Shift-based Working Routine

Typically, aluminium production facilities operate based on a shift-based working routine. To this end, they divide the potroom into sections. In a section, one type of activity is ongoing during a

2.2. Common Factory Characteristics 25 shift. Repetitive activities are performed for a series of cells resulting in scale advantage. That is, each cell in the section undergoes the same activity and requires similar equipment and machinery. As the cells are (usually) positioned nearby each other in a section, the required machines, vehicles, and equipment can be shared and only the moment of job execution per cell slightly differs. For example, time-consuming overhead crane movements are then limited to the corresponding section only.

This subsection starts with clarifying how sections are defined, after which the shift-based work- ing routine is discussed. We end this subsection with a discussion on how a continuous way of working may be realized by the introduction of AGAPTVs.

2.2.3.1 Potroom Sections

Each potroom is divided into sections. Each section holds a number of electrolytic-cells and an electrolytic-cell is dedicated to one section only. In a section, one type of activity is carried out for the entire group of cells. Basically, three types of activities sequentially alternate each other in shifts. These activities are: anode changing, metal tapping, and pot tending. So, for example, in the first shift anode changing takes place for a dedicated group of cells and in the second shift these cells undergo the metal tapping process, etc. We discuss these activities in the shift-based schedule explanation later on. In the sequel, we elaborate on the way sections are arranged in smelters.

To the best of our knowledge, there is no general consensus about how sections are defined in smelters. Nevertheless, the potrooms are usually divided such that either the sections contain ap- proximately the same number of cells or the workload per section is roughly the same. However, there are also plenteous plants that classify their sections based on historical reasons (e.g., experience) or pragmatic approaches. A potential cause of the irregular section division is the plant expansion over the years.

There is a variety of possibilities to distribute the potroom into sections. One commonly used approach is to divide potrooms into three sections. Figure 2.13 sketches a smelter containing two potrooms with three sections each. For relative small plants, this might be an option, but when the plant size is enormous it may be unmanageable due to (practical) complications such as excessive congestions in sections or workforce and equipment limitations. Moreover, cell segmentation based on three sections could negatively impact the production throughput. Another widely used approach is to segregate a potroom into sections based on clear physical separations such as road crossings. The plant managers experience in addition to the physical plant layout properties usually lead to a section scheme.

FIGURE2.13: Example of a section layout containing two potrooms with three sections each. Adapted from Eick, Vogelsang, and Behrens (2001).

Plant managers sometimes decide to adjust their section schemes based on new findings. Espe- cially when occasions such as holidays or weekends occur, it might be convenient to follow a different section scheme. This requires a proper way of managing the section activities.

Thus, the size and layout of the plant play a major role in the section distribution but also the plant managers’ experience and impact on the production should be considered when deciding how to arrange the sections. It is a model requirement to incorporate the versatility in approaches to designate sections. Insights could be obtained by studying alternative section schemes.

2.2.3.2 Shift-based Schedule

Primary aluminium smelters operate based on shifts. Usually, each day is divided into three shifts of 8 hours. In the weekend, it is common to have two shifts of 12 hours each. Recall that the following three different activities are identified:

1. Anode changing; 2. Metal tapping; 3. Pot tending.

Before we explain these activities in detail, some more elaboration about the shift-based planning approach is necessary. Multiple teams work during a shift to finish the operations on-time. An op- erating team exists of machines, vehicles, and equipment needed to perform its tasks. However, for example, the AGAPTVs needed for the anode changing activity, might be shared among teams. This may happen, for example, when the tasks in one section are completed earlier. Figure 2.14 illustrates the shift-based planning approach using the example smelter layout we used before in Figure 2.13. So, the planned activities are based on a repetitive scheme.

Remark that there are also smelters that base their shifts on two activities only (anode changing and metal tapping). Pot tending activities are then integrated within the anode changing and metal tapping activities. Time window differentiation of the three activities can then provide an outcome to still maintain the three shift division. An example of this often used approach, is changing anodes and tapping metal every 8 hours but perform the pot tending activities every 2 hours. This will require a more sophisticated working approach then having three sections per potroom with non-overlapping activities, because more equipment is blocking pathways and traffic density increases in sections. On the other hand, this may increase the overall production output. An appropriate coordination is required to handle multiple activities that must be performed at the same cells within (almost) the same time window.

In the sequel, we respectively discuss the three aforementioned activities (anode changing, metal tapping, and pot tending). Thereafter, Subsection 2.3 focuses on the internal anode transportation, which is indispensable for the anode changing activity. Subsection 2.4 presents typical factory lay- outs.

