Process and arrangement for extracting a metal from slag a containing said metal

Abstract

The invention is directed to a process for extracting a metal from a slag containing said metal, wherein the liquefied metal-containing slag is heated in at least one electric arc furnace (1, 2). In order provide an improved method for recovering especially copper from slags, the invention provides that the metal-containing slag is heated in a first furnace (1) constructed as an AC electric furnace or a DC electric furnace, and the melt is introduced from the first furnace (1) into a second furnace (2) which is constructed as a DC electric furnace. Further, the invention is directed to an arrangement for extracting a metal from a slag containing said metal.

Claims

1. A process of extracting a metal from a slag containing said metal, comprising the steps of: heating the metal-containing slag in a first furnace designed as an alternating current electrical furnace, and carrying out pre-reduction of slag and precipitation of metal matte in the first furnace; conveying the metal matte into the second furnace designed as a direct current electrical channel furnace, and carrying out a thorough slag reduction and removal of inclusion in the second furnace, with separating the to-be-extracted metal by electrolytic precipitation accompanied by an electromagnetic stirring of the melt.

2. A method according to claim 1, wherein the to-be-extracted metal is copper contained in the metal-containing slag.

3. A method according to claim 1, wherein the to-be-extracted metal is lead, zinc, platinum, or nickel contained in the slag.

4. A method according to claim 1, wherein at least one electromagnet acts on the melt in the second furnace in order to generate the electromagnetic stirring.

5. A method according to claim 4, wherein the at least one electromagnet generates a magnetic field between 50 and 1000 Gauss, and in that the magnetic field covers at least a portion of a cross section of the melt and of an area of electrodes in the second furnace.

6. A method according to claim 1, wherein at least one permanent magnet acts on the melt in the second furnace in order to generate the electromagnetic stirring.

7. A method according to claim 1, comprising the step of introducing a reducing agent into the first furnace during heating.

8. A method according to claim 7, wherein the reducing agent is coke.

9. A method according to claim 1, comprising the step of adding carbon-containing material to the surface of the melt in the second furnace in such a way that a horizontally extending layer of the carbon-containing material is formed with a substantially constant thickness and forms a horizontally extending anode layer in contact with an electric connection.

10. A method according to claim 9, comprising the step of maintaining a horizontally extending layer of the metal matte with a substantially constant thickness in the second furnace in the bottom area below the melt, and the layer, acting as a cathode, makes contact with an electric connection.

Description

(1) The drawings show:

(2) FIG. 1 a schematic view of a three-phase electric arc furnace and a DC reduction channel furnace arranged downstream of the latter; and

(3) FIGS. 2a and 2b the sectional front view and sectional side view of the DC reduction channel furnace for thorough slag reduction and removal of inclusions using a coke bed and liquid copper matte as electrodes.

(4) FIG. 1 shows a first furnace 1 in the form of an AC furnace to which a second furnace 2 in the form of a DC furnace is connected. The melt of copper slag prepared in the furnace 1 is guided into the second furnace 2 via connection means 8 in the form of a melt channel.

(5) Two electrodes 9 and 10 in the form of graphite electrodes connected to an AC source 11 penetrate into the first furnace 1, namely, into the molten slag located in this furnace.

(6) The second furnace 2 has a slag inlet 16 for the slag 15 and a slag outlet 17. Two plate-shaped electrodes 4, 5 are located in the second furnace 2. Both electrodes 4, 5 are coupled to a DC source 12 via electric connections in the form of a graphite contact electrode 6 and 7, respectively. The top, horizontally extending electrode 6 is connected to the positive pole of the DC source 12 and serves as an anode. The bottom electrode 5 which is also arranged horizontally is connected to the negative pole of the DC source 12 in a corresponding manner and accordingly serves as a cathode. The copper is extracted by means of an electrolytic process.

(7) As can be seen from FIG. 2, the second furnace 2 is constructed as a channel furnace. At the sides, electric coils 13 and 14 are arranged around metal cores which accordingly form electromagnets 3. An electromagnetic stirring effect which stirs the melt in the second furnace 2 (see below) is generated by these magnets.

(8) In the process according to the invention, liquid slag is processed substantially in the AC electric arc furnace 1 (AC furnace). Magnetite and cupreous oxide in the slag react with the carbon of the graphite electrodes 9, 10 and the added coke according to the following equations:
Fe.sub.3O.sub.4+CO=3FeO+CO.sub.2
Cu.sub.2O+CO=2Cu+CO.sub.2
CO.sub.2+C=2CO

(9) The reduction of cupreous oxide is limited by the magnetite coreduction. The conditions for the coreduction are determined by the equilibrium of this reaction:
(Cu.sub.2O).sub.slag+3(FeO).sub.slag⇄2(Cu).sub.metal+(Fe.sub.3O.sub.4).sub.slag

(10) The copper content in the molten slag is between 2% and 10% and the magnetite content is between 10% and 20% depending on the smelting method and the grade of matte that is generated.

