Phosphate Mining Offshore New Zealand
Authors: Grundlehner Gertjan , Castle Chris , Falconer Robin K.H. , Wood Ray 
Published: April 2012

Phosphate nodules are present at 400 m water depth on the Chatham Rise, 240nautical miles east of the South Island New Zealand, within the New ZealandEEZ. They occur as nodules of 1 mm to 150 mm size within a silt/sand layergenerally less than 70 cm thick. Their P2O5 content averages about 22%. Theycan be ground and applied as direct application fertilizer or used for superphosphate fertilizer manufacture.

Mining design studies have examined a range of options including seabedseparation of the nodules from the silty sand, self propelled crawlers, towedsleds and on site mining vessel to transport barge transfer. The preferredoption is trailer suction hopper dredging, with all the material being broughtto the surface. The drag head plus underwater pumps will be suspended fromwinches, some of which will be motion compensated. The riser system will be aflexible hose. Separation will take place onboard with the silt/sand tailingsreturned to the seabed. The mining vessel will mine for several days recovering30,000 tons of phosphate, then deliver it to New Zealand ports. Annualproduction of 1.5 million tons is planned. Current plans are for productionmining in 2014.

Existing data of 1200 samples has identified 25 million tons over 380 sqkm. Anextensive program of geotechnical sampling, additional resource detail andenvironmental studies is planned for early 2012. Detailed engineering designstudies are under-way. Details of this work will be presented.

When the Chatham Rise phosphate mining commences it is likely to be the deepestapplication of trailer suction hopper dredging and the deepest marineproduction mining to date.

Introduction

To date New Zealand supplies its fertilizer phosphate demand by import of rockphosphate which is then processed into super phosphate. Inside its EEZ however, on the Chatham Rise offshore eastern NZ, there are large submarine phosphatedeposits. Chatham Rock Phosphate Ltd (CRP) is now developing commercial miningof these deposits. CRP selected Royal Boskalis Westminster nv (Boskalis) todevelop the mining method further in close cooperation with themselves. Thispaper gives an overview of the project, selection of the mining method and howit is presently being developed towards actual operation

Phosphate deposits on the Chatham Rise

Phosphate deposits on the Chatham Rise, 240nm offshore eastern New Zealand, atwater depths of 400 m, may be the first deep shelf mineral deposits mined. Figure 1 shows the location. The phosphate occurs as nodules ranging in sizefrom 1 mm to 150 mm. The nodules are within a silt/sand layer, generally lessthan 50 cm thick, overlying an ooze/chalk layer. The nodules formed 7-11million years ago through replacement phosphatization of the calcium carbonateooze. The nodules average about 22% P2O5. They can be ground and applied asdirect application fertilizer or processed into super phosphate fertilizer. Economic concentrations of nodules are present over a 380 square km area, within which the average concentration is about 65,000 tons per square km, fora total of 25 million tons.