PRODUCTIVITY IMPROVEMENTS AT CONTINUOUS CASTING ROD PLANT OF STERLITE COPPER AT SILVASSA

Balachandran P. Kamath, Raghavendra Adiga, Lokesh Kumar Sharma, Sameer GuptaSterlite Industries (India) Limited, Silvassa.

Abstract

Sterlite copper operates two continuous cast copper rod plants located at Silvassa, India. The plants are using Continuus Properzi technology and have capacities of 12 MT/Hr and 10 MT/hr.

Over the last few years, several improvements have been carried out at the rod plants, which has resulted in higher productivity and efficiency level. This was possible due to modernization, application of new technology and continuous improvement in operating practices in a systematic manner.

The paper covers the strategies adopted, steps taken and changes made over a period of three years in operations in order to improve the quality of C.C. rods. Drastically improved cathode quality has also played a major role in enhancing the quality of rods and have been able to meet the quality requirements of highly demanding multi-wire drawing customers in a big way. The journey of quality improvement has also resulted in reduction of cost of production and decrease in down time.

The propane consumption per Mt has been drastically reduced by 25 %(from 45 kg / Mt to 34 Kg/ Mt). The casting wheel life has been increased by more than 300 % (from 3500 Mt/wheel to 15000 Mt/wheel). Changes in casting parameters including frequent wheel cleaning, use of better quality of water for cooling, have resulted in increase in life of casting wheel.

Several innovative practices like filtration of molten metal in tundish, changes in soluble oil system purification and automation of flow control during casting, commissioning of Defectomat and tightening of operating parameters have resulted in improvement in quality. The improvement in rod quality as is evident by reduction in wire breaks frequency from 1.4 breaks/Mt to 0.12 breaks/Mt. The downtime for daily maintenance has been brought down from 2 hr/day to 1.5 hr/day.

Paper published at Cobre 2003, Santiago, Chile

Introduction:

The copper unit of Sterlite produces continuous cast copper wire rods at Silvassa, India. The present cumulative capacity of plants is 1,50,000 MTA.

At Sterlite several improvements has been done and implemented to bring down the operating cost without sacrificing the quality. This was achieved through various tools like TQM. This has helped us to contain the competition.

The Process

Copper rods are being produced in continuous melting , casting , and rolling operations. Sterlite has ASARCO shaft furnace for the melting of copper cathodes. For casting and rolling, Sterlite has Italian Continuuos Properzi technology. In order to meet the requirement of different market segments Sterlite produces three different sizes of rods i.e. 8.00 mm, 11.42 mm and 12.45 mm.

Copper cathodes which are melted in shaft furnace to make rods are produced at Sterlite’s own Electrolytic Refinery which is situated in the same premises at Silvassa. Shaft furnace utilizes LPG / Propane as fuel. Molten metal is transferred to the holding furnace, which acts as buffer and provides a constant flow of molten copper to casting machine through tundish. Melting, holding and transfer of copper is done in controlled atmosphere achieved by controlling of CO % (carbon mono oxide) at all the burners using LPG or Propane as fuel. Molten copper is poured in one end of trapezoid shaped casting wheel – band cavity through a pouring spout at a desired flow rate of 12 Mt/hr. Molten copper gets solidified in the casting wheel and the solidified cast bar is extracted from the other end. Hot cast bar ( 850 0C) is dressed and guided to Continuus Properzi rolling mill. The cast bar is rolled in predetermined sequences at different rolling stands. Typical three-roll configuration of Properzi mill breaks the cast structure totally and imparts better metallurgical properties in copper stock during rolling. New uniformly refined grains develop in the bar stock during hot rolling. Rod coming out of the finishing stand is subjected to alcohol pickling. Finally the rod is quenched, whipped and waxed before coiling. Coils are compressed strapped and stretch wrapped to protect from atmospheric tarnishing.

Besides online flaw detection, rods undergo several quality checks to ensure chemical, mechanical and metallurgical properties. The endeavor is to facilitate customers, excellent drawability to finer sizes.