(11) The first step in the slag treatment in the AC electric arc furnace 1 concentrates on the magnetite reduction to a value of 7% to 8% and a copper content of 0.8% to 1.2%, which requires a unit energy consumption of 50 to 70 kWh/t depending on the original slag composition. The degree of slag reduction indicated above allows the reduction time to be decreased by about 50%, which corresponds to a twofold increase of the furnace handling capacities. The slag is tapped off to the second DC reduction channel furnace 2 (DC furnace) continuously or at regular intervals.

(12) The coke bed 4 on the slag surface by which the graphite electrode 6 makes contact with the DC source 12 functions as the anode, and the liquid matte 5 contacting the graphite block 7 is a cathode in the DC reduction channel furnace 2.

(13) On the inlet side of the furnace, two permanent magnet blocks are arranged in the window of the furnace vessel, namely, at half of the height of the slag layer. The cooperation of a nonuniform, horizontal magnetic field with a nonuniform, vertical, constant electric field induces the gradient of the Lorentz force acting on the slag.

(14) The Lorentz force which acts in the crossed constant electric and permanent magnetic fields in every elementary volume of conductive liquid such as, e.g., liquid slag, obviously changes the relative density of the liquid:
γ.sub.A=γ±j×B, where γ.sub.A—apparent relative density in N m.sup.−3, γ—relative density in N m.sup.−3, j—current density in a liquid in A m.sup.−2, B—magnetic induction in T.

(15) With the above-mentioned force at a current density of 200 to 2000 A/m.sup.2 and a magnetic field strength of 0.005 to 0.1 Tesla, the slag velocity is 1 to 2 orders of magnitude greater than the natural convection velocities. It sets the slag in intensive rotation in the area of the magnet so that the magnetite transfer to the coke surface is improved and the reduction is accelerated. At the high temperature of the slag reduction (1200° C. to 1300° C.), the reactions during the reduction of the magnetite and the coreduction of the cupreous oxide are controlled by mass transfer, the stirring of the slag substantially increases the reduction speed.

(16) Further, the stirring of the slag prevents the formation of stagnant liquid and homogenizes the slag. The stirring of the slag in the first step of the process for removal of inclusions is advantageous for increasing the probability of their collision and coalescence.

(17) The slag movement increases the probability of the collision of matte inclusions and metallic copper so that their coalescence and precipitation are improved. The second part of the channel furnace 2 does not experience any intensive slag movement and allows a steady sedimentation of the inclusions.

(18) Due to the ion structure of the liquid slag, the DC current excites the slag electrolysis. Cathodic reduction and anodic oxidation result in the magnetite reduction, copper precipitation and formation of carbon monoxide on the electrodes corresponding to the following reactions:
Cathode: Fe.sup.3++e=Fe.sup.2+
Cu.sup.++e=Cu.sup.O
Anode: SiO.sub.4.sup.4−+2C=SiO.sub.2+2[CO]+4e
O.sup.2−+C=[CO]+2e

(19) The cathodic decomposition of magnetite and the precipitation of copper increase the overall rate of the magnetite reduction and the removal of copper. The precipitation of CO as anodic product forms additional centers of the magnetite reduction.

(20) The additional force acting on metallic inclusions as a result of the apparent change in the relative density of the slag and the interaction of the current in the metal and of the magnetic field are equal:
F.sub.EMB=2πjBr.sup.3, where F.sub.EBF—buoyancy in N, j—current density in A/m.sup.2, B—inductance of the magnetic field in T, r—radius of the inclusion in m.

(21) The interaction of the electric field with the electric surface charge on the surface of the inclusion allows the metal drops to migrate along the electric field lines; the migration velocity, known as a phenomenon of electrocapillary motion, is described by Levich's formula:

(22) $V_{\mathrm{EM}}=\frac{\mathrm{.Math.}Er}{2{\eta }_s\left(1+\frac{r}{2\kappa w}\right)},$ where V.sub.EM—migration velocity in m s.sup.−1, ε—surface charge in coul m.sup.−2, E—strength of the electric field in V m.sup.−1, η.sub.S—slag viscosity in Pa s, κ—specific conductance of the slag in Ω.sup.−1 m.sup.−1, W—resistance of the metal/slag interface in Ω m.sup.2.

(23) Based on the electric charge density, the migration velocity of the metal or of the matte inclusions decreases with the drop radius based on the preceding formula. With smaller inclusions, the migration velocity is considerably higher than the precipitation resulting from the force of gravity.

(24) The slag processing in the crossed electric and magnetic fields makes use of a number of phenomena by which the slag cleaning process becomes very intensive and effective. Electromagnetic stirring of the slag increases the mass transfer so that the slag reduction is accelerated and the coalescence of the inclusions is fostered. Simultaneous slag electrolysis acts as an additional reductant during cathodic reduction of magnetite and copper oxide and anodic formation of carbon monoxide. Electrocapillary migration of the inclusions promotes their coalescence and leads to the removal of inclusions from the slag.

