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The owner of the glass stockpile that critics call "Mt Visy" will build a 120,000-tonne-capacity sorting plant to boost the rate of recycling.

Recycling company Visy announced the joint project with glass container-maker Owens-Illinois to build a glass-sorting plant next to Visy's $21 million Onehunga recycling facility.

Spokesman Lee Smith said the sorter, combined with a glass smelter Owens-Illinois is building, would increase the amount of glass that could be used to make new bottles.

The plant will be finished early next year and will use a beam of light across a conveyer belt to sort glass into three colours for recycling at Owens-Illinois' manufacturing plant at Penrose.

The Onehunga plant takes the contents of wheelie bins - which include glass, paper and cardboard and aluminium cans - from households in Auckland and Manukau and prepares them for recycling.

The new sorter will be able to recognise 17 million glass colours and is sensitive enough to tell a Heineken beer bottle from another green bottle.

One of the problems facing wheelie-bin collectors is that glass can be broken up into small pieces that are difficult to sort into colours.

So instead of being made into new bottles, they are often ground into a sand substitute and used in footpaths or roads.

Turning used glass into bottles is generally considered better for the environment because recycling uses less energy than making bottles from raw materials.

The glass industry has criticised Auckland's co-mingled recycling scheme because it says too much glass is broken up and contaminated during kerbside collection and sorting.

Yesterday, David Carter, chairman of the Glass Packaging Forum, said the new sorter would help.

But it was likely there would still be too little high-quality recycled glass in Auckland to service the new smelter, which is due to be built by the end of next year.

Mr Smith said the sorting equipment was from a specialist manufacturer in Europe and would be able to colour-sort 100,000 pieces of glass the size of a fingernail each minute.

It would separate glass into three main colours - white, brown and green.

The Visy plant has been under scrutiny this year after thousands of tonnes of material piled up unrecycled at its Onehunga site.

Mr Smith blamed the pile on more-enthusiastic-than-expected glass recycling efforts by Aucklanders.

He said the Onehunga pile would be re-sorted to remove paper, plastic and other material and any glass of sufficient quality would be made into new bottles.

A 12,00-tonne pile of glass next to the mixed pile could go straight to the new plant for colour sorting.