Belt Conveying in Building Materials Plants

Belt conveying moves the bulk of the tonnage in a quarry, a cement works or an aggregate plant, and it does it over long outdoor routes at high rates. Three costs dominate and none of them is the belt.

The first is spillage. At these tonnages a small percentage escaping at loading points and along the carry side is tonnes a day on the ground, and somebody has to move it. It also wears the return idlers, loads the structure and builds up where it cannot be cleaned.

The second is dust. Transfer points liberate fines, and in a cement or aggregate plant those fines include respirable crystalline silica, which carries an occupational exposure limit. A plant that relies on general ventilation in an open structure is not controlling it.

The third is weather. The route is outdoors: rain makes material stick and belts slip, wind blows dust off the carry side, and winter brings frozen build up on pulleys and in chutes.

The general case, where a belt conveyor is chosen against tonnage and lift, says little about the abrasion and the spillage that decide the design here.

Loading points and skirting arrangements that keep material on the belt, so spillage is a designed out condition rather than a daily labour cost.

Dust captured where it is generated at transfer points, with capture velocity at the opening as the design criterion and the respirable fraction measured where silica is present.

A route detailed for the weather it will see: covers where wind matters, drainage where water collects, and heating or scraping arrangements where freezing is credible.

In food production the belt itself is rarely the problem: a food grade belt mounted on a frame of closed box sections leaves a cleaning problem the belt specification never mentions.

Spillage is attacked at the loading point first, because that is where most of it starts. A chute that delivers material centrally, at belt speed, in the direction of travel; skirts of adequate length with the correct clearance and a sealing arrangement that can be adjusted as it wears; and an impact bed to stabilise the belt line through the loading zone.

Belt cleaning then deals with what sticks. Primary and secondary scrapers at the head pulley, tensioned and maintained, because a scraper that is not adjusted is a wear item doing nothing. Carryback on the return side is the second largest source of spillage and build up.

Dust control is designed at the transfer points as enclosure plus extraction, or as a dust suppression arrangement where water is acceptable. Capture at the opening is what matters; a large fan on a leaky hood moves air and not dust.

Weather is then designed for explicitly. Covers over the carry side where wind is a factor, drainage out of chutes and enclosures, and attention to what happens on the first cold morning: frozen build up on a pulley is how belts get damaged.

• Loading chute delivering centrally, at belt speed, in the direction of travel.
• Skirts of adequate length with adjustable sealing and an impact bed through the loading zone.
• Primary and secondary belt scrapers, tensioned and maintained, to control carryback.
• Dust captured at transfer points, with capture velocity at the opening as the criterion.
• Respirable silica fraction measured where it is present, with exposure limits applied.
• Covers, drainage and freezing protection designed for an outdoor route.

• Tonnage, which turns a small spillage percentage into a daily labour cost.
• Abrasiveness and top lump size, which set belt grade, liners and impact protection.
• Crystalline silica content, which makes dust control an exposure duty.
• Weather exposure along the route, including wind, rain and winter.
• Carryback behaviour of the material, which decides the scraper arrangement.
• Access for cleaning and maintenance, especially along elevated runs.