Sterlite Industries (India) Ltd., Tuticorin, India produces 400,000t/year of copper. During the process, around 800,000t of copper slag is generated within a year. The joint study between Sterlite Industries (I) Ltd. and NCCBM, New Delhi, India aims to utilise copper slag as raw mix component in Ordinary Portland Cement (OPC) and as a blending material for Portland Slag Cement (PSC).
Introduction
Sterlite Industries (India) Ltd., Tuticorin, Tamil Nadu produces 400,000t/year of copper and during the process, around 800,000t of copper slag is generated in a year. The copper is being produced from a copper concentrate containing around 30 - 35% of copper, iron and sulphur each along with around 12% of silica and 5% of calcium. While producing copper the anode, a slag with rich iron and moderate silica content is also generated.
The chemical analysis of the slag indicated that its matrix is compatible with the cement system and can be used in the manufacture of OPC. The copper slag has been found to contain high iron oxide and may be used as a source of iron in the manufacture of OPC. However, the presence of high iron in the slag hinders its bulk utilisation. The effect of copper present in the copper slag is invisible in the formation of clinker as the quantity of copper slag used in the raw mix for OPC may not be enough to percolate the effect of copper on the formation of OPC clinker.
| LOI | SiO2 | Fe2O3 | Al2O3 | CaO | MgO | Na2O | K2O |
| 0.0665 | 0.2514 | 0.6836 | 0.0178 | 0.0109 | 0.0029 | 0.002 | 0.0019 |
Above: The chemical composition of granulated copper slag.
The National Council for Cement and Building Materials (NCB) has done pioneering work in the area of utilisation of industrial by-products for cement manufacture. The joint study aims to utilise the copper slag as raw mix component in the manufacture of Ordinary Portland Cement (OPC) and as a blending material for Portland Slag Cement (PSC). Hence while designing the raw mixes, every effort was made to keep the level of utilisation of copper slag to the maximum possible extent.
The Central ElectroChemicals Research Institute, a Council of Scientific and Industrial Research, Karailudi, India had been consulted to confirm the leaching behaviour of copper slag and to obtain the recommendations from CECRI.
The paper is divided into three parts as below.
1. Using copper slag as a raw material for the manufacture of OPC.
2. Using copper slag as a blending material for the manufacture of PSC.
3. Understanding the leaching behaviour of copper slag and confirm the suitability of using the same in cement manufacturing.
It is also found that the utilisation of copper slag in manufacture of cement offers a significant saving in fuel.
Using copper slag as a raw material for the manufacture of OPC
Materials and methods: The raw materials - limestone, copper slag, granulated blast furnace slag and bauxite were characterised for their chemical and mineralogical properties using standard procedures. The results are presented in Table 1 and Figure 1.
| Sl. No. | Raw Materials | LOI | SiO2 | Fe2O3 | Al2O3 | CaO | MgO | Na2O | K2O |
| 1 | Granulated copper slag $ | 6.65# | 25.14 | 68.36 | 1.78 | 1.09 | 0.29 | 0.2 | 0.19 |
| 2 | Blast furnace slag | 0.76# | 33.08 | 0.49 | 19.17 | 32.3 | 11.92 | 0.23 | 0.84 |
| 3 | High limestone, LS-HG | 41 | 3.31 | 0.82 | 1.1 | 49.6 | 3.25 | 0.26 | 0.21 |
| 4 | Avg. limestone, LS-AG | 36.1 | 12.57 | 1.03 | 2.05 | 45 | 1.98 | 0.23 | 0.34 |
| 5 | Bauxite | 25.09 | 2.68 | 21.78 | 47.69 | 0.27 | 0.19 | 0.02 | 2.22 |
| 6 | Coal ash* | 0.88 | 65.51 | 4.08 | 21.85 | 0.86 | 0.96 | 0.18 | 1.31 |
| *SO3 content is 0.96%, #Gain on ignition, $ IR =28.56%, Cu=0.58%, Mn2O3=0.32, Sulphide-sulphur=0.18% | |||||||||
Table 1: Chemical composition of raw materials.
