Golden Cat Co. (formerly Lowe Industries, Inc.) manufactures Kitty Litter® and Tidy Cat 3® brand cat litter at several plants across the country. The process, in general, consists of the following:

Clay granules are collected from an open stockpile and screened to three sizes before being stored in individual bins. After processing, the final product is directed to one of several downpipes. A slide gate at the bottom of each downpipe is used to control the flow of material to a chute that is designed to remove dust generated during handling. The material is then placed in a surge bin which in turn discharges to a packaging machine.

Particle segregation [220K QuickTime video] was causing a large volume of smaller particles ("fines") to discharge toward the end of each run. This typically resulted in several bags of unacceptable product. Since these bags could be detected only by spot checking, some of the bags were not being discarded. To increase product quality, segregation had to be minimized and any discharge of fines at the end of each run had to be accurately predicted. The result would be fewer bags with high quantities of fines, which would therefore be easier to locate and more consistently discarded.

Roderick J. Hossfeld, a senior project engineer from Jenike & Johanson visited several of Golden Cat's plants to analyze their handling systems and determine the cause of the segregation. Segregation is a common problem when handling a free-flowing mixture that contains various particle sizes, such as cat litter. When a bin is filled by dropping material into it, a pile forms below the inlet. Fine particles tend to sift through the voids of the larger particles and remain at the center of the pile. Coarse particles tend to roll or slide down the pile to its periphery. In Golden Cat's systems, both the downpipes and surge bins were subject to this type of segregation.

The flow pattern of material through equipment such as downpipes and surge bins dictates the extent of the segregation that will occur in the discharged material. Mr. Hossfeld observed that both pieces of equipment at Golden Cat were discharging in a funnel flow pattern. Because of this flow pattern, a steep flow channel formed above the outlet of each surge bin, with non-flowing material remaining around this flow channel. As the level of material in the bin decreased, layers of the non-flowing material slid into the flow channel.

In the downpipes, the fines collected above a ledge of a partially opened slide gate. The coarse material discharged first, followed by a high concentration of fines when the slide gate was fully opened at the end of each run. Unfortunately, the design of the downpipes could not be modified, so the segregation that occurred there could not be reduced. This left only the surge bin for modifications.

Engineers at Golden Cat had attempted to correct this segregation problem at one of their plants by installing an inverted conical insert in the surge bin. While this helped to expand the narrow, funnel flow channel, material was withdrawn fairly uniformly only as long as the level in the bin remained in the cylinder section. Whenever the level dropped into the hopper section, the insert was no longer effective and the fines, forced to the outside of the bin by the insert, remained until the bin was emptied, resulting in unacceptable quantities of fines in the bags.

We conducted flow properties tests in one of our laboratories on representative samples of cat litter. These tests confirmed the site observations of our engineer: the surge bins were providing a funnel flow pattern. Based on our test results, we developed recommendations to minimize the segregation in the surge bins by converting them to mass flow bins.

In a mass flow bin, all of the material is in motion whenever any is withdrawn. Material from both the center and the periphery moves toward the outlet simultaneously. A material which has segregated in a side-to-side manner tends to remix at the outlet.

Mass flow was obtained by the use of our patented BINSERT® (cone-in-cone) design. This had two major advantages over a simple mass flow cone for this application. First, the walls of the BINSERT® outer cone could be made less steep than would normally be required for mass flow, resulting in a larger capacity. Second, with a BINSERT® there is better control over the velocity profile of the material. We concluded that a uniform velocity profile was important, particularly in the hopper section. If the center portion were to flow faster, this would aggravate the problem of fines discharge, especially since there would be a layer of fines located on the top surface due to segregation in the final discharge of the downpipe. Uniform flow of material through the bin would minimize the number of bags with high levels of fines and allow Golden Cat to accurately determine the number of bags to discard at the end of each run.

An integral part of this BINSERT® system is the distributor which we designed to be located at the top of the surge bin. The distributor ensures that both the center cone and outer annulus fill with equal amounts of material. This minimizes side-to-side segregation during filling of the bin. The distributor also spreads the final discharge of fines from the downpipe across the top surface of the material, ensuring that the fines will discharge last.

After providing recommendations to minimize the segregation problem, we were asked to supply the equipment. This ensured the correct interpretation of our recommendations, and allowed us to offer a written warranty that the system would operate as intended.

The equipment was installed and it worked as designed and warranted. However, Golden Cat wanted to further reduce the number of discarded bags. After determining that a small amount of material could segregate below the new BINSERT® system, we designed and supplied an additional small BINSERT® and pant leg chute to be placed below the original BINSERT®. This resulted in a further reduction of the number of bags discarded due to high concentrations of fines.

Jenike & Johanson's modification of the manufacturing equipment effectively controlled the fines during production, and enabled Golden Cat to achieve the quality of cat litter it desired.