EXAMPLE

(25) Slag from the smelting of concentrate in a flash melting unit contains 4% Cu and 15% Fe.sub.3O.sub.4. The slag is tapped off every 3 hours and is transferred by a channel to the 9.5 MVA three-phase electric arc furnace 1. The slag production quantity amounts to 30 t/h which corresponds to a processing of 90 t in each cycle. The coke consumption comes to about 8 kg/t and the energy consumption to approximately 70 kwh/t corresponding to an average power consumption of 6.3 MW. After one hour, the slag is tapped out to the electric arc furnace over a period of 2 hours. The slag with a Cu content of 1.1% and 7% Fe.sub.3O.sub.4 is transported through the channel 8 into the DC electric arc furnace 2 with a chamber of 4 m in length and 1 m in width. FIG. 2 shows the reduction channel furnace for semi-continuous slag cleaning. The slag flows continuously through the reduction channel furnace 2 for 2 hours. With a slag level, of 1 m, the average holding time is about 30 minutes. With furnace heat losses of 1 GJ/h, the unit current consumption is approximately 35 kWh/t and the required power consumption is 1 MW. At an estimated voltage of 100 V, the current strength is in the order of magnitude of 10 kA. The estimated coke consumption is about 2 kg/t. The final slag contains 0.5% Cu and 4% magnetite. The total energy consumption amounts to 105 kWh/t and the coke consumption to 10 kg/t.

(26) The process according to the invention works according to the embodiment example, that is, as a two-step copper slag cleaning in electric arc furnaces.

(27) The first electric arc furnace 1 can be charged with slag periodically or continuously. In this furnace 1, the graphite electrodes or carbon electrodes are introduced into the molten slag and current is supplied via these electrodes. Coke or another reducing agent is added to the slag surface. The regulation of the slag temperature in the slag cleaning furnace is carried out by regulating the power consumption. Finally, the extracted metals are tapped off in the form of copper matte and metallic copper.

(28) A periodic or continuous tapping off of slag can also be carried out in the DC channel furnace 2. A DC current is applied between the coke layer functioning as anode at the slag surface and the liquid matte functioning as cathode. The superimposed, locally limited magnetic field which is generated by electromagnets or permanent magnets is used to set the slag in motion. Coke is added to the slag surface to keep the layer thickness of the coke layer constant and to maintain favorable conditions of electric contact with the graphite electrodes or carbon electrodes. Here, also, a continuous or periodic tapping off of the cleaned final slag can be carried out. Similarly, the tapping off of the copper matte or of the copper matte together with metallic copper can be carried out periodically. Further, a copper matte (copper) layer is maintained on the furnace bottom as a liquid cathode, wherein the cathode is in contact with a graphite block.

(29) The copper slag can be the slag that is extracted by smelting copper concentrates to form copper matte or to form blister copper directly as well as the slag that is extracted by conversion of copper matte.

(30) A conventional AC three-phase electric arc furnace or a DC electric arc furnace can be used as first electric arc furnace 1.

(31) The induction of a magnetic field generated by permanent magnets or electromagnets is preferably in the range of 50 to 1000 Gauss, wherein the permanent magnetic field covers part of the cross section of the liquid slag in the area of the electrode or electrodes in contact with the coke bed.

(32) Graphite electrodes or carbon electrodes are preferably used as electrodes. The placement of the electrodes allows the current lines to cross the magnetic field lines. As a result of the optimal positioning of the electrodes, the current lines extend perpendicular to the magnetic field lines.

(33) As was mentioned, the layer of liquid metal or metal matte under the slag contacts a graphite electrode or other electrode functioning as the cathode, while the carbon or coke layer at the slag surface is in contact with a graphite electrode or other electrode having the function of anode.

(34) The strength of the DC current is preferably in the range of 500 to 50,000 A depending on the size of the slag cleaning unit, the slag quantity, and the temperature.

(35) Although the suggested method is preferably provided for the extraction of copper, it can also be applied for other metals such as lead (Pb), zinc (Zn), platinum (Pt) or Nickel (Ni).

(36) By means of the two-step slag reduction and the removal of the copper in two electric arc furnaces, the first three-phase electric arc furnaces can be used for the pre-reduction of the slag and precipitation of copper matte, followed by a thorough slag reduction and removal of inclusions in a DC reduction channel furnace with electromagnetic stirring. The use of electromagnetic stirring which improves the mass transfer on the reduction surface and the coalescence of the inclusions, together with slag electrolysis and electrokinetic phenomena, enable an efficient slag cleaning and a high recovery of copper.

LIST OF REFERENCE NUMERALS

(37) 1 first furnace (AC furnace) 2 second furnace (DC furnace) 3 electromagnet 4 electrode (anode) 5 electrode (cathode) 6 electric connection (graphite electrode) 7 electric connection (graphite electrode) 8 connection means 9 electrode 10 electrode 11 AC source 12 DC source 13 electric coil 14 electric coil 15 slag 16 slag inlet 17 slag outlet