The bond indices for copper slag, granulated blast furnace slag and clinker and moduli values of granulated blast furnace slag and copper slag were determined and the results are presented in Table 2 and 3 respectively.
| Sl. No. | Sample details | Bond Index (KWH/T) |
| 1 | Copper slag | 17.6 |
| 2 | Granulated blast furnace slag | 16.7 |
| 3 | Clinker (CL-3) | 10.2 |
Table 2: Bond Grindability Index.
| Sl. No. | Formula | Moduli Values | |
| Granulated blast furnace slag | Copper slag | ||
| 1 | CaO+MgO+1/3Al2O3 SiO2 + 2/3Al2O3 |
1.104 | 0.075 |
| 2 | CaO+MgO+Al2O3 SiO2 |
1.916 | 0.126 |
Table 3: Moduli values of granulated blast furnace slag and copper slag.
Five raw mixes were designed with the above raw materials keeping the coal ash absorption level at 4.75%. The proportions of raw materials are given in Table 4. Raw mixes RM-1 to RM-5 were prepared as per the raw mix designs.
| Sl. No. | Raw Mix No. | LS-HG | LS-AG | Copper Slag | Bauxite | Iron Ore |
| % | ||||||
| 1 | RM-1 | 1.5 | 95.5 | 1.5 | 1.5 | - |
| 2 | RM-2 | 1.5 | 95 | 2 | 1.5 | - |
| 3 | RM-3 | 1.5 | 94.75 | 2.5 | 1.25 | - |
| 4 | RM-4 | 1.5 | 94.5 | 3 | 1 | - |
| 5 | RM-5 | 1.5 | 94.75 | - | 0.75 | 3 |
Table 4: Utilisation of copper slag as a component of raw mix. (Coal ash absorption +4.75%).
The contents were thoroughly blended and ground to a fineness of 10% residue on 170 mesh. Nodules of about 1cm in diameter were prepared and dried in an oven at 105±5°C for 2 hours before subjecting them to burnability studies. The burnability studies were carried out by introducing the nodules in a laboratory electric furnace at ambient temperature, which was gradually raised to 1450°C.
The raw mixes were fired at 1300°C, 1350°C, 1400°C and 1450°C with a retention time of 20 minutes. The clinkers CL-1, CL-2, CL-3, CL-4, and CL-5 prepared from RM-1, RM-2, RM-3, RM-4, and RM-5 respectively were room cooled and free lime determined. The results are presented in Table 5 and Figure 2.
| Sl. No. | Sample No. | Temp. (°C) | CaOfree(%) |
| 1 | RM-1 | 1300 | 0.35 |
| 1350 | 0.31 | ||
| 1400 | 0.19 | ||
| 1450 | 0.1 | ||
| 2 | RM-2 | 1300 | 0.45 |
| 1350 | 0.34 | ||
| 1400 | 0.22 | ||
| 1450 | 0.1 | ||
| 3 | RM-3 | 1300 | 0.35 |
| 1350 | 0.3 | ||
| 1400 | 0.23 | ||
| 1450 | 0.11 | ||
| 4 | RM-4 | 1300 | 0.41 |
| 1350 | 0.27 | ||
| 1400 | 0.13 | ||
| 1450 | 0.08 | ||
| 5 | RM-5 | 1300 | 0.71 |
| 1350 | 0.51 | ||
| 1400 | 0.35 | ||
| 1450 | 0.21 |
Table 5: Burnability studies of raw mixes (retention time: 20min).
Based on burnability results, raw mix RM-3 was selected and bulk clinker (CL-3) was prepared from it and characterised. The results are presented in Figure 3 and Table 6
| Sl. No. | Constituent Determined | % |
| 1 | LOI | 0.08 |
| 2 | SiO2 | 21.54 |
| 3 | Fe2O3 | 4.38 |
| 4 | Al2O3 | 4.86 |
| 5 | CaO | 63.45 |
| 6 | MgO | 2.87 |
| 7 | SO3 | 0.06 |
| 8 | Na2O | 0.38 |
| 9 | K2O | 0.51 |
| 10 | CaOf | Nil |
Table 6: Chemical analyses of bulk clinker sample (CI-3..
