For years, Lafarge’s Joppa, Illinois cement plant has experienced severe flow and operational problems with their gypsum bin. These problems significantly impacted productivity, created housekeeping and safety issues, and required the limited available manpower to spend significant time tending to these problems.

The gypsum bin has had several modifications over the years. However, even with the modifications, severe flow problems were still experienced with this bin. The original bin was designed to hold approximately 590 tons of gypsum, and consisted of a 20 ft. diameter by 40 ft. tall vertical cylinder section, which mated to a 40° (from vertical) conical hopper that converged down to a 10 ft. diameter. At the 10 ft. diameter, the conical hopper terminated with a flat bottom and had two 5 ft. diameter openings. Each opening had a 20° (from vertical) conical hopper that converged down to a 4 ft. diameter Matcon Buls discharger valve. Beneath each of the valves were weigh belt feeders that modulated the flow of gypsum (ranging from 5 to 10 tons per hour) into a grinding mill.

The Matcon units, with their vertically vibrating internal cones, were installed in an attempt to promote flow of the gypsum. However, the inner cones were removed by Lafarge because they left only a 4 in. annular gap, which could not discharge the large 5 in. diameter lumps contained within the natural gypsum.

The gypsum bin had a number of serious design flaws from a bulk solids flow perspective. The major problem with the bin was the flat-bottomed section where the dual outlets began. This alone created a large region of stagnant gypsum, resulting in a funnel flow pattern. In addition to this, the conical hoppers were neither steep enough nor smooth enough to ensure reliable gypsum flow.

Samples of natural and synthetic gypsum were provided by Lafarge for flow properties testing by Jenike & Johanson. Since the gypsum was stored outdoors and thus exposed to the elements, its moisture content periodically approached saturation. In order to capture worst case handling conditions, sub-samples were tested at saturation conditions and moisture contents between 70% and 90% of saturation. The established flow properties would then provide a basis for bin modifications.

Based on the constraints presented by Lafarge and the results of the gypsum flow properties tests, only a few alternatives were possible. In order to provide for mass flow, utilize the existing cylinders, and have two independent outlets with feed control, all the existing equipment from the bottom of the cylinder section to the inlet of the mill feed conveyors had to be replaced.

Multiple outlet mass flow bins present additional problems, since it is unlikely that each outlet will always operate at the same time and at the same rate. Under these conditions, significant non-symmetric loads may be exerted on the bin structure.

When the limitations of a multiple outlet, mass flow bin were presented to Lafarge, they agreed with Jenike & Johanson's assessment that a single outlet would be more practical than two independent outlets. This would eliminate the concern about stagnant regions of material forming within a hopper when one outlet remained unused.

Jenike & Johanson engineers recommended the design of a transition hopper, lined with 304 stainless steel sheet with a #2B surface finish, and the design of the interface between the hopper and belt feeder. A properly designed interface was needed to ensure uniform withdrawal of material over the entire length of the hopper outlet.

A multiple screw feeder was considered for feeding the gypsum from the hopper, but there were several drawbacks to its usage. The hopper outlet would have to be considerably shorter to accommodate a screw feeder (due to the limits of an increasing capacity screw design). To provide a shorter outlet, the hopper would have to be much taller, causing a larger reduction in bin capacity. In addition, there was concern about being able to handle the large natural gypsum particles (4 to 5 in.) with a screw feeder.

Lafarge implemented the mass flow technology on one of their gypsum storage bins in 1998. As a result of the Jenike & Johanson recommended modifications, Lafarge has been able to operate this bin reliably while handling both natural and synthetic gypsum. The transition hopper’s robustness (due to planar geometry) allows the design to handle varying properties of gypsum, whereas a conical hopper geometry would have been much more sensitive to product variations and would have taken up more headroom.

Since the modified bin has been in operation, substantial time savings and product quality improvements have been realized because operators are not required to use brute force methods (e.g., hammers, lances, rods, etc.) in an attempt to initiate flow. Furthermore, based upon the success of this gypsum bin, Lafarge has already started retrofitting other troublesome bins at their plant, which handles cement raw materials.