MAINTENANCE OF CRUCIBLES AND SIPHON PIPES

Transcription

1 MAINTENANCE OF CRUCIBLES AND SIPHON PIPES Introduction Molten metal and molten bath crucibles used in aluminium smelters must be cleaned and maintained to optimize metal transportation from the potroom to the casthouse. As an increase in cell amperage leads to higher potroom production and metal transportation, this cleaning step becomes more crucial. Crucible cleaning involves the removal of metal/bath deposits and must be followed by crucible preheating. Tube/siphon cleaning and preheating are usually conducted in a common location within the smelter. STAS Inc., which is known worldwide for the manufacture of advanced technology equipment for the aluminium industry, can provide a fully integrated solution for the cleaning and maintenance of crucibles and siphon pipes/tubes where cost and safety considerations are taken into account. Supplies include equipment and integrated control/information systems. Hot and Cold Crucible Cleaner (HACC) Metal and bath crucibles need to be cleaned on a regular basis to remove deposits of bath material and aluminium that accumulate on the interior walls and bottom and which reduce the crucible capacity over time. Crucibles are sometimes cleaned manually or with mechanised tools after cooling to room temperature. These operations have limited throughput and expose the plant personnel to noise and dust while the use of mechanised tools often leads to refractory damage. STAS has developed and built several crucible cleaners that are efficient to clean not only cold, but also hot crucibles within a few minutes. Due to the automatic operation of this equipment, there are considerable productivity gains and savings since full time operators are no longer required. Both single and dual head machines are available for different crucible sizes. The dual-head cleaner is designed to clean both metal and bath crucibles one at a time, with no tools replacement since the augers (or cleaning head) for both crucibles are mounted on the same shaft. The sequence of operation is the same for both types of crucibles. Figure 1: STAS Crucible Cleaner (Single Head) (I:\AAI\02\Ventes\Communications_Client\ STAS Crucible Cleaning and Maintenance-R07.doc) 1

2 Main components of the equipment: 1. Base of machine. 2. Carriage module and cleaning augers. 3. Auger hydraulic rotation system. 4. Electrical system. 5. Automation system. 6. Tilting table(s) with dust collection hood(s). Base of Machine The base of the machine is a sturdy structure that holds the rails on which the carriage module travels. The base ensures the stability of the machine (Figure 2). There are a top rail and a bottom rail on both sides of the frame supporting the carriage module. The wheels move precisely to provide the highest achievable precision to the cutting tools. Rails and wheels can be easily replaced when need be. Automation System The automation system uses an industrial PLC to perform the sequential and combinational logic controlling the complete locking and cleaning steps. The controller allows communication through an industrial network with level-2 system in the plant where the equipment is installed. An exchange table that contains the main operational variables and alarms is programmed in the PLC memory for the supervisor s control system. The automation system takes care of the crucible cleaning sequence between the moment the crucible is detected on the tilting table and the moment the tilting table returns to its horizontal position. The horizontal speed of the cleaning head is governed by the control system which ensures optimum cleaning efficiency at all time. The automated operation ensures an optimized cleaning of each crucible. Tilting Table(s) with Dust Collection Hood(s) The cleaning process for a dirty crucible can take place once the crucible is properly tilted in front of the cleaning head. This way, residues and dust are simply pushed out of the crucible by the rotation of the auger down to the residues bin located underneath the machine. A tilting table (Figure 3) is used to tilt the crucible into the cleaning position. On dual-head machine, there are two tilting tables one at each end of the machine to allow two different types of crucibles. Generally, one tilting table is dedicated to the metal crucible, while the other is dedicated to the bath crucible. Figure 2: Base and Carriage Module of HACC Carriage Module and Cleaning Augers The carriage module consists of a main shaft equipped with one or two augers that are factory sized according to the specific crucibles to be cleaned. Behind each auger, deflector plates are installed to remove the crushed material from the crucible and to direct it to a chute. Each auger is equipped with a series of cutting tools that scrape the sides and bottom of the crucibles. Auger Hydraulic Rotation System The auger rotation is activated through a heavy-duty, helical-gear unit. It is powered by an industrial high-speed, low-torque hydraulic motor. A low rotational speed ensures efficient operation and high durability of the cutting tools. Electrical System Though partly hydraulics, electricity is the primary source of energy for this equipment. Furthermore, various accessories such as detectors, actuators, junction boxes and push-button stations are integrated to ensure safe operation and ease of troubleshooting. Figure 3: Tilting Table (Metal Side) Three hydraulic functions are required at each table: i) locking of the crucible, ii) tilting of the table and iii) locking of the table in cleaning position. Displacement of the carriage module is also performed hydraulically. The HACC has been designed to clean crucibles as large as 15 tonne and could accommodate larger crucibles if required. A dust hood is strategically positioned above each cleaning head and crucible opening. The hood is fitted with slots to favour even distribution of extracted air and efficient evacuation of the dust that (I:\AAI\02\Ventes\Communications_Client\ STAS Crucible Cleaning and Maintenance-R07.doc) 2

