Hide Curing

April 30, 2002

We had an interesting visit to Pacific Hide and Fur Depot in Nampa, Idaho. The plant processes 3,000 hides a day, for which they pay $15 for a #3 grade, $20 for a prime. The hides are circulated in a large pit containing a salt solution which cures the leather. The plant only cures hides (that is, they do not do leather tanning). The cured cow hides are shipped to China where they are made into leather goods that are exported back to the States, while the thicker bull hides are sold to Mexican firms for use as shoe soles.

An important byproduct from the plant is #1 Yellow Fancy oil. This high quality oil is extracted in cookers that alternately heat and cool the waste flows at the plant. The oil is used to soften tobacco, among other things. (Smokers can ponder that one.)

Oil recovery involves pumping the plant's flushing water first to a 30' long skimmer tank, then to a settling tank, and finally to a DAF. The sludges from the skimmer, settling tank, and DAF are all directed to the cookers. Heating and cooling cycles separate the oil. The solids float, water goes to the bottom, and oil stays in the middle. Once the process is complete, the water is drained and the oil is recovered. The solids that remain are an additional source of oil.

The main application for a screw press at this plant is to squeeze oil and water from these remaining cooker solids. The solids are pumped to a gravity screen, from which they fall into a screw press. The press in service is an old, but very effective, wine press. This press has a 12" diameter screw with a tapered shaft, starting at about 5" and going up to 10" at the cake discharge. The flights on the screw have reducing (tightening) pitch, adding to the squeezing power of the press. The drive was about 15 hp going 10 rpm, which is a lot of torque. The bar screen looked homemade, with bars laid into staves and TIG welded in place. Surprisingly, the drive shaft diameter was only 2". There was not much motivation to replace this old press.

Another screw press application involved five pounds of material that are trimmed from each of the hides. The trim is piled, allowing water to drain, and then it is sent to landfill. (This trim cannot be sold to a rendering company because it contains too much salt.) We were asked about supplying a screw press to improve the water separation, but the financial incentive was insufficient.

Another press need was to dewater what is referred to as manure mud. Unfortunately it is more mud than manure. The particles were so small that it clearly would not dewater without a press aid. We discussed using the press cake from the wine press as a press aid for the manure mud, but even with that the solids capture rate is likely to be unacceptable.

Plant wastewater goes to a DAF unit that handles 5,000 gpd. The filtrate flows by gravity to 50 and 30 micron cartridge filters. The plant did test a Model FF-6 Fiber Filter as a replacement for these cartridge filters, but the testing was not successful.

Issue 127