Saturday 24th Jul, 2021

Weigh batching controls compensate for terra cotta variations

Flexicon’s bulk bag dischargers and flexible screw conveyors are blending clay and grog to create the ideal mix for terra cotta.

Flexicon’s bulk bag dischargers and flexible screw conveyors are blending clay and grog to create the ideal mix for terra cotta.

Gladding, McBean is an American manufacturer of terra cotta products with a century long history. The company mines clay from its own reserves and combines traditional methods with modern technology to create roof tiles, floor and paving tiles, clay pipe, ornamental pieces, and architectural elements for buildings.

To streamline batching and mixing of clay blends for different products, the company installed five bulk bag dischargers and five pairs of flexible screw conveyors from Flexicon to automatically deliver weighments of bulk ingredients for blending of clay compounds. The system cut dispensing and weighing time for each batch by half and reduced out-of-spec material and scrap by 95 per cent.

Compensating for raw material variations

Clay is a natural material that varies in composition, so the company must determine the ideal ratio of ingredients for various products.

Joe Parker, Gladding, McBean Operations Manager, says the company has to ensure what it creates in the lab will translate into the final material

“The only way to accomplish that is to maintain precise control of production,” Parker says.

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Clay is sourced from the company’s nearby mine, classified, and loaded into bulk bags at the plant. Crushed, recycled ceramic material called grog is the other major component used in the manufacturing of terra cotta.

Previously, operators retrieved clays and grogs from bins, weighed them on a scale, and transferred them to the mixer using an open trough conveyor.

The new batching system, supplied by Flexicon, integrates bulk bag dischargers, flexible screw conveyors and a central weigh hopper, all of which are actuated by programmable controls.

The system allows Gladding, McBean to vary bulk bag discharging, conveying, weighing, and mixing on a batch-by-batch basis according to recipes developed in the lab. A human machine interface (HMI) includes options for automatic or hand-mode operation, as well as setpoints, adjustments, status, start/stop, completion and other parameters.

Once the recipe is programmed for a batch, each ingredient is conveyed by a flexible screw conveyor from a bulk bag discharger to a central weigh hopper. Load cells supporting the hopper transmit weight gain amounts to the controller, which steps-down the conveyor’s feed rate to dribble before stopping it once the precise batch weight has been gained. The system weighs up to 30 batches per day, with improved accuracy and reduced labour.

How clays and grogs are batched and blended

Five, forklift-loaded BULK-OUT BFF model bulk bag dischargers stand side by side. An operator attaches a bulk bag to the steel lifting frame by sliding the bag straps into four Z-Clip strap holders. A forklift raises the lifting frame with the 909-kilogram bag of clay or grog intact, into four self-centring cradles atop the discharger frame.

With the bag in place, the operator opens a 305-millimetre-diameter iris valve, pulls the bag spout through it, and closes the valve before untying the spout. The operator can then open the iris valve gradually to prevent bursts of material into the hopper, and displaced dust from escaping the enclosed system.

FLOW-FLEXER massaging devices on each discharger raise and lower opposite edges of the bag at timed intervals to promote flow. If more flow is required, each floor hopper is equipped with a pneumatic turbine vibrator.

The clay or grog is transferred to the 1133-litre capacity central weigh hopper by flexible screw conveyors in two stages to accommodate the plant’s limited floor area and headroom. Each conveyor extends either 4.6 or 6.1 metres, depending on the position of the discharger and the need to route the conveyors under the roof trusses or between the truss components.

Each conveyor consists of a polymer tube with an inner flexible screw, which is driven above the discharge point, avoiding material contact with seals.

Material moves through the transition discharge adapter of the lower conveyor into the inlet of the upper conveyor. Custom-engineered masts support the top end of the first conveyor and the bottom end of the upper conveyor.

The upper conveyor gravity discharges into the weigh hopper, which is 1.2 metres in diameter and suspended 5.7 metres above the floor.

Parker says most batches of clay mix weigh 454 kilograms or 341 kilograms, with a typical loading/blending cycle requiring 10 to 15 minutes. After blending with water, the batch is delivered to the appropriate process elsewhere in the plant.

“The system allows us to blend up to five different types of clays or grogs,” Parker says. For example, the dischargers might handle bulk bags of two kinds of clay and three grogs, delivering the correct amount of each for a given batch.

Weighing in on batching performance

The versatility of the system makes it cost efficient,” says Egidio Modolo, plant manager. “It’s a simple, straightforward process, and an efficient way to measure and transfer clay to the blender.”

He says that no maintenance was required in the six months since installation, and that throughout discharging, conveying, and mixing the enclosed system prevented dusting.

“We can make a small or large batch, and alter the recipe and raw materials,” Parker says. “Currently we have two different clays and three different grogs. We can change that any time.”

“It gives us the ability to be accurate in spite of clay’s unpredictability. Now we have a tool for our team in the lab to create a mix that will allow us to be very precise with our final product specifications.”

Microscopically, clay is very abrasive and can destroy just about any equipment used over time. Parker says one of the reasons the company chose the system was for simplicity of maintenance due to few moving parts, minimising downtime.

With the system in place, Parker says it will open up possibilities for other types of products and materials that need the same type of batching accuracy.