3 is produced. It also plays an important role in noise reduction. For those reasons, it must fit the crucible flange as tight as possible. The HACC is designed with an emphasis on robustness, reliability of components as well as on maintainability. Main features and benefits of the equipment are: 1. Fastest available on the market, with a cleaning time of less than 20 minutes per crucible. 2. Very flexible; can clean any type of crucible at any temperature. 3. Precise control and structural rigidity of the cleaning head, increases the life of the refractory. 4. Eliminates worker exposure to dust, hot dross/bath and noise. 5. With PLC control, the equipment is operating at optimum in relation with speed and durability. 6. Fully automatic cleaning operation; no operator is required during the cleaning cycle. 7. Low installation cost. 8. Low operation and maintenance cost. Figures 4 and 5 show the cleaning efficiency of the STAS HACC. It is worth noting how well the metal/bath deposits which would otherwise reduce the crucible capacity after several operations have been removed after cleaning. Figure 5: Cleaned Crucible Crucible Preheating-Curing Systems Before filling them with molten metal or bath, cold crucibles need to be preheated to avoid moisture entrapment. Also, from time to time, both metal and bath crucibles need to be relined with new refractory material. It is important that the new refractory material be properly cured before putting the crucibles back into service. Several electrical preheating and curing systems have been designed and built by STAS. It is worth mentioning that electrical systems are well adapted to meet the strict requirements imposed by refractory manufacturers when initial curing is performed. They also provide very uniform heat distribution that ensures that all areas within the refractory wall are properly heated. Finally, the process is virtually noise free and produces no pollutants. Figure 6 shows a standard system available from STAS. To perform the heating cycle, the heating device is inserted into the crucible by means of a jib crane and a hoist. The system offers the possibility of positioning the crucible on the plant floor or within a pit which provides some additional thermal insulation. Figure 4: Dirty Crucible Figure 6: STAS Crucible Preheating-Curing Equipment (I:\AAI\02\Ventes\Communications_Client\ STAS Crucible Cleaning and Maintenance-R07.doc) 3

4 STAS also offers the possibility of integrating a monitoring system to supervise and report the heating conditions of each crucible. The basic equipment is composed of the following modules: a heating device with lid, a rotating jib crane, a lifting device, and a power and control panel. Heating Device with Lid The heating device is designed to fit the shape of the crucible. The upper section (lid) is made of 5 mm thick plates and is properly reinforced to ensure appropriate rigidity. It is insulated with ceramic fibre wool. The heating elements are made of nickelchromium resistance wires. The wires are protected by a metallic sheath. The heating elements are surrounded by a heavy-duty frame that also acts as a guide for lowering the heating device onto the crucible. The total heating power is set to meet the process requirements. The maximum process temperature of up to 750 C (for curing purposes) can be achieved. Rotating Jib Crane The standard STAS preheater-dryer comes with a rotating jib crane that operates over two stations. With this configuration, the setup time is reduced and productivity is improved. When a crucible is being preheated, another crucible can be positioned in the second station. The slewing jib crane is designed for heavy-duty service. The rotation of the crane is achieved by sprockets and a chain, and the top load is handled by an oversized bearing mounted on top of the vertical mast. The bottom load is taken by a two wheel assembly. Lifting Device Several configurations are available. In a typical system, the heating lid is moved up and down by means of a hoist. The lug-mounted type electric chain hoist is fixed to the end of the horizontal arm of the jib crane. The up/down movements are limited by rotary limit switches integrated in the hoist. Power and Control Panel The heating control system uses a temperature controller programmed to operate a contactor or an SCR power controller for temperature modulation. This is done according to the temperature setpoint using a high and low limit deadband. Curing is carried out by modulating the SCR output according to the programmed curing profile. A thermocouple located through the lid is used to control the process temperature, which is displayed on the control panel. The heating elements are protected against overheating by a high limit controller. This controller reads the temperature from another thermocouple also located through the lid. The indicating lights, push buttons, temperature display, relays, timer and other accessories are installed in the control panel. The jib starter, the main contactor, the SCR power controller, the control transformers and the main disconnect switch are housed in the power panel. Main features and benefits of the equipment: 1. It can be used for curing new refractory. 2. It provides precise temperature control for curing requirement. 3. Low operating, maintenance and capital costs. 4. Automatic operation: no need for dedicated operator. 5. No need for natural gas or crude oil. 6. No pollutants; noise free. 7. Excellent heat distribution, eliminating cold spots and thermal shocks. 8. Reports preheating conditions for all crucibles. Straight and Curved Siphon Cleaners Deposits of aluminium and bath built up on the interior walls of crucible siphon tubes or pipes necessitate cleaning on a regular basis to maintain the conduit free of obstructions. Manual cleaning of siphon pipes can be very dangerous: a cleaning tool is normally pushed manually in and out of the siphon tube which make the operator liable and to come into contact with rotating equipment or hot surfaces. To improve the operation and make it much safer for the operator, STAS has designed and built several siphon pipe cleaners with two types available for either curved or straight pipes. Curved Siphon Cleaner (CSC) STAS has developed a semi-automatic, pneumatic siphon cleaner that can be installed in the potroom or any adjacent building. The STAS equipment can clean rapidly straight or curved siphon pipes with a few minutes only. Due to the extremely rapid setup time, pipes can be cleaned between every siphoning run, which reduces pipe blockages caused by total obstruction. This preventive maintenance operation considerably extends the life of the siphon pipes or tubes. Figure 7: STAS Curved Siphon Cleaner in Potroom STAS curved siphon cleaners are designed to clean pipes whilst they are still attached to the crucible lid (either on the crucible or on a stand). Maintaining the siphon pipe bolted to the crucible lid offers the advantage of saving time thus increasing productivity and reducing maintenance requirements. Main features and benefits of the equipment: 1. It eliminates exposure of operator to rotating parts. 2. It can clean siphon pipes in hot condition. (I:\AAI\02\Ventes\Communications_Client\ STAS Crucible Cleaning and Maintenance-R07.doc) 4