Performance characteristics of OPC-3 and OPC-5 prepared from above clinker CL-3 and CL-5 were studied and the results are presented above in Table 7.
| Sl. No. | Property | Results |
|
| OPC-3 | OPC-5 | ||
| 1 | Fineness, m2/kg | 301 | 304.1 |
| 2 | Setting Times, Minutes | ||
| Initial | 84 | 79 | |
| Final | 152 | 160 | |
| 3 | Compressive Strength, MPa | ||
| 3 days | 34.9 | 34 | |
| 7 days | 51.2 | 52.1 | |
| 28 days | 65.8 | 64.2 | |
| 4 | Soundness | ||
| Le-Chatelier Exp., mm | 1 | 1 | |
| Autoclave Exp. (%) | 0.021 | 0.026 | |
Table 7: Performance of ordinary Portland cements prepared from clinker CI-3 and CI-5.
Observation and analysis: Chemical analysis results (Table 1) indicate that high grade limestone (LS-HG) and average grade limestone contain 49.60% and 45.00% of CaO respectively, 3.31% and 12.57% of SiO2 and 3.25% and 1.98% of MgO. Optical microscopic analysis indicated the presence of calcite as a major mineral with veinlets of quartz. These limestones are fine grained and a few exhibits inclusions of clayey matter. These results indicate that both limestones can be considered suitable for the manufacture of OPC.
While designing the raw mixes, efforts were made to keep the level of utilisation of copper slag to the maximum possible extent. But because of presence of the high Fe2O3 content (68.36%), its proportion in the raw mixes could not be increased beyond 3.0%.
The results of free lime determination (Table 5) indicate that all the five raw mixes have good burning characteristics (Fig. 2) and are capable of yielding quality clinkers when test fired at 1450°C with retention time of 20 minutes. The choice of selecting the raw mix was limited and varied in terms of the utilisation level of copper slag and bauxite content. Raw mix RM-3 was selected for the obvious reason of utilising copper slag 2.5% and requiring only 1.25% of bauxite. The free lime content in all the clinker samples was found to be less than 0.20% at 1350°C. However, to eliminate the possibility of free silica and also to provide a safety margin, 1400°C was considered more appropriate temperatures for clinkerisation with a retention time of 20 minutes.
Raw mix, RM-3 yielded good clinker with the moduli values almost as per the design at 1400°C with a retention time of 20 minutes. The raw mix RM-3 was selected to avoid process problems attributed to the presence of high proportion of fluxing agent (68.36% Fe2O3) in the raw mix. Raw mix, RM-3 was therefore considered more promising and selected for the preparation of bulk clinker. The effect of copper present in the copper slag (0.58% only) is invisible in the formation of clinker as quantity of copper slag used in the raw mix RM-3 is only 2.5%.
Further bulk clinker (~10kg) was prepared from raw mix RM-3 and characterised for chemical and mineralogical composition. There was no free lime in the clinker sample. SiO2 and CaO contents were found to be 21.54 and 63.45%. The phase composition calculated by Bogues formulae indicates that the quality of clinker is good and is capable of yielding good quality cement.
Optical microscopic investigations indicated homogeneous distribution of well developed alite (51%) and belite (18%). X-ray diffraction studies of clinker CL-3 indicated the presence of 56% C3S and 19% C2S phases in it (Fig. 3).
The results of performance of OPC (Table 7) from these clinkers indicate that both OPC samples conformed to all the requirements of IS:12269-1987 for 53 grade OPC.
Using copper slag as a blending material for the manufacture of PSC
Materials and methods: PSC blends were prepared by using both granulated blast furnace slag and copper slag separately between 25 to 50%. The performance characteristics of control OPC and PSC blends were determined and are presented in Table 8 and results of the effect of fineness on performance of PSC are presented in Table 9.