Continuous improvement is a habit at Sterlite. Emphasis is on team work and efforts are always on to achieve lowest cost and to ensure best quality. Several projects have been

carried out in the past and several are on, to achieve the targets. A summary discussion of some of them follows.

The Copper Cathode

The quality of raw material always is the single larger parameter which has the greatest impact on process and product quality. At Sterlite melting furnace charge comprises of copper cathodes essentially conforming to ASTM B 115-95 or other similar standard. Plant reverts like crop bars, process rejected coils are also charged in the furnace along with the cathodes. Copper cathodes are produced at Sterlite’s own refinery. Chemistry is one of the important factors, which plays a vital role in producing good quality rods. Elements like Bi, Se, Te, S etc. are having significant effect on the properties and performance of the rods. Se and Te ,the most harmful of these elements can significantly decrease electrical conductivity, increase the mechanical strength of the annealed wire, retard recrystallization, & Bi if beyond the specified limit induces hot shortness during the hot rolling process in the production of the CC Rod. Sulphur results in the formation of SO2 bubbles in the cast bar and subsequently results in wire breaks during wire drawing. Sulphur above 2.5 PPM starts showing its effect on Spiral Elongation Number. We at Sterlite have observed direct relationship between the sulphur content in the cathode and the average values of spiral elongation number.

Enhanced detection capacity and regular cross check of results with internationally reputed laboratories, assures chemical composition of cathodes and rods. The rods produced from each batch of cathodes are subjected to SEN ( Spiral Elongation Number ) test. Efforts at tank house to reduce impurity level in cathodes have helped to maintain a high average of SEN value more than 440.

Up-gradation of analysis facilities and improvements carried out at refinery resulted in the consistency in chemical composition of copper cathodes. Apart from chemical composition physical appearance and cleanliness of cathode plates have also taken care of. Visual inspection practices and washing of cathode with the help of high pressure water jet ensures clean and dirt free cathodes which are almost free from any nodule to the melting furnace. This in-turn ensures supply of quality copper melt to the casting line with minimum slag generation.

Melting and molten metal handling

Melting of cathodes is carried out in ASARCO make shaft furnace. Molten copper is transported to holding furnace and then to tundish for casting through launders.

Beside concern for productivity and fuel consumption which affects cost of production, gas pickup, refractory erosion and slag contamination are some of the problem normally associated with copper melting, holding and transport, which has a greater impact on rod quality and its performance during manufacturing of downstream products.

Team efforts of various kinds have evolved solutions to increase productivity virtually to optimum level and to ensure desired quality of molten metal.

The initial launders employed Alumina refractories, which resulted in occurrences of refractory inclusion in rods hence wire breaks at customers’ end. Lining of the furnace since inception was made of silicon carbide refractory which offers excellent erosion resistance. We after analysis decided to change the launder refractory which was replaced by silicon carbide. Besides, design of the launder and bricks were also changed to minimise the flame impingement on molten copper and flame erosion of refractory. Re-designing of slag skimming pot location and it's design change has further resulted in supply of clean melt to tundish. Initial installation cost of the changed refractory was high but it gave advantages of increased life of lining there by reduced shutdowns and increased productivity of plant with added benefit of inclusion free rods.

Introduction and in-house standardization of metallographic test of tundish button sample, with a microscope equipped with an image analyser helps in identifying any slip in the molten metal operating system.

The main contributor in cost of conversion of cathodes to rods is fuel. The fuel used at CC Rod plant is LPG or Propane The Fuel efficiency of melting furnace plays the important role. Liquid petroleum gas or Propane is used as fuel in shaft furnace, holding furnace and launders. It gives sufficient heat input to prevent the freezing of metal at the same time provides controlled atmosphere to protect the molten metal from gas pick-up and oxidation.

Fuel efficiency of the combustion system is governed by the effective control of carbon mono oxide percentage in the combustion gases. This high percentage of carbon mono oxide in the combustion system has two fold effects,

1. The increase in fuel consumption2. Increase in concentration of the gases dissolved in the molten metal.

Whereas the first one increases cost of conversion the second has an impact on the quality of product, as molten copper has strong affinity towards hydrogen. Flame length increases because of the high concentration of CO results in improper combustion and there are chances of gas pickup in molten copper which ultimately result in porosity in copper rods and results in wire breaks at the time of manufacturing of downstream products.

