The Water-Atomizing Process: Part One

Sommario:

Water atomizing of metals is now a commercially important methodology to achieve fine particle distribution for a range of materials.
The general process is effective by impinging a falling stream of molten metal with jets of water which immediately solidify the metal into granules (>1mm) or powder (<1mm). Compared to less modern techniques like crushing and grinding water atomization presents a cost effective and efficient approach to producing metallic powders.

Atomization is the most commercially used process to produce the largest tonnage of metal powders. High-pressure water atomization has proven to be a viable, low-cost process to achieve fine particle size distributions for iron, stainless and low-alloy metal powders. The economic advantages and pre-alloying capability provide desirable advantages over competing technologies. Previous shortcomings relative to powder characteristics, i.e. irregular particle shape, lower tap densities, oxidized surfaces, have been refined to more closely replicate gas atomized powder properties.

In principle the technique is applicable to all metals that can be melted, and is commercially used for the production of iron, copper, including tool steels, alloy steels, brass, bronze and the low-melting-point metals, such as aluminum, tin, lead, zinc, cadmium.

Figure 1 shows water atomized iron and copper powder products.



Figure 1: Water Atomized Iron and copper powder products

If a falling stream of molten metal is impinged by jets of water, then it is broken up into droplets which rapidly freeze to form granules (>1 mm) or powder (<1 mm), depending on the composition of the metal alloy, and the water pressure. Classical granulation uses pressures in the range of 2–5 bars (200–500 kPa) and typically produces 1–10 mm size granules. Very large flow rates can be accommodated (in the t/min range), as pumping costs are modest.

To make finer powders, one needs higher pressures (we find median size is approximately inversely correlated with pressure), so it is necessary to use more controlled streams, typically from a tundish arrangement instead of a launder as in many granulators, and to run more modest flow rates, typically using nozzles from 3 mm to 30 mm in diameter to give flow rates from 5 kg/min to ~500 kg/min (up to 30 t/h). Pressures range from 20 bars for coarser powders (say 0.3 mm) to 200 bars for finer powders (say ~50 μm). Elements like sulphur in the melt strongly affect (reduce) the required pressure by reducing melt surface tension.

As shown in Figure 2, the atomized slurry can be pumped, either directly to leach tanks, or to a dewatering system which can deliver either a thickened slurry at ~20% moisture or a damp solid at ~5% moisture. This can then be fed to the leach tanks if the water balance is critical. Drying adds significant energy and cost, but may be done if, for instance, a smelter is selling to a refiner.

Water atomization has been used in the production of iron/steel powders since the 1950s, following its development in the 1930s for copper powders. Median particle sizes of ~50–100 μm are readily produced from high-purity Fe-0.1–0.3%C melts using pressures of ~100 bars (10 MPa). Today, about 80% of the world’s iron powder, some 1 Mt/a, is produced by water atomization, typically using ladle sizes from 30 to 110 t and atomizing speeds of 10–50 t/h.

Atomizing Systems Ltd (ASL) has supplied scores of atomizers for all sorts of metals, from copper and silver, to Ni, Co, Fe alloys, and has worked with Bateman Engineering Projects on the Chambishi project, which applies atomization to an Fe-Cu-Co alloy for hydrometallurgical refining. In total, 11 projects for atomizing in metal refining applications, including Ag, Au, Pt, Rh, Co, and Ni values, have been supplied. The size of particles requested by clients has ranged from <50 μm to sub-millimetre, and operating pressures have varied from 500 bar (50 MPa) to 20 bar (2 MPa).

Compared to crushing and grinding, atomization has an advantage in lacking any serious wearing parts; the water jets do the work and maintenance costs are modest. The cost of atomization is generally much less than the cost of melting, especially if drying of the product is not necessary, as is normally the case for leaching feeds.



Figure 2: Water atomization-schematic flow sheet

Typical specifications and size distribution of copper powder are given in Tables 1 and 2.

Composition
(by weight %)
Particle
shape
Apparent density
(g/cm3)
Oxygen
(ppm)
Surface
area
Min 99.9 Irregular spherical 2.7 < 800 low


Table 1: Properties of water atomized powder

+100 -100+150 -150+200 -200
Trace 5 10 85


Table 2: Particle size distribution % (Approximate)

Applications of Powdered Metals:

- Chemical Industries
- Sintering Industries
- Metallurgical Engineering
- Moulds Manufacturing
- Pigments Paste and Flakes
- Flames and