| Property | Control OPC | Portland Slag Cements with GBFS/(copper dlag) | |||
| 25 | 35 | 40 | 50 | ||
| (slag content%) | |||||
| Setting Times, Minutes | |||||
| Initial | 90 | 112/ (105) | 140/ (128) | 160/ (149) | 192/ (190) |
| Final | 150 | 172/ (156) | 220/ (230) | 265/ (252) | 292/ (265) |
| Compressive Strength, MPa | |||||
| 3 days | 33.1 | 27.4/ (24.1) | 29.9/ (25.7) | 30.1/ (25.3) | 24.2/ (20.9) |
| 7 days | 38.2 | 32.0/ (30.0) | 36.0/ (33.8) | 36.4/ (31.5) | 31.6/ (26.1) |
| 28 days | 57.2 | 52.2/ (45.5) | 58.3/ (48.6) | 59.0/ (47.1) | 56.5/ (42.0) |
| Soundness | |||||
| Le-Chatelier Exp., mm | 1 | 1.0/ (1.00) | 1.0/ (1.0) | 1.0/ (1.0) | 1.0/ (Nil) |
| Autoclave Exp., (%) | 0.087 | 0.014/ (0.082) | 0.010/ (0.066) | 0.010/ (0.051) | 0.019/ (0.031) |
Table 8: Performance of slag cements prepared with varying contents of granulated blast furnace slag and copper slag. (Fineness: 350m2/kg).
| Sl. No. | Property | Portland Slag Cements with Copper Slag | ||
| 300 | 350 | 400 | ||
| (Fineness in m2/kg) | ||||
| 1 | Setting Times (min) | |||
| Initial | 141 | 128 | 109 | |
| Final | 244 | 230 | 208 | |
| 2 | Compressive Strength (MPa) | |||
| 3 days | 22.2 | 25.7 | 26.9 | |
| 7 days | 30.1 | 33.8 | 35 | |
| 28 days | 43.6 | 48.6 | 50.9 | |
| 3 | Soundness | |||
| Le-chatelier (mm) | 1.0 | 1.0 | 1.0 | |
| Autoclave (%) | 0.053 | 0.066 | 0.101 | |
Table 9: Performance of slag cements prepared by using 35% copper slag at different fineness.
Observation and Analysis: Chemical analysis of granulated blast furnace and copper slag are given in Table 1. The results indicate that granulated blast furnace slag conforms to all the requirements of Indian Standard Specification IS: 12089-1987 for the manufacture of PSC unlike copper slag. This is due to the presence of higher insoluble residue (28.56%). Similarly, the glass content in granulated blast furnace slag was found to be 96% as against 1.5%. It also does not conform to the requirement of moduli value laid in the standard (Table 3). Mineral compositions of granulated blast furnace and copper slags were determined by X-ray diffraction analysis and the results indicated the presence of mainly amorphous material in both the slag samples. The diffractogramme of copper slag sample is presented in Fig. 1.
PSC blends were prepared using OPC sample (OPC-Control) collected from a major cement plant and both the slags separately in the range of 25 to 40%. The results of their physical performance are presented in Table 8.
With 25% slag content:
1. 3 days compressive strengths of PSC's from granulated blast furnace slag and copper slag were lower by 17.2 and 27.2% respectively compared to the control OPC.
2. At 7 days the compressive strength of PSC from granulated blast furnace slag was lowered by 16.2% but in case of copper slag, the compressive strength was lowered by 21.47% of compressive strength of the control OPC.
3. At 28 days the compressive strength of PSC's prepared from granulated blast furnace slag and copper slag were lowered by 8.7% and 20.45% respectively compared to control OPC.
When the slag content was raised from 25% to 35% level:
1. PSC samples indicated a decrease of 9.67% and 22.36% in compressive strength at 3 days compared to OPC for blends prepared with granulated blast furnace slag and copper slag respectively.
2. At 7 days the compressive strengths of PSC blends were of the similar trend (5.75% and 11.52% respectively).
3. However, at 28 days the compressive strengths of PSC blend prepared with granulated blast furnace slag was marginally superior to control OPC but with copper slag there was a drop of 15% in the strength.
At 40% and 50% addition of the slags:
The strengths of the PSC blends prepared with granulated blast furnace slag were comparable to the control OPC. But in the case of copper slag addition of 40%, there was an all round further drop in the strength at all ages compared to the addition of 35% of copper slag. At 50% level of slag content, the PSC samples prepared from both the slag samples showed an all round drop in compressive strength at all the days. The level of drop in strength with copper slag was very high compared to the granulated blast furnace slag at all the ages.