During the year of 2000 the gas consumption was around 45 Kg/Mt of copper melted. Fuel cost was high at the same time the problem of porosity in the wire break made it imperative to look in to the shaft furnace operation. The task force conducted several studies to follow the heat balance of the furnace.

Various studies of Energy Conservation Team and Quality Improvement Groups highlighted tht carbon mono oxide sampling, measurement and control, as the major area for improvement. The points of sampling were increased and the process was automated. The design of launders were also changed. The height of shaft furnace stack was increased. With the help of nozzle mix burners and controlling of Carbon Mono Oxide through modified PLC system in individual burner the fuel efficiency increased and the gas consumption is brought down to 37 Kg / mt of copper melted.

Further fine tuning of controls of carbon mono oxide, through complete automation of its sampling, measurement and control adjustments, helped achieving reduced fuel consumption. Carbon monoxide percentage was brought down to an average of 0.5 % from 2.5%. Thus the consumption was further reduced form the level of 37 Kg/MT to 34Kg/MT of CC rods produced.

During manufacturing of ETP copper it is required to maintain a slight positive trend of oxygen content in metal from shaft furnace to tundish with a precise control of atmosphere. Automation of the system eliminates the possibility of localized reducing atmosphere from where the hydrogen gas may get entrapped in the liquid metal. The oxygen trend maintained gradually positive from shaft furnace to tundish, which helped in minimising the chances of gas pickup in molten copper. This parameter is chaked at various points by collecting samples at various locations and analysing using LECO oxygen analyser.

Increase of shaft furnace stack height for utilisation of waste heat in preheating of charge and change in launder dimensions also assisted in lowering the fuel consumption..

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Modified design of slag skimming pot helped in reduction of slag inclusions in copper and introduction of ceramic foam filter eliminated any remote possibility of inclusion entrapment in the molten copper.

Casting Properzi is the originator of wheel and belt casting system. Casting system uses trapezoidal grooved wheel and a steel belt which forms a end less mould cavity to produce a continuous cast bar. A continuous layer of soot is applied on the walls of wheel and belt by burning of acetylene. This soot acts as mould releasing agent and prevents direct contact of molten copper and wheel. Lot of work is done on the pyrolysis of acetylene at SIIL. Introduction of oxygen in acetylene helped in uniform coating of soot layer and maintaining the temperature of casting wheel, which results in sound casting and uniform columnar growth through out the cross section of the bar.

Continuous casting operation requires a constant laminar flow of molten metal in mould. At times manual regulation of molten copper were resulting in non-uniform and turbulent flow. By the installation of new auto flow regulator system constant laminar flow in the casting wheel is ensured which eliminated the possibility of air entrapment during casting. Th operator watches the flow stream regularly and takes correcive action to ensure that it is smooth.

Close control of casting cooling parameters and regular monitoring of water quality resulted in increase in casting wheel life by 500 % i.e from 4000 mt copper production to 20000 mt copper production. Others steps like better control while fixing the wheel, bufing of wheel in between casting and removal of scale from water side of the casting wheel have also helped.

Bar Preparation Unit During casting slightest overflow results in small protrusion in the hot cast bar. These protrusions or fins which are highly oxidised, if enters in the rolling mill and gets rolled along with the bar result in a overfill type defect and the rod fails at the customers during cold drawing. To overcome this problem the top edges of the trapezoidal cast bar are trimmed.

Since inception steel brushes were provided after the edge trimming unit to remove any surface inclusion. These brushes continuously cleaning the bar before it enters in the rolling mill. Despite of the presence of ferro detector Sterlite used to receive complaints of wire breaks from their customers. After analysis of the break samples it was observed that most of the samples showed presence of steel particles. Such type of samples collected and analysed on EPMA and EDAX . At the same time all steels which were directly coming in contact with copper also analysed. Almost all the wire break (sample containing steel inclusion) test results confirmed the analysis of cast bar brushes. In order to overcome the wire break problem due to ferrous inclusion the brushes are replaced by

a high-pressure jet, which cleans the cast bar before it enter in the rolling mill. As a result of this study the number of complaints have been reduced down to almost zero level.

