Minggu, 28 Februari 2016

Types of Nickel Laterite Deposits

Nickel laterite deposits that are exploited for the extraction of nickel, cobalt and related metals generally fall into three distinct types as described below. At places, all three types of deposits may be associated with a single weathering profile occupying different horizons. At most locations, a saprolite zone may underlie an oxide zone.

Oxide Deposits

Consisting essentially of sesquioxides of iron and to a less extent of aluminium, oxide nickel laterite deposits occur at the top portion of the weathering profile in the ferruginous zone. Nickel occurs mostly in the goethite/limonite structure where it generally replaces the Fe. Some nickel may be present associated with asbolane and lithiphorite (manganese hydroxides). Nickel enrichment in the oxide deposits is generally less than in other types.

Upgrading may be possible in oxide nickel ores through screening out of a distinct low-grade fraction (such as barren silica). Such protocol is used Ravensthorpe deposit of BHP-B and is proposed for the Vermelho project in Brazil.

Due to the very high iron content, oxide nickel deposits are not suited to pyrometallurgical extraction. At the QNI facility at Yabulu, Australia, oxide ores are first calcined and then subjected to ammonia leaching. Nickel and cobalt are precipitated as mixed hydroxides. In Cuba and at Cawse, oxide ores are directly leached by sulphuric acid at high temperatures and pressures. The Goro orebody in New Caledonia has a thick oxide profile overlying a saprolite zone.
Types of Nickel Laterite Deposits
Nickel laterite deposits.

Clay Deposits

In some nickel laterite deposits nickel is tied in the smectite-nontronite clays. Such deposits are generally associated with poorly drained terrains and wet/dry climate. Such horizons generally lie below the ferruginous zone and above the saprolite zone. Potential for beneficiation through simple screening is generally limited. Smectite nickel deposits are generally exploited as part of the oxide or saprolite nickel mining. Nickel enrichment in smectite deposits is generally in between that of oxide and saprolite deposits. Examples of smectite-nontronite nickel deposits include Bulong and Murrin Murrin in West Australia.

Saprolite (Silicate or Garnierite) Deposits

These deposits are generally associated with freely drained terrains, commonly with an oxide deposit developed at the top of the weathering profile. In saprolite or silicate nickel deposits, nickel is in the form of nickeliferous serpentine, nickeliferous talc and nickeliferous chlorite. In these silicates, nickel replaces Mg as the latter is leached out of the original minerals. Silicate deposits yield some of the highest nickel grades in the world.

Potential for beneficiation in nickel silicate ores lies essentially in the screening out of the fresh and unaltered bedrock or zones of massive silica. Significant upgrading can be achieved by simple screening. Due to high magnesia content, silicate nickel ores are not ideally suited to acid leaching and nickel is generally extracted through pyrometallurgical treatment. Examples of silicate nickel deposits include several locations in New Caledonia (including Goro), and Sorowako, Bahodopi and Pomalaa in Sulawesi, Indonesia.