7. Producing glass using soda ash

7.1  Overview

The manufacture of glass using soda ash involves crushing, blending, melting and reaction of materials which primarily comprise silica sand (SiO₂), soda ash (Na₂CO₃), limestone (CaCO₃) and recycled glass (cullet). Process emissions result mainly from the decomposition of soda ash (sodium carbonate) and calcium carbonate during melting of these raw materials. The SEIP regulations also include the possible use of magnesium carbonate in the form of dolomite.

7.2  Information you are required to collect

Quantities are likely to be determined by direct weighing, with sampling and analysis to determine the carbonate content. Weighing devices would be expected to be calibrated and traceable back to New Zealand national measurement standard, where possible. Accreditation to ISO17025 for the relevant test methodology is recommended.

7.3  Example calculation

Oamaru Glass Company Ltd manufactures glass bottles from raw materials and cullet. The following materials were used in its glass furnace during the reporting year: 3500 tonnes of soda ash, with a purity of 99 per cent; 3000 tonnes of limestone which also contains dolomite; and 10,000 tonnes of cullet. All materials were weighed using a calibrated weigh bridge. Fortnightly analysis of the limestone in the on-site ISO17025-certified laboratory showed the weighted average composition of the material used was 80 per cent of CaCO₃, 15 per cent of (Ca.Mg(CO₃)₂), and 5 per cent of non-carbonate impurities.

Using the formula provided in the regulations:
E = (3,465 × 0.4152) + (2,400 × 0.4397) + (450 × 0.4773)
E = 2709 tonnes CO₂-e