Rolling Mill

Sterlite has Continuus Properzi technology for the rolling of hot cast bar. Sterlite Properzi mill contains 11 (eleven) stands. First three stands are roughing mills in which the rolling profile is circular rest eight stands have alternate triangular and circle profile. Three configuration rolls the bar from three sides at 1200 angle. The portion which gets rolled in the stand is known as tangent and the unrolled portion is called witness. In the next pass the witness portion gets rolled and so on. This triangle-circle rolling destroys the cast structure completely and a new uniform grain structure is formed.

In past ,Sterlite used to receive complaints of wire break, besides ferrous inclusions some break showed the presence of oxide stringers. In order to eliminate the problem of oxide to get rolled in a through study was conducted and very high pressure jets were applied on bar as well as on rolls to remove the residual layer of oxides on the rolls and bars. This development helped in not only in reduction in wire breaks but also the residual surface oxide layer is also reduced from 1500 A 0 to less than 300 A 0. Use of sand blasted rolls in first three roughing mills and co-current quenching (with alcohol) of hot rod also helped a lot in reducing the surface oxide layer. .

Considerable attention was given to the soluble oil filtration system. A hydro-cyclone before the Hoffman Filter helped in reducing the load on the filter and seggergating the heavy particles from fine perticles. This also drastically minimised the suspended copper fines in the system.

Up-gradation of soluble oil filtration system helped us in two fold

1. Dust formation during the drawing of rods is minimised.2. Roll life was increased

Pickling and Waxing

At Sterlite besides normal twist test, we regualrly carry out scotch tape test ato detect any rolled in oxide. To some extent we have been able to identify causes for high dust generation at customer’s works. We regularly collect dust from the customer and identify the causes by microscopic analysis. Since we keep archives of our rods on a lot to lot baisis till the rod is used by the customer, a correlation has been made possible. With steps taken on the basis ofcustomer feed back we have been able to sole irritants which cause problems at customer’s works.

Hot rolled rod when comes out the rolling mill immediately is subjected to alcohol pickling and quenching in order to reduce the copper oxide in copper. When the rod comes out of the quenching solution its temperature remains about 60 oC. this rod passes through air drier and them a water soluble wax is applied on the rod continuously to prevent its surface from atmospheric tarnishing. With proper adjustment of Alcohol Pickling system we were able to get a surface oxide value of less than 350 Å .

In recent past some of the enameled wire manufacturer observed that there is a change in shade of the wire after drawing 8 mm rods. In the same drawing conditions some rods showed bright copper colour and some showed slight dull or dark colour after drawing. It was a mystery as there was no alteration in drawing parameters and different rods were giving different shades. A detailed study was carried out and the problem was cracked by identifying the root cause of the problem. It was the wax compound which contained azole group. When the % of this wax increased by more than 2.5 % it was forming a soap type residue which was showing the difference in shade. The problem is resolved and now there is no shade change is observed in drawn copper.

Power (Electrical energy)

Continuous efforts on power saving resulted in reduction of power cost required per Mt of copper. Installation of frequency drives on pumps, impeller trimming of pumps helped in reduction of power cost tremendously.

CONCLUSION

We at Sterlite with team work and brains stroming have been able to improve productivity, reduce costs and improve quality. Customer is the focus for all our efforts and the emphasis is to ensure that all his requirments are met or exceeded. We have learnt through interaction with our customers and have found that quality led improvements automatically bring in reduction in cost and improvement in productivity since the drivers for all thes parameters for a rod plants are the same.

Sterlite is brimming with the optimism about its prospects in the next century. While efforts are on for consistent product quality fordomestic use the industry is conscious of the need for exploring & developing overseas markets. We are fully aware that quality and cost leadership is the esence of success.

ACKNOWLEDGMENT

The Authors wish to express their deep sense of gratitude to the management of Sterlite Copper for giving the support to us with their endeavor to improve quality of rods to international standards & the permission to present the paper.

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