5 3. It can clean siphon pipes while still attached to the crucible lid. 4. Rapid setup and short cleaning times (preventive maintenance tool). 5. No need for electricity (can be used in potroom). 6. It can be adapted to different shapes and sizes of pipes. 7. Fixed or mobile models are available. Tube Cleaning Machine (TCM) STAS has deployed a lot of efforts to complete its product line by developing the more powerful siphon tube cleaning machine available on the market. The equipment is designed to clean straight siphon tubes (pipes) only. The STAS Tube Cleaning Machine (TCM) comprises an electrically-driven cleaning head with power control through a variable drive controller. The rotational speed is optimized as a function of the pipe dimensions as well as the level and type of obstruction. The equipment is composed of a ventilated soundproof casing, a pipe holding mechanism and a drive-cleaning module. The straight siphon tube is clamped to the casing, and the carriage moves the cleaning head inside the pipe. Main features and benefits of the equipment: 1. It can clean siphon tubes plugged with bath or partially plugged with a mix of bath and aluminium. 2. Automatic operation; no operator is required, except for set up and for taking the siphon pipe out. 3. It eliminates exposure of operators to rotating parts, dust and noise. 4. Rotation of cleaning tool and translation of carriage are electrically driven and controlled. 5. Robust and easy to maintain. 6. Powerfull machine that can process most of dirty tubes. Figure 8: STAS Tube Cleaning Machine (TCM) Siphon Tube Preheating (STP) System In some plants, furnace operation involves molten metal siphoning. In such plants, siphon tubes are used to transfer metal from crucibles to casting furnaces and they need to be cleaned on a regular basis to remove the metal build ups accumulated inside. Once the cleaning operation is completed and before being returned into service, the tubes need to be preheated to eliminate moisture entrapment. This operation is efficiently carried out using the STAS siphon tube preheating system. Figure 9: STAS Siphon Tube Preheating Unit Main components of the equipment: 1. The siphon tube support, which is a steel structure onto which two tubes can be mounted. Both positions are equipped with collector systems. Two siphon tubes can be preheated at the same time. 2. A blower/heater assembly, mounted underneath the siphon tube support and providing heating power. 3. A collector system (distribution boxes, gates, flexible connections, clamps), designed to connect the siphon tube to the blower/heater assembly. It also allows heated air circulation inside the siphon tubes. 4. A platform, used by the operator to connect the siphon outlet. 5. A dust deposit box with baffles allows recovery of largest dust particles. 6. A control system, which includes: a local control panel (with indicating lights, push buttons, temperature display and warning strobe lights); a power panel (heating contactor, control transformer, main disconnect switch); a heater thermocouple, mounted in the flange of the blower/heater unit); and process thermocouples mounted at the outlet and inlet of the blower/heater assembly. 7. A PLC is often added to the equipment for retransmission of the requested information (operating status, cumulative number of operating hours, etc.) to the Level 2. Conclusion Equipments described in this paper constitute fully integrated solutions for the cleaning and maintenance of crucibles and siphon pipes and tubes. These technologies have been developed by STAS to operate separately or all together with integrated control systems. They offer a high degree of adaptability to each particular application. The automatic or semi-automatic operation of the process equipment offered by STAS reduces or eliminates operator exposure to dust and noise, thus reducing the occurrence of injuries and offering a safer working environment. (I:\AAI\02\Ventes\Communications_Client\ STAS Crucible Cleaning and Maintenance-R07.doc) 5

6 During the recent years, the efficiency and reliability of those technologies have been proven at the satisfaction of customers such as Aluminerie Alouette, Canada; MOZAL, Mozambique; Hillside Aluminium, South Africa; Aluminium Bahrain, Kingdom of Bahrain; Rio Tinto Alcan, Alma, Canada; Sohar Aluminium, Sultanate of Oman; Rusal Taishet and Boguchany, Russia, Emirates Aluminium, in the United Arab Emirates. It is worth mentioning that in some cases, the high production rate of the STAS equipment has allowed to accommodate increased capacities following plant expansion or amperage increases without requiring further investments in additional equipment. For more information, please contact: STAS Inc Outarde Chicoutimi (Québec) Canada G7K 1H1 Telephone: Website: (I:\AAI\02\Ventes\Communications_Client \ STAS Crucible Cleaning and Maintenance -R07.doc) 6