Anode Changing

Anode changing activities are initialized by the shift scheme. That is, during the anode changing pro- cess, all electrolytic-cells in the concerning section that require anode replenishment are considered. The process of anode changing may be carried out by various team compositions. Often used team compositions are:

• one Pot-Tending Crane (PTM) (tools are swapped, which is a time-consuming task); • two PTMs;

• one PTM and one Anode Changing Vehicle (ACV); • one PTM and one Cavity Cleaner (CCL);

• one ACV and one CCL.

The process of changing one anode takes approximately 10 to 20 minutes (excluding the tool swapping task when having only one PTM) and consists of the following activities:

1. Break the crust to release the anode.

2. Take the old anode (anode butt) out and place it on an (empty) anode pallet position. An exam- ple of an used anode is shown in Figure 1.5a.

3. Clean the cell and put the bath material in an empty bucket.

4. Take a new anode from an anode pallet. An example of a new anode is shown in Figure 1.5. 5. Place the new anode on the correct height inside the cell.

6. Cover the anode with fresh bath material.

The Automated Guided Anode Pallet Transport Vehicle (AGAPTV) provides a supportive role by assisting those operations. Pallets filled with new anodes are delivered to nearby the cell and empty pallets or pallets with burned anodes are picked-up. The drop-off of pallets with new anodes may already start before the actual anode shift starts. The goal for the anode changing team is to start swapping anodes as soon as the anode changing shift starts. Cranes get their anodes from the pallets

2.2. Common Factory Characteristics 27

(A) Shift 0:00-8:00.

(B) Shift 8:00-16:00.

(C) Shift 16:00-24:00.

FIGURE2.14: A simplified example of the shift-based working routine. The scheme is representing the smelter layout from Figure 2.13 and depicts two potrooms including three sections each. Arcs denote lanes and nodes lane crossings. The daily working routine consists of three shifts with the same timespan. One activity (i.e., either anode setting, pot tending or metal tapping) takes place in each section during such a shift.

After a shift has been ended, the successive activity in that shift will start.

and likewise drops them off in the pallets. In addition to the team compositions, there might be some other supportive machinery/vehicles involved as well (e.g., forklifts, hammer crust breakers, drumfeeders, etc.). However, detailed movements of these are not considered in this thesis.

Let us explain the commonly used anode changing approach in sections using an example. We consider a section containing 20 electrolytic-cells in total thus 10 cells on each side. There are no intersecting aisles and the cells are positioned in an end-to-end layout. Each cell holds 13 anodes on both sides, so 26 in total (see Figure 2.15). Anodes can only be reached from one side, either the center aisle or the back aisle. Recall that anodes must be replenished as soon as (or slightly before) the setting cycle is elapsed. We consider a setting cycle of28days for all anodes in this example. Suppose the repetitive changing scheme for cells is determined such that one anode is changed daily per cell starting on day one. Then, on the26thday, the last anodes have been interchanged. As we consider a similar changing scheme for each cell, there are two days left in which no anodes are changed (the so-called anode free days).

As introduced with the simplified example above, the repetitive anode changing pattern depends on the number of cells in a section, number of anodes per cell, and setting cycle. A larger number of cells or more anodes per cell, require more anodes to be transported. An increased setting cycle, results in requiring less anodes because anodes can sustain longer in cells. The sequence in which anodes are interchanged may differ inside a cell and is based on information from the Manufacturing Execution System (MES). Likewise, the setting cycle may be initialized on different moments per cell. That is, for example, an anode in the first cell of the section should be replaced on the first day, but the one in the second cell should not be replaced on that day. So, this results in fluctuating anode demand per shift. An appropriate modeling technique should be used to cover a variety of possible anode changing schemes.

(A) Top view of an electrolytic-cell. (B) Top-right view of an electrolytic-cell.

FIGURE2.15: Electrolytic-cell orientations. This cell contains 13 anodes on each side, so 26 in total.

Metal Tapping

Metal tapping comprises the process of tapping liquid aluminium from an electrolytic-cell and the logistics involved. Tapping could be done by a crane holding a vacuum crucible (e.g., see Figure 2.4) or metal tapping vehicles (e.g., see Figure 1.1a).

The time it takes to (practically) empty one cell depends on several factors like the size of the cell, the number of anodes, the used equipment, and the desired residual amount. Crucibles are usually full after a certain amount of taps and are then placed in main passages for transport. Crucible transporters transport the load to the casthouse. Empty crucibles are likewise transported by means of the crucible transporters. Tapping duration, tapping quantity, and crucible capacity are client specific. When the metal tapping is in progress, it is desired to not have any other vehicles, machines, and equipment, which are not strictly necessary for this smelter process, in the corresponding section. Aluminium tapping involves special attention as the liquid aluminium is intense heat and therefore

2.3. Internal Anode Transportation 29