In view of the above discussion, copper slag content of 35% is considered optimal for the manufacture of PSC using copper slag. The blend conforms to all the requirements, physical and chemical laid down in Indian Standard Specification for Portland slag cement, IS:455-1989. In order to optimise the fineness of grinding, different samples of PSC were prepared using 35% copper slag ground to fineness ranging from 250-400m2/kg Blaine and their compressive strengths at 3, 7 and 28 days were determined. Results are presented in Table 9. The result of compressive strength indicates that there is a significant gain in strength with increase in fineness from 300 to 400m2/kg at 3 days, which is well expected. However, the gain in the increase in compressive strength beyond 350m2/kg Blaine is only marginal. Based on these investigations the fineness of grinding of 350m2/kg Blaine is considered optimum.
Leaching behaviour of copper slag
Materials and methods: Copper slag was subjected to leaching studies in three aqueous media such as tapwater, rainwater and sea water. The composition of aqueous media is as shown in Table 10.
| Tap water | Rain water | Sea water | |
| pH | 7.2 | 2.4 | 8.68 |
| Free Cl | 120ppm | ||
| Permanent hardness | 230.40ppm | ||
| Temporary hardness | 91.6ppm | ||
| Sulphuric acid | 3.185mg/l | ||
| Ammonium sulphate | 4.62mg/l | ||
| Sodium sulphate | 3.195mg/l | ||
| Sodium nitrate | 2.125 mg/l | ||
| Sodium chloride | 8.484mg/l | ||
| Salinity | 2.45ppm | ||
| Dissolved oxygen | 4.11ml/l | ||
| Chloride | 22000ppm | ||
| Calcium | 840mg/l | ||
| Magnesium | 1276mg/l | ||
| Alkanity | 112ppm | ||
| Alkanity due to CO3 | 16ppm | ||
| Alkanity due to HCO3 | 107ppm | ||
| Inorganic phosphate | 6.48μgm/l | ||
| Organic phosphate | 0.054μgm/l | ||
| Dissolved organic | |||
| phosphate | 3.302μgm/l | ||
| Total phosphorous | 9.784μgm/l | ||
| Nitrite | 0.394ppm | ||
| Nitrate | 0.670ppm | ||
| Iron | 0.002ppm | ||
| Potassium | 324.2ppm | ||
| Sodium | 1821.63ppm |
Table 10: Composition of leachant.
The leachability of eight toxic elements was determined by atomic absorbtion spectrometry. The data is tabulated in Table 11.
| Element | Limit | Tap Water | Rain Water | Sea Water |
| mg/l | ||||
| As | 5 | 0.389 | 0.334 | 0.745 |
| Se | 1 | NF | NF | NF |
| Pb | 5 | NF | NF | NF |
| Cd | 1 | 0.029 | 0.003 | 0.007 |
| Cr | 5 | NF | NF | NF |
| Ba | 100 | NF | NF | NF |
| Ag | 5 | NF | NF | NF |
| Hg | 0.2 | NF | NF | NF |
| Cu | 5 | 0.107 | 0.931 | 0.239 |
Table 11: Leachability study of copper slag for a period of six months with various leachants.
Observation and analysis: The powdered copper slag was mixed with various leachants and was mixed in the ratio of 1:1 and allowed to stand for leaching period from 24 hours to 3600 hours. It is observed that the amount of leaching increases with increase in time and become constant after a period of time. The maximum amount of various elements leached in a period of 150 days is well within the permissible limits.
Leaching studies of copper from copper slag admixed mortar: Cylindrical mortar (1:3) specimens of size Ø5cm and height 10cm were cast using copper slag. After 24 hours, the mortar specimens were demoulded and kept immersed in various aqueous media such as tapwater, rainwater and sea water. The surrounding solution near the mortar is taken and analysed for leaching of copper for various systems under different exposure periods. The results of the study are shown in Table 12.
| Sr. No. | Days | Hours | Tap Water | Sea Water | Rain Water |
| 1 | 1 | 24 | 0.0013 | 0.0638 | 0.0076 |
| 2 | 3 | 72 | 0.015 | 0.0816 | 0.015 |
| 3 | 7 | 96 | 0.0169 | 0.0842 | 0.0093 |
| 4 | 14 | 336 | 0.237 | 0.1823 | 0.2234 |
| 5 | 28 | 672 | 0.0203 | 0.194 | 0.02 |
| 6 | 150 | 3600 | 0.007 | 0.131 | 0.006 |
Table 12: Leaching of copper in copper slag admixed mortar.
