COMPANY PROCESS AND PROFILE
4.4 The Phosphating Step
the sides of the floor of the spray tunnel. These 30 cm wide grills run along each side of the tunnel and are used to drain the sprayed solutions back into the tanks. The risers run up the sides, across the top, and down the center of the tunnel. These pipes are fitted with nozzles through which the solution is sprayed onto the workpiece surface. There is a gutter on either side of the tunnel in which the solution returns to the tank to begin the re- circulation process again. The number of risers and nozzles situated in the tunnel were counted and recorded in Table 4.1.
Table 4.1 Structure of the spray system Tank
1 2 3 4
No. of Risers In the
floor pipe 7 4 8 4
In the centre
8 4 8 4
In the side 7 (pairs) x 2
4 x 2 8 x 2 4 (risers) x 2
No. of Nozzles In the floor
pipe 7 x 2 4 x 2 8 (riser) x 2 4 (risers) x 2
In the centre 6(pairs) x 2 per
riser 6 (pairs) x 2 6 (pairs) x 2 6 x 2 per riser
In the side (per riser)
6 6 6 6 There are two hatches which allow personnel access into the tunnel. One hatch is situated between Ti and T2, while the other is situated between T3 and T4. The four tanks below the tunnel contain the solutions used in the phosphating step and collect the solution returning from the spraying operation. There are also two local exhaust ventilators system attached to the top of the spray tunnel. One is positioned at the start of the tunnel and one at the end. These ventilators extract the fine mists which are emitted in the tunnel during the spraying operation and transfers it to the outside of the building. There are brushes on either side of the conveyor rail. The bristles of the brushers are there to sweep back the solution escaping from the tunnel via the conveyor system back into spray tunnel.
Chemcoater 755 (Chemcoater) is the commercial phosphating solution used to make up the tank solution. This chemical is supplied by the chemical supplier Chemrox Chemicals in 25 L drums. Four 25 L drums are manually emptied into Ti and two 25 L drums are emptied into T2 at solution make-up. The contents of Ti and T2 are then diluted to volumes of 3.14 m3 and 1.71 m3, respectively. T3 and T4 are filled up with clean water from the mains supply. No formal records have been kept as to when these tank solutions were disposed of and replaced. Records of the chemical additions made to top up the
solutions, replacing the chemicals used and wasted in the pre-treatment process are kept by the line supervisor. The company throws away spent solution as full or partial dumps.
A full dump is where the entire contents of the tank are thrown away. In a partial dump only half of the tank's contents are dumped. Only the rinse tanks are partially dumped.
Process tanks are usually fully dumped twice a year. There is a drain pipe (see Figure 4.14) situated at the bottom and back of each tank. This pipe is used to empty the contents of the tanks into the main drain which in turn flows to the municipal drain.
Figure 4.14 Main pipe inlet into Tank 2
A crude sludge filter has been fitted inside each of three of the tanks. This filter sits almost vertically in the tank. There is a gap between the bottom of the filter and the base of the tank. This allows the re-circulating process solution to flow through the filter while the sludge drops out and is swept through the gap and collects against the tank wall. The purpose of this filter is to stop any sludge from entering the risers and thus from being deposited onto the workpiece and eventually blocking the nozzles. The sludge is removed manually when the process tanks are fully dumped. Half way along the tunnel, there is a control panel, which is used to operate the spray tunnel, pumps, heat exchangers and
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extractor fans. There are three heat exchangers which are manually cleaned with soap and water twice a year. One heat exchanger is attached to Ti, one is attached to T3 and the other is attached to T4. However, the process solutions were generally run at around 40 °C, the first rinse tank was generally run hot (28 to 57 °C) and the final rinse is run as a cold rinse. Figure 4.15 shows the pipes that enter and leave the heat exchanger. These pipes are approximately 80 cm long and have their ends bent up at a 60° angle. Hot water from the boiler enters the heat exchanger through the bottom and exchanges heat with the solution from the tank. Cold water then exits the heat exchanger through the top. A third pipe, attached at the front of the tank, brings water from the bottom of the tank into the heat exchanger. A fourth pipe then transfers the heated solution back into the tank. The water flowing to the heat exchangers flows through a closed loop system. This water is drained and refilled only during maintenance which takes place once a year.
Cold water out
Hot water in
Figure 4.15 Piping system through the heat exchanger
Each tank has a pressure and temperature gauge attached to it. The pressure gauges are set at 2 bars for heavy items such as uprights and beams, and at 1 bar for small items such
as shelving and deck panels. The temperature gauge is set at 40 °C. Each tank has an overflow pipe connected to the main drainage pipe. It has a ball and cock system which automatically opens when the level of solution in the tank becomes too high.The overflow is directed to the drain.
The water feed to the heat exchangers is heated by paraffin from boilers. The paraffin tank is situated underground outside the building (see Figure 4.16).
Steel scale
Paraffin tank
Figure 4.16 Paraffin tank situated on the outside of the factory
A paraffin cleaner additive, Pyron 7 Industrial Fuel Additive, is added to the tank before the paraffin is put in. This is done because the paraffin must be lubricated in order to be pumped around the factory. This paraffin is piped from the paraffin tank into the building to supply the following seven burners:
• one burner on the drying oven
• two burners on the curing oven
• four burners for the heat exchangers.
The paraffin levels are measured manually using a steel scale (see Figure 4.16) which is calibrated in units of 100 L. The scale starts at 500 L and goes up to 14 038 L.
Once the workpiece passes through the drying oven, it is transported to the powder coating booth.