, the copper sinter that is formed is cooled to a temperature of 50 - 100 ⁰ C. When the plant capacity is 50 000 t Cu/year and it is desired to implement the reduction capacity in accordance with the example in a two-furnace system, the length of the belt will be 27 m, but if the solution comprises three furnaces, the length of each belt will be 18 m.

The situation according to the example is presented in Figure 3, which illustrates the effectiveness of reduction as a function of the layer thickness and the length of the reaction area. The reaction area refers to the drying, heating and reduction zones.

The temperature of the oxide fed into the furnace is 25 ⁰ C and its moisture content 10% i.e. it contains 10% water. The amount of oxide fed into the drying zone is 4062 kg/h and after drying the temperature of the oxide entering the heating zone is 100 ⁰ C and the amount 3662 kg/h. After the heating zone the temperature of the oxide is 300 ⁰ C and the amount the same as before. After the reduction zone the temperature of the copper is 800 ⁰ C and the amount 3200 kg/h. The copper sinter is cooled to a temperature below 100 ⁰ C. Reduction gas is fed into the furnace from the

opposite end to the oxide so that gas, which is 100% hydrogen, is fed into the cooling zone at 654 Nm ³ /h, and 1 150 Nm ³ /h gas is removed from the drying zone. The exhaust gas contains 10.1 % hydrogen, water vapour and evaporated hydrochloric acid i.e. hydrogen chloride.

The amount of heat required in the drying zone is 356 kW, and in the heating and reduction zones only less than 40 kW. 465 kW of heat is released in the cooling zone and binds itself to the hydrogen fed into the cooling zone, which is heated up in this zone from a temperature of 25 ⁰ C to 400 ⁰ C.

If the copper (I) oxide fed into the furnace is dried to a moisture content of 2% before being conveyed into the furnace, in order to achieve a copper amount of 3200 kg/h the amount of oxide fed into the drying zone is 3736 kg/h. The amount of gas fed into the cooling zone is still 654 Nm ³ /h of 100 % hydrogen, but the amount removed from the drying zone is only 744 Nm ³ /h, of which the amount of hydrogen is 10.1 %.