Observation and analysis: It is observed that in all three systems, the amount of copper leaching is found to be less up to 7 days of exposure and reaches a maximum at 14 days and get reduced at 28 days except in the case of sea water. Compared to solution studies (Table 11), the amount of leaching is less in copper slag admixed mortar and the same can be attributed to the complex formation with cement mortar. The amount of copper leaching in ppm for various systems follows the order of sea water > tap water ≈ rain water.
Leaching studies of copper from copper slag admixed concrete: 10x10x10cm3 concrete cubes with water to cement ratio of 0.44 were cast using copper slag. After 24 hours, the mortar specimens were demoulded and kept immersed in various aqueous media such as tapwater, rainwater and sea water. Periodically copper leaching is estimated using AAS. The results of the study are shown in Table 13.
| Sr. No. | Days | Hours | Tap Water | Sea Water | Rain Water |
| 1 | 1 | 24 | -0.0157 | 0.0214 | -0.0128 |
| 2 | 3 | 72 | -0.0143 | 0.0251 | -0.0215 |
| 3 | 7 | 96 | 0.0015 | 0.0032 | 0.0153 |
| 4 | 14 | 336 | 0.2414 | 0.2657 | 0.2504 |
| 5 | 28 | 672 | 0.063 | 0.295 | 0.017 |
Table 13: Leaching of copper in copper slag admixed concrete (all the nos. are in ppm).
Observation and analysis: It is observed that in all three systems, the same trend is observed, that of the mortar. However, initially leaching is observed to be very less (below detectable limit) indicative by the negative sign. Tap water and rain water showed no leaching of copper up to 3 days of exposure and sea water showed the leaching of copper for the entire period.
The amount of copper leaching in ppm for various systems follows the order of sea water > tap water ≈ rain water.
Conclusion
Characterisation of copper slag indicates that it contains constituents compatible with OPC system along with impurities of copper. The insoluble residue was higher and the glass content was on the lower side of the limit laid down in Indian Standard Specification IS:12089-1987 for utilisation of slag in manufacture of PSC.
As the copper slag is found to contain an appreciable amount of Fe2O3, a maximum of 2.5% of copper slag is recommended in the raw mix for manufacture of OPC.
The performance of OPC prepared with 2.5% copper slag as raw mix component indicates that at a fineness of 301.0m2/kg, the cement satisfies all the requirements of Indian Standard Specification IS:12269-1987 for 53 grade OPC and hence can be suitable for OPC.
Copper slag can be gainfully utilised up to 35% in the manufacture of PSC which conforms to all the requirements of Indian Standard Specification IS: 455 -1989.
It is observed that the leaching of elements is well within permissible levels over a period of 150 days. Among three aqueous media tested, tap water showed the least amount of leaching when compared to sea water and acid rain water. Copper slag admixed mortar and concrete showed a lesser amount of leaching than solution due to the complex formation with cement mortar.
Hence, copper slag can be used (2.5%) in raw mix for OPC and (35%) in manufacture of PSC. The material does not have any adverse impact on the performance of OPC and PSC. The users of copper slag in OPC / PSC can be assured of the non-leachability of copper and other elements. This further ensured the performance of cement and using copper slag in OPC and PSC shall also help manufacturers to reduce the cost of production.
Acknowledgement
We at Sterlite Industries (I) Ltd., Tuticorin, India acknowledge the research carried out by National Council For Cement and Building Materials, New Delhi and Central Electrochemical Research Institute, Karaikudi, India for us and thank them for their kind guidance.
References
1. Sharma, K. M., Sharma, P. S. and Yadav, D.: Utilisation of Lead-Zinc Slag for the Manufacture of Cement. 8th NCB International Seminar on Cement and Building Materials, Vol. III, IX, 67-73 (1980).
2. Sharma, K. M., Sharma, P. S., Yadav, D., Sharma, J. M. and Mohan, K.: Technical suitability of Copper Slag for the manufacture of cement. for M/s Sterlite Industries (India) Ltd., Tuticorin (Tamil Nadu) sponsored project-1324, July 2005.
3. Central Electrochemical Research Institute, Karaikudi, Corrosion and Leaching Studies on Blended Copper Slag in Concrete, March - 2004.
