DESC:FORM 2
THE PATENT ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006

COMPLETE SPECIFICATION
(See section 10 and rule 13)

TITLE OF THE INVENTION
A method of improving metal recovery from discarded slag.

APPLICANTS
Aditya Birla Science and Technology Company Pvt Ltd, Aditya Birla Centre, 2nd Floor, ‘C’ wing, S. K. Ahire Marg, Worli, Mumbai 400030, Maharashtra, India.

PREAMBLE TO THE DESCRIPTION
The following specification particularly describes this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
[001] The present invention relates to a method of improving recovery of metal from the slag obtained from slag cleaning furnace (SCF). More particularly, the invention describes a method of improving recovery of copper from discarded slag, by using a novel reductant and optimum slag tapping cycle.
BACKGROUND OF THE INVENTION
[002] Copper is most commonly present in the earth’s crust as copper-iron-sulphide minerals, e.g. chalcopyrite (CuFeS2), bornite (Cu5FeS4) and chalcocite (Cu2S). Copper concentrates contain 20 to 30% copper, ~ 30% iron, ~ 30% sulphur and remaining gangue materials such as alumina, calcia and magnesia. It is extracted from the concentrates by pyrometallugical as well as hydrometallurgical means.
[003] Pyometallurgical production of molten copper by conventional method, as shown in Fig. 1, generates two slags, smelting and converting. In general, smelting furnace slag contains 1 to 2% copper, whereas converting furnace slag contains 5-10% copper. The presence of high amount of magnetite (Fe3O4) in both the slags restricts the settling of copper/matte from slag during smelting and converting. As a result, a significant amount of copper is present in both the slags. Hence, both the slags in molten state are further treated in slag cleaning furnace for copper recovery.
[004] There are plenty of pyrometallurgical processes available for copper extraction. One of such process route is flash smelting. In this process, silica is added as a flux to remove the iron as slag during smelting and converting stages as well. The slags generated during smelting and converting stages contain high amount of copper, i.e. 1 to 2% in smelter slag and 5-10% in converter slag. Both the slags are therefore further processed/treated in the slag cleaning furnace (SCF) in order to recover the copper in the form of matte. Thereafter, copper is recovered in the form of matte containing 60 to 70% Cu, which settles at the bottom of the furnace. This matte is recycled back into the converter and the slag which is copper-lean floats over the matte. It is tapped at regular frequency and finally discarded off. The discarded slag is also referred as slag cleaning furnace slag (SCF slag) and further contains copper less than 0.7%. Since the slag is discarded off, any amount of copper present in slag is referred to as “copper loss”. Both these slags contain high amount of magnetite (Fe3O4), which restricts the settling of cooper matte from slag in SCF and causes high copper loss in slag.
[005] Copper is present in slag in two forms: entrained matte (Cu2S.FeS) and cuprous oxide (Cu2O). Copper in the form of entrained matte is a result of poor settling, due to high content of magnetite precipitates. Copper in the form of cuprous oxide is a result of higher oxidation potential of slag. Hence, reducing agent is often required to reduce the magnetite and cuprous oxide in slag. Coke is added in SCF for the following reduction reactions:
C + Cu2O (slag) ? 2 Cu° (1)
C + Fe3O4 (slag) ? CO + 2FeO (2)
[006] Thus addition of coke improves the copper recovery from slag by (1) improving the settling of entrained matte particles through magnetite reduction, and (2) reducing cuprous oxide. As a result, matte is formed and settles at the bottom. The matte is tapped out in a ladle and recycled back in the converter for copper recovery. The slag which floats over the matte is tapped out and then discarded off. The matte is referred herein as SCF matte and slag as SCF slag in order to distinguish them from smelter slag, converter slag and entrained matte. The upper limit of copper loss in SCF slag is 0.7%.
[007] In conventional methods, coke is used as a reductant. Coke particles are charged in SCF to reduce the magnetite and cuprous oxides (Cu2O) in order to enable the settling of copper/matte from slag. However, due to the lighter density of coke, the charged coke particles form a coke bed and this floats over the molten slag in SCF. As such, coke particles take part in the reduction reaction with magnetite present at the top layer of slag. However, the extent of magnetite in slag is higher at the slag-matte interface than at the top layer of slag owing to the reaction between matte and dissolved oxygen of slag. The magnetite at slag-matte interface does not get effectively reduced by coke. As a result, the reduction reactions are not effective in SCF and the copper loss is above 0.7%, i.e., 0.74%.
[008] However, over a period of time, the copper loss from SCF slag has increased far up to 0.75% owing to the following reasons:
1. The grade of copper concentrates (%Cu) has decreased over a period of time, resulting in high content of iron and gangue (Al2O3, MgO, CaO) in the concentrate. Hence, overall flux requirement and thus slag volume have also increased. The overall residence time of slag in SCF has thus decreased, causing high copper loss.
2. Insufficient reduction of slag: Though coke is a very good reducing agent for magnetite and cuprous oxides, it has its own limitation in slag reduction due to its lower density (0.77 gm/cc). After coke is charged in SCF, it floats over the slag layer. Most of the coke is immediately burnt without taking part into reduction reactions. The remaining coke takes part in the reactions only at the top layer of the slag as shown in Fig. 2. Most of the magnetite precipitates are present at the slag-matte interface owing to the oxidation of matte (FeS) by slag oxygen. Consequently, the magnetite present at the slag-matte interface does not get reduced by coke particles floating over slag layer thereby causing high copper loss.
[009] Therefore, there is a need of a method which solves some of the problems present in the prior art as mentioned above.
BRIEF DESCRIPTION OF THE DRAWINGS
[010] Figure 1 shows conventional flash smelter, converter and slag cleaning furnace for treatment and recovery of copper in the form of matte from slag;
[011] Figure 2 illustrates the conventional process of slag reduction by coke as a reductant, which floats over the slag, and by iron carbon briquette, which penetrates through slag layer to reduce magnetite present at the interface between slag and matte;
[012] Figure 3(a) shows the front view of coke, solid charge (Revert) and Iron-Carbon Briquettes feeding system in slag cleaning furnace, in accordance with an embodiment of the present invention;
[013] Figure 3(b) shows the top view of slag cleaning furnace with locations of six bins, three electrodes, smelter slag feed port, converter slag feed port, SCF slag tap-holes and SCF matte tap-holes, in accordance with an embodiment of the present invention;
[014] Figure 4 shows the comparison between previous slag tapping cycle and optimum slag tapping cycle, in accordance with an embodiment of the present invention;
[015] Figure 5 depicts the top view of the vector plot of slag velocity at 120 cm of bath level, emphasizing by-pass of converter slag from SCF tap-hole during simultaneous charging of converter slag and tapping of SCF slag, in accordance with an embodiment of the present invention;
[016] Figure 6 illustrates the variation in copper loss with the residence time of slag in SCF during the field trial, in accordance with an embodiment of the present invention;
[017] Figure 7 shows the reduction of copper loss in slag with Iron-Carbon Briquettes charging in SCF during the three days of field trial, in accordance with an embodiment of the present invention;
[018] Figure 8 shows the reduction of copper loss in slag with Iron-Carbon Briquettes charging in SCF during the sixty days of field trial incorporating the variation of operating parameters, in accordance with an embodiment of the present invention;
[019] Figure 9 depicts the recovery of selenium with copper from SCF slag, in accordance with an embodiment of the present invention.
BRIEF DESCRIPTION OF THE EMBODIMENTS
[020] Accordingly, the embodiments of the present invention provide a method of improving the recovery of metal from discarded slag. There is provided a method for improving copper recovery from slag cleaning furnace (SCF) slag by using a novel reductant, the method of charging a novel reductant and optimum slag tapping cycle.
[021] In an embodiment of the invention, ‘Iron-Carbon Briquettes’ is used as a new reductant due to its density higher than that of slag and less than that of matte. The optimum size of the reductant is that which allows the submergence of new reductant in the slag layer up to the slag-matte interface. Preferably, it is ensured that the new reductant does not submerge in the matte layer in order to avoid the build up at bottom of the furnace. The optimum size of the new reductant is determined by balance of gravity force, drag force and buoyancy force acting on the reductant. Preferably, the slag density is usually between 3.5 to 3.7 gm/cc. The matte density is usually between 4.5 to 4.8 gm/cc. The Iron-Carbon Briquettes are used as a new reductant in this invention with a density between 4.5 to 5.0 gm/cc.
[022] In an embodiment, the Iron-Carbon Briquettes contain 80-90% of iron and remaining carbon, sulphur and the like, which are useful for reduction. Preferably, Iron-Carbon Briquettes is used as an alternative reducing agent to the coke. Iron-Carbon Briquettes can also reduce the magnetite and cuprous oxides as follows:
Fe (IRON-CARBON BRIQUETTES) + Fe3O4(slag) = 4FeO(slag) (3)
Fe (IRON-CARBON BRIQUETTES) + Cu2O(slag) = 2Cu(matte) + FeO(slag) (4)
