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Case Study CS0002:
MINIMISATION OF VARIATION IN TAPPING INTERVAL
Preface:
Tapping of metal from pots is an important activity in the electrolysis process of aluminium extraction. In our system, tapping is done every alternative day (after 48 hrs).
On an average 772 kg of metal is tapped from one pot. Deferred tapping or advance tapping effect directly to the thermal balance/efficiency of the pot. When tapping is deferred, pot needs more energy, take less alumina and frequency goes up. The heating zone comes nearer to the cathode if metal tapping is done earlier, hence pot becomes hot and instability increases. To eliminate the above problem and operate the line in a steady condition, the metal should be tapped from pots in a regular interval of time.
This prompted an SPC project to minimise variation in tapping interval.
Problems faced by the tapping department :
- Power problem.
- Improper planning.
- Crucible condition.
- Workers preferring breaking of side pots for tapping.
- Man power shortage (single tapping).
- Casting plant (down stream process) demand variation.
- Material handling equipment break-down.
- Month-end advance tapping.
Flow diagram before SPC project :
Data collected before SPC project :
| Month |
22 - 29 hrs |
30 - 39 hrs |
40 - 55 hrs |
56 - 66 hrs |
Above 66 hrs |
Total VC |
| Jan - Feb | 6 | 70 | 378 | 71 | 2 | 527 |
| March | 18 | 22 | 192 |
37 | 0 | 269 |
Data collected after taking steps :
| Month |
22 - 29 hrs |
30 - 39 hrs |
40 - 55 hrs |
56 - 66 hrs |
Above 66 hrs |
Total VC |
| April | 6 | 16 | 211 |
31 | 6 | 270 |
| May | 4 | 27 | 220 |
35 | 2 | 288 |
| June | 3 | 27 | 219 |
22 | 12 | 283 |
| July | 5 | 35 | 215 |
29 | 1 | 285 |
| August | 1 | 17 | 267 |
17 | 1 | 303 |
| September | 0 | 14 | 236 |
19 | 1 | 270 |
| October | 0 | 21 | 227 |
17 | 5 | 270 |
Analysis and corrective actions :
| Cause |
Effect |
Corrective actions |
| Man power shortage |
Single tapping |
- Reduce absenteeism
- Councelling
- Skill development |
| Improper planning | Non-adherence to tapping schedule | - VC condition monitoring
- Monitoring previous tapping time
- Planning for the scheduled tapping
- Interaction with casting plant (down-stream process) |
After taking corrective actions, the tapping crew adapted the following work flow.
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