A recycling plant is a washing plant in disguise
A plastics recycling line receives baled material, sorts it, shreds it, washes it, dries it, and extrudes it into pellets. The process moves from dry and dusty to soaking wet to hot, and the building has to hold all three states.
Sorting is dusty and manual
The sorting stage is where people pick material off a conveyor. It needs light, ventilation and reasonable temperature, because people work there for a full shift.
It also produces dust and debris from the bales, which settles on every surface. The sorting hall is the part of the building most concerned with human comfort and least concerned with corrosion.
Washing is a wet room with a lot of water
The wash line uses hot water, sometimes with caustic or detergent. It is a continuously wet process: the material enters dirty and leaves wet, and the machinery sprays and leaks.
The building around a wash line is essentially an industrial wet room. It needs falls in the slab, gullies sized for the flow, protection at every column base, and a wall detail that survives splash to a metre or so above the floor.
It also needs an effluent route. Wash water from recycling carries labels, dirt and process chemicals, and its disposal is a site design question that arrives back at the building as a level problem.
Drying and extrusion are hot
Drying removes the surface water, and extrusion melts the material. Both stages release heat into the hall, and extrusion releases volatile compounds that have to be extracted.
The extraction is not optional: the vapours are unpleasant at low concentration and harmful at higher ones, and they condense on cool surfaces as an oily film that then collects dust.
The three zones need walls or at least boundaries
An open shed containing a dry sorting area, a wet wash line and a hot extrusion line is a building that is humid where it should be dry and dusty where it should be clean.
Walls between the zones cost little at design stage. A curtain or a partition will do for some boundaries, but the wet-to-dry boundary usually wants a wall.
Fire load is the bales and the product
Baled plastic is a significant fire load and it burns hot. Storing bales and finished pellets in the same open volume as the process is how recycling plants are lost.
A separate store, or at least a fire wall, is the mitigation. It is also worth noting that a plastics fire produces smoke that damages the neighbourhood, so the separation has an external dimension too.
What I want before drawings
- The process stages and where the dry, wet and hot zones begin and end.
- The wash water volume, temperature and effluent route.
- The extraction required at the extruder, and who is designing it.
- The bale and pellet storage volumes.
- The sorting hall's occupancy and comfort requirements.
- The wind speed for the site in km/h from the applicable local code, not a round number.
Recycling plants are usually built to a budget and on a short programme. That makes the zone boundaries and the effluent route the two things worth insisting on, because both are cheap now and disruptive later.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: a wet process producing a dry product · the floor as the primary design element · dust from a dry process · corrosion class as a selection input
