Harvesting the sea

Is it quarrying or mining? There are elements of both in the sea-salt harvesting operation at Grassmere in Marlborough.    HUGH DE LACY REPORTS

Gavin Williams has to scratch his head when he’s asked if the operation he manages for Dominion Salt can really be described as mining.

The half-century old Dominion Saltworks centred on Lake Grassmere 25 kilometres south-east of Blenheim on SH1 certainly harvest a valuable mineral, but since the wind – and to a lesser extent the sun - is the core machinery involved, Williams struggles to find a more apt description for it than harvesting. But you couldn’t label it manufacturing either, and there are certainly elements of both mining and quarrying at Grassmere, where more than 1.6 million tonnes of salt has been recovered from the sea since 1953.

And like a mining mother-lode that may exist in one place and no other, Grassmere is unique in New Zealand in that there’s nowhere else with the climatic conditions to make the large-scale harvesting of sea-salt viable.

Grassmere, where Williams manages a permanent staff of 42, is a 1782 hectare shallow saline lake on a flat surface of impervious silt that is battered through much of the year by the nor’west winds funnelled down through Cook Strait. The winds combine with low rainfall and high sunshine hours and temperatures to create an environment which, at 42 degree south, is the world’s highest latitude for a sea-salt operation.

Dominion Salt’s Grassmere works produces around 60,000 tonnes of salt a year against a national consumption of 120,000. Most of the rest of the nation’s requirements are met by imports of Australian rock-salt (Solar salt) that are processed at the company’s other plant at Mount Maunganui, but nearly 10,000 tonnes of Grassmere industrial salt is exported annually to Australia, the Pacific islands and South-east Asia.

The idea of producing sea-salt by evaporation from a site at such a high latitude was first canvassed during World War Two by one of new Zealand’s most prominent early industrialists, George Skellerup, who emigrated to New Zealand from Australia in 1902. Skellerup had worked for the Dunlop Pneumatic Tyre Company in Melbourne before coming to New Zealand and founding the rubber products company that bore his name. It has since gone through several corporate evolutions, but continues today as the Para Rubber Company.

Skellerup was keen to address the rubber shortage here during the war by re-cycling old tyres, but he needed salt to make the necessary caustic soda. He quickly saw possibilities at Lake Grassmere, a sea-side mud-bath in winter and a wind-blown desert-scape in summer with an annual rainfall of about 610mm, of which only around 250mm falls over the summer months.

The worst winter floods in 30 years initially frustrated Skellerup’s plans to start a sea-water evaporation system on 400 hectares on the north side of the lake, but the Coalition Government of the time saw merit in the scheme. It bought a share of Skellerup Solar Salt Ltd, and the name was changed to Dominion Salt.

However, it was not until 1953 that the first 45 tonnes of salt was scraped up from the lake-bed by shovel, purified in brine in a concrete mixer, then crystallised, packaged and sold. By trial and error the system was refined over subsequent years by the addition of further evaporation ponds. Production vaulted to a record 116,500 tonnes in 1973, though a nil harvest in 1986 showed the process is still ultimately subject to the vagaries of the weather.

Today, the operation begins with sea-water being pumped at the rate of 40 tonnes a minute throughout the summer into the 688 hectare main lake where initial evaporation increases the salt concentration from 2.8 percent to five percent. The brine then enters a further 10 concentrating ponds covering 486 hectare, and on to five final ponds, totalling 81hectares,  which bring it up to saturation strength - the point at which the salt starts crystallising.  It’s then fed into 22 crystallising ponds, covering 93 hectares where the salt is deposited on the bottom as a crust that varies in depth from 25mm to 76mm, depending on the amount of summer rain, wind and sun.

The Grassmere site also includes four winter storage ponds that are between three metres and five metres deep and cover 20 hectares. There are also 40 hectares of re-concentrating ponds whose job is to get rain-diluted brine back to saturation strength. A further eight hectares of ponds is used for washing the salt during harvest, and for re-washing stockpiled salt during the refining process. The brine in the ponds is notable for its pink coloration that is caused by the algae Dunaliella Salina.

The actual salt-making season, which lasts just six months, starts in early October when the crystallising ponds are filled to a depth of 350mm from the deep winter storage ponds. Over the summer months 508mm of water evaporates from the saturated brine, and in March or April the crystallising ponds are drained, with the spent brine, called bitterns, being pumped back into the sea.

The crust of salt left behind on the surface of the crystallising ponds is harvested at the rate of 4000-6000 tonnes a day by a 12-tonne purpose-built machine that looks a bit like a bulldozer going backwards. It runs on tracks to spread the weight and prevent it breaking through the crust into the silt below. It has a driver up the front and another operator at the back who controls the cutting depth of the three metre-wide chain-driven tines that peel back the layer of crusted salt. The tines can be moved up or down, depending on the thickness of the crust, or angled horizontally to compensate for loading on the discharge side. As the harvester moves along, a  flighted chain (or bucket elevator) discharges the chunks of salt crust onto four, self-propelled conveyors that take salt to the trucks, which cart them to the washery.

Grassmere operates two harvesting machines. The larger one was built to Dominion Salt’s design in 1982 by J. Johnson and Son of Invercargill. A lighter one of about eight tonne works the edges of the crystallising ponds and was built in 1962 by Burns and Twigg of Rockhampton, Queensland. The conveyors are mounted on Kobelco excavator track units and were purpose-built in the early 1980s by various companies and suppliers of which Cuddons Engineering in Blenheim was the principal.

It may seem remarkable that old machines should continue to operate in such a saline and windy environment. While the maintenance is certainly “significant”, says Williams, an effective maintenance routine prior to each harvesting season has been developed over decades of operation that includes plenty of quality paint. Every five to seven years the machines are sand-blasted and re-painted.

After being washed and screened through one of two washery plants, the salt is stacked in 20 metre piles that can contain as much as 100,000 tonne and are a landmark to motorists on SH1. Dominion Salt doesn’t have its own fleet of trucks, but instead contracts that part of the operation out. Over the four-to-six weeks of harvest, when casual labour swells the salt company’s workforce to about 55, a dozen trucks work 11 hrs per day, five days a week.

Once stockpiled, the harvested salt provides year-round work for the other four main items of plant at Grassmere. The largest of these is the main solar refinery which takes the salt from the stacks through a washing, crushing, drying and screening process that produces a range of particle sizes. A second plant serves the retail packing and salt-shaker market, while a third makes blocks for livestock salt-licks. Finally there is the vacuum refinery and crystallising evaporator which produces highly purified and fine-particle salt for cheese and butter-making, baking and the salting of animal casings.

The biggest salt-users in the country are meatworks which take nearly 30 percent of Dominion Salt’s total Grassmere and Mount Maunganui production for the processing of hides and pelts, while the paper-making industry takes a further 26.5 percent. The remainder goes to a multiplicity of uses from bacon, smallgoods and confectionary manufacture to home-cooking, fertiliser, cleaning and pharmaceutical products.

About half the world’s supply of salt comes from underground deposits that are mined in pretty much the same way as coal and other minerals. The rest comes from systems like that at Grassmere.