Glass wool

The raw materials for glass wool manufacture are mainly delivered by road tankers and pneumatically conveyed into storage hoppers. Each process will use a range of raw materials and the precise formulation of the batch may vary considerably between processes. The basic materials for glass wool manufacture include sand, soda ash, dolomite, limestone, sodium sulphate, sodium nitrate, and minerals containing boron and alumina.

Most processes also use process cullet as a raw material. This is shattered glass, which has been produced by quenching the molten stream from the furnace in water when the fiberising operation has been interrupted. Process cullet has the same precise formulation as the final product, and is readily recycled back to the furnace. Other forms of glass cullet, e.g. container glass and soda-lime-silica flat glass are also extensively used as a feedstock. This type of material is more difficult to recycle and its use depends heavily on cost, composition, purity and consistency of supply. One limiting factor in the use of cullet as a raw material is represented by glass-ceramics. As for other types of glass, the presence of glass-ceramics in recycled cullet is becoming an increasing problem. Several manufacturers also recycle processed fibrous waste and the dust collected from the furnace waste gas stream to the melter.

The fibrous nature of much of the waste makes it impracticable to recycle without further treatment. Glass furnace raw materials are charged as powders or in granular form and so waste material must be ground or pelletised before charging. This is usually achieved by some form of milling operation. The waste product and the filtered waste contain significant levels of organic binder. In a glass furnace, the carbon content of the waste presents a number of potential problems including: reduced heat transfer; foaming; destabilisation of melting conditions; and alteration of the furnace chemistry. These problems can be mitigated but there is a limit to the amount of waste that can be recycled back to the furnace. Furthermore, it can be necessary to add sodium or potassium nitrate as an oxidising agent, and the decomposition of these materials can add significantly to the emissions of nitrogen oxides.

The various raw materials are automatically weighed out and blended to produce a precisely formulated batch. The blended batch is then transferred to an intermediate storage hopper before it is added to the furnace.

The furnace (with a few rare exceptions) will either be an electrically-heated furnace, a traditional gas-fired recuperative furnace, or less commonly an oxy-gas furnace. These techniques are described in Section 2.3 above.

A stream of molten glass flows from the furnace along a heated refractory-lined forehearth and pours through a number (usually one to ten) of single orifice bushings into specially designed rotary centrifugal spinners. Primary fiberising takes place by means of centrifugal action of the rotating spinner with further attenuation by hot flame gases from a circular burner. This forms a veil of fibres with a range of lengths and diameters randomly interlaced. The veil passes through a ring of binder sprays that release a solution of phenolic resin-based binder and mineral oil onto the fibres to provide integrity, resilience, durability and handling quality to the finished product.

The binder is highly diluted with water to enable it to adequately coat the fibres which have a very high surface area. The water acts as a carrier for the binder and is then evaporated.

The resin-coated fibre is drawn under suction onto a moving conveyor to form a mattress of fibres. This mattress passes through a gas-fired oven at approximately 250 C, which dries the product and cures the binder. The product is then air-cooled and cut to size before packaging.

Edge trims can be granulated and blown back into the fibre veil, or they can be combined with the surplus product to form a loose wool product. Some products are produced without oven curing, e.g. microwave cured, hot pressed, uncured or binder-free products. Also, certain laminated products are made by the application of a coating, e.g. aluminium foil or glass tissue which is applied on-line with an adhesive.

Water is sprayed into much of the downstream process ducting to prevent the build-up of fibre and resinous material, which could cause fires or blockage; and to remove entrained material from the flue-gas. Water is also used for cleaning the collection belt and other parts of the plant.

The process water system is generally a closed loop; it is collected, filtered and reused for duct sprays, cleaning water and binder dilution. A typical glass wool process water circuit is shown in Figure 2.11 below. A significant portion of water evaporates from the following production operations: binder spraying, waste gas scrubbing, cooling and equipment cleaning.

Sanitary water Internal treatment

plant Filtration plant Collection

reservoir

Collection reservoir (pit) Process wash water:

-Fume scrubbing/duct sprays -Equipment cleaning

Cooling water

Cullet quenching water Binder plant

Wet-fibrous

scrap Clean water blow-down

Clean water blow-down

Clean or polluted sanitary water Internal

treatment plant

Clean water blow-down Evaporation

Natural sources

City mains

Forming Curing

Figure 2.11 Typical glass wool process water circuit

A global water balance for a typical glass wool plant in normal operation gives a consumption of 3 to 5 m³ of water per tonne of wool produced (see also Section 3.8.3). Almost all of this water leaves the plant as steam or gas-borne water droplets, either through the stacks or through general evaporation.

However, water is constantly recirculated within the process wash water system so that the internal flow of water actually used in the glass wool process is much higher and may reach up to 100 m³/tonne of glass. The majority of this water flow (typically 70 %) is used in the forming sections and their associated pollution control equipment.

This process wash water contains dissolved organics and solids (mainly fibres). Undissolved solids are removed in a plant by using cyclones, fixed or vibrating screen filters, centrifugal filters or similar equipment. In order to prevent an over-concentration of the dissolved organics, a proportion of water is abstracted from the process wash water, refiltered and introduced to the binder mix to be combined with the product. By this means, an equilibrium of dissolved solid content is established for a given binder formulation and product binder content.

The characteristics of wash water are periodically monitored, particularly because the efficiency of flue-gas scrubbing depends upon the concentration of dissolved solids; variations can be important, depending on such parameters as the formulation and quantity of binder used and the weather/season of the year.

For other water uses, treatment systems such as air cooling, reverse osmosis, ion exchange and de-oiling are applied.

Process effluents arising from binder plant cleaning, tank farm bunds or secondary cleaning operations may be recycled internally into the wash water system or settled and treated before discharge to a sewer depending upon local arrangements. Often there is no effluent discharge from a facility except under agreed emergency conditions, or there is discharge to a foul sewer according to permitted conditions. The typical maximum emission is 50 tonnes per day of water.

(see also Section 3.8.3).

A range of secondary products can be formed from manufactured glass wool. These include granulated insulation wool for blown installation, packaged uncured wool for supply to customers for further processing, and laminated or faced products. Pipe insulation is a significant secondary product usually manufactured by diverting uncured wool from the main process for press moulding and curing. Alternatively, the wool may be wound onto retractable heated mandrels to form the bore, and heat processed to form the outer wall before transfer to an overall curing stage.

The binder is prepared by mixing the partially polymerised resin with certain additives that improve application efficiency, promote resin adhesion to the wool, suppress dust formation, confer water resistance and assist binder dilution. The binder is diluted with a substantial amount of water (process water, where available) prior to application in the veil.

The most commonly used resin is a thermoset product of phenol, formaldehyde and a catalyst.

The resin is water-based and typically contains up to 50 % solids. A more detailed description of the binder chemistry is given in Section 4.5.6.1. Resin may be imported from specialist manufacturers or may be made on site by the mineral wool manufacturer. On-site resin production usually consists of a batch process where the raw materials are reacted under thermal control to give the desired degree of polymerisation and solids. Resin manufacture is considered a chemical process and is not covered in this document.