[023] The advantage of Iron-Carbon Briquettes over coke is its higher density i.e., 5 gm/cc. This ensures the complete submergence of Iron-Carbon Briquettes in the slag layer. However, there is a possibility of bottom build up with Iron-Carbon Briquettes as its density is higher than matte. So in order to ensure the submergence of Iron-Carbon Briquettes up to the slag-matte interface, the optimum size of Iron-Carbon Briquettes is determined by calculating various forces acting on Iron-Carbon Briquettes in the slag layer. The forces such as slag buoyancy, particle drag and gravitational are considered in calculation. The optimum size of the Iron-Carbon Briquettes was found to be between 20 to 30 mm but may range from 5mm to 30mm. In addition, the reduction in electrode power consumption is 5 kWh per ton of slag cleaning furnace slag is observed when using iron-carbon briquette as reductant.
[024] In another embodiment of the invention, the optimum method of charging the Iron-Carbon Briquettes in SCF is determined. There are 6 bins available for charging of a new reductant in SCF. These bins are located all over the SCF covering the entire periphery. The Iron-Carbon Briquettes is charged in such a way as to ensure the effective utilization of Iron-Carbon Briquettes in reduction reactions 3 and 4. The Iron-Carbon Briquettes feeder arrangements are shown in Fig. 3(a). The charging of the new reductant is enabled from the bins which are located above the converting slag charging runner and smelting slag charging runner. One hopper is separately arranged for charging Iron-Carbon Briquettes into the furnace. Iron-Carbon Briquettes is charged through the bins, which are six in numbers and arranged as shown in Fig. 3(b). The new reductant is charged from the identified bins during the charging of smelter and converter slags. This enables effective mixing of the new reductant with slag and thus avoids floating over coke bed.
[025] In another embodiment, the charging of the new reductant is avoided during tapping of SCF slag from furnace so as to avoid the by-passing. Bin-1, Bin-2 and Bin-3 are located in the vicinity of the slag tap-holes. So there is a possibility that Iron-Carbon Briquettes after being charged may by-pass and exit through SCF slag tap-hole during tapping. Therefore, Bin-4, Bin5 and Bin-6 are identified for charging the Iron-Carbon Briquettes. The charging of the Iron-Carbon Briquettes is enabled from the bins which are located above the converting slag charging runner and smelting slag charging runner. The Iron-Carbon Briquettes is charged during the charging of smelter as well as converter slag in SCF. This enables effective mixing of the new reductant with slag and thus improves the reduction reactions. It also avoids Iron-Carbon Briquettes floating over coke bed. The charging of the new reductant is avoided during tapping of SCF slag from furnace so as to prevent the by-passing of Iron-Carbon Briquettes with SCF slag.
[026] In yet another embodiment of the invention, the optimum slag tapping cycle is determined such that it provides the sufficient residence time to the converter slag in SCF. Due to the degradation of copper concentrates, the iron and gangue material contents have increased in the concentrates. As a results, the generation of smelter and converter slags has also increased in the process. Due to the fixed size of SCF, the overall residence time of these slags to achieve desired settling of entrained matte particles and desired reduction of cuprous oxides has reduced significantly, causing higher copper loss.
[027] Proper understanding and control of drainage of slag from SCF is therefore essential for stable operation and lower copper loss. The slag and matte levels need to be known in advance to take the decision on slag tapping. Direct measurement of the slag and matte levels is extremely difficult and inaccurate due to the hostile conditions. Hence, estimation of slag and matte levels needs to be simulated based on the operating parameters available. The dynamic mass balance based model for prediction of slag and matte levels in SCF is developed. The model takes the mass flow rates of converter slag, smelter slag, solid charge (revert), SCF matte and SCF slag into the calculations for the given size of SCF. It solves ordinary differential equations to predict the instantaneous levels of SCF slag and matte. The model has been simulated using process data and validated with the level measurements of slag and matte in SCF. Based on the model predictions, the optimum tapping cycle of SCF slag has been developed in this invention. The optimum slag tapping cycle is compared with the previous slag tapping cycle and shown in Fig. 4.
[028] In general, 0.1 to 0.15 tons of converter slag per ton of SCF slag is charged five to six times in SCF per day. However, 0.8-0.9 tons the smelter slag per ton of SCF slag flows almost continuously into SCF for about 20 hours per day. 0.1- 0.16 tons of SCF matte per ton of SCF slag is tapped per day. In the previous slag tapping cycle, simultaneous charging of converter slag and tapping of SCF slag is observed. As a result, the converter slag does not get sufficient time for reduction and thus tends to bypass through SCF slag tap-hole. This causes high copper loss i.e. > 0.7% in SCF slag. In the proposed tapping cycle, minimum 60 minutes of residence (holding) time is provided between the converter slag charging and SCF slag tapping.
[029] Experimental Data:
Advantages and benefits of the embodiments of the present invention would become more apparent from the below experimental details to a person skilled in the art.

Example-1
[030] The converter slag contains 5 to 10% copper, out of which 70% is in cuprous oxide form (Cu2O). Hence it is necessary to provide sufficient time for reduction of converter slag by Iron-Carbon Briquettes. There are two SCF slag tap-holes: one is at 120 cm of bath level (lower tap-hole) and other at 160 cm of bath level (upper tap-hole). Both the tap-holes are of 80 mm diameter. The minimum and maximum bath level is 120 cm and 180 cm. The SCF process was studied to determine the flow pattern of smelter and converter slag in SCF during slag tapping at different bath levels in SCF i.e. 120 cm, 140 cm, 160 cm and 180 cm. The flow pattern of slag at 120 cm of bath level during simultaneous charging of converter slag and tapping of SCF slag is depicted in Fig. 5. The converter slag finds the least resistance path and is by-passed through SCF slag tap-hole. This is depicted by high velocity vectors of slag flow from converter slag feed port to SCF slag tap-hole. The residence time distribution of the converter slag in SCF is also determined. The study involved the tracer injection through converter slag entry and measurement of its concentration at SCF slag tap-hole. From the known amount of concentration at the SCF slag tap-hole, the fraction of converter slag by-passed is determined and shown in Table-2.
Table-2 Bypassing of converter slag from SCF tap-hole at different bath levels
Bath level, cm Converter slag by-pass (%)
120 10.5
140 5.9
160 4.6
180 7.76
[031] It is observed that the converter slag by-passed through SCF tap-hole is minimum at 160 cm of bath level and SCF slag tapping at 120 cm level. Hence, 40 cm of slag height should be maintained above the lower tap-hole during simultaneous charging of converter slag and tapping of SCF slag.
Example 2
[032] A trial was conducted in SCF with the optimum tapping cycle and bath level. This trial was conducted without Iron-Carbon Briquettes charging. Using the model, the levels of slag and matte were predicted continuously during the process for the given volume of incoming smelter slag and converter slag in SCF, and 0.05-0.07 tons solid charge per ton of SCF slag. The SCF slag tapping was carried out only after the bath level had reached 160 cm. The simultaneous charging of converter slag and tapping of SCF slag was avoided. For each cycle, the SCF slag tapping was kept on hold after the converter slag was charged in SCF. The residence (holding) time for converter slag in SCF was varied and the copper loss in SCF slag was measured in each cycle. The results of the copper loss at different residence time is shown in Fig. 6. It is observed that the copper loss is significantly reduced after 60 minutes of residence time between the converter slag charging and SCF slag tapping.
Example 3
[033] Another trial was conducted with Iron-Carbon Briquettes addition wherein, 0.011 tons of Iron-Carbon Briquettes per ton of SCF slag was charged in SCF per bin per shift. Iron-Carbon Briquettes were first transferred to the hopper as shown in Fig. 3. From the hopper, Iron-Carbon Briquettes were charged into SCF through Bin-4, Bin-5 and Bin-6, as shown in Fig. 3. Thus, total 10 tons of Iron-Carbon Briquettes per day were charged into the furnace. Iron-Carbon Briquettes was charged into the furnace from Bin-5 and Bin-6 at the start of every shift before the coke charging in order to ensure Iron-Carbon Briquettes submergence in molten bath. Iron-Carbon Briquettes were charged from Bin-4 only during the charging of converter slag in SCF in order to ensure better mixing between Iron-Carbon Briquettes and converter slag. The Iron-Carbon Briquettes charging trial in SCF was conducted continuously for 3 days. Samples from all incoming and outgoing streams of SCF were regularly collected for analysis.
[034] Fig. 7 shows the variation in Cu loss and magnetite content of SCF slag before and during the trial. Before the trial the Cu loss and magnetite content of SCF slag were 0.74% and 4.1% on average. After the start of the trial, both the Cu loss and magnetite content fluctuated for a day. This fluctuation is attributed to the fluctuation in SCF & FSF slag temperature and magnetite content of FSF slag. Moreover, the amount of Iron-Carbon Briquettes charging per day in SCF was calculated to reduce the fixed amount of magnetite in the slag. Furthermore, it took almost a day to stabilize SCF bath with available quantities of slags mixture with the addition of Iron-Carbon Briquettes at above-mentioned rate. So this period is termed as the stabilization period in Fig. 7. After this period, the Cu loss as well as the magnetite content of the SCF slag were reduced to less than 0.7% and 3.5%, consistently. The average Cu loss reduced to 0.68%. The reduction in Cu loss along with the magnetite content proved that the Iron-Carbon Briquettes did effectively take part into reduction reactions with slag, thus reducing the slag viscosity, matte entrapment and resultant Cu loss. The reduction in magnetite content proved the effective reduction reactions with Iron-Carbon Briquettes. The copper loss was reduced from 0.78% to even 0.66%.
Example 4
[035] Yet another trial was conducted with Iron-Carbon Briquettes addition in SCF for 60 days. This trial for a longer duration was conducted in order to establish the impact of Iron-Carbon Briquettes on copper loss with a variation in concentrate grade from 24 to 28% Cu and operating conditions such compositions, amount and temperature of converter slag and smelter slag, power and SCF matte grade. 60 days of duration for trial is sufficient enough to capture the variation in above parameters. In addition, the holding time of minimum 60 min for converter slag before SCF slag tapping was maintained. The amount of Iron-Carbon Briquettes charged and the charging bins were same as used in example-2. The samples of SCF slag and matte were collected during every tapping and analysed for copper. The results of copper loss are shown in Fig. 8. The copper loss data are daily average data. The copper loss data before the trial and during the trial are compared in Fig. 8. The copper loss was reduced to less than 0.7% consistently with Iron-Carbon Briquettes addition for the period of 60 days. It was reduced to 0.66%. With variation in concentrate grade and operating data, the Iron-Carbon Briquettes addition in SCF has established the reduction of copper loss to less than 0.7% consistently.
[036] During this trial, the power consumption was reduced by 5 kWh per ton of SCF slag. This is due to the exothermic reaction no. 4. Combining reaction (3) and (4), the total change in enthalpy of reaction is -100 KJ per mole Fe.
[037] The low & high Cu loss slag samples were collected and analysed for selenium concentration. The selenium content of slag increased with copper loss in SCF slag, as shown in Fig. 9. The Se content was reduced by 100 ppm with reduction in Cu loss from 0.78% to 0.66%. From the regression analysis, the reduction in selenium loss was calculated to be ~ 22 ppm for 0.03% reduction in Cu loss.
[038] The foregoing description of specific embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obvious modifications and variations are possible in light of the above teachings.

,CLAIMS:Claims
We Claim:

1) A method for improving recovery of copper from slag in a slag cleaning furnace (SCF), comprising:

(a) charging said slag cleaning furnace with smelting slag and converter slag, wherein said slag cleaning furnace comprises of
a smelting slag feed for charging smelting slag,
a converter slag feed for charging converter slag,
a plurality of bins positioned across periphery of said slag cleaning furnace,
a plurality of tap holes for tapping slag from slag cleaning furnace,
(b) contacting said smelting slag and converter slag with a reductant, thereby allowing reduction of said smelting slag and converter slag;
(c) separating copper from slag in the form of matte;
(d) tapping slag cleaning furnace slag (SCFS) from slag cleaning furnace through said plurality of tap holes;
(e) charging of another batch of converter slag through converter slag feed; and
(f) charging of reductant from said plurality of bins positioned across periphery of said slag cleaning furnace;
wherein, the residence time between tapping of SCFS and charging of converter slag when reductant is in contact with said slag in SCF is in the range of 60-70 minutes,
wherein said charging of converter slag and reductant is performed when said tap holes are in closed condition,
wherein said reductant is an iron-carbon briquette.

2) The method as claimed in claim 1, wherein the density of iron-carbon briquettes is in the range of 4.5 to 5.0 gm/cc.

3) The method as claimed in claim 1, wherein the size of iron-carbon briquettes is in the range of 5mm to 30mm.

4) The method as claimed in claim 1, further comprising maintaining a predetermined bath level in slag cleaning furnace to prevent loss of converter slag during tapping of SCFS.

5) The method as claimed in claim 1, wherein the predetermined bath level of slag cleaning furnace is 160 cm.

6) The method as claimed in claim 5, further comprising charging said reductants through bins 4, 5 and 6.

Dated this 14th Day of December 2020

Digitally Signed
M. Kisoth
IN/PA-2259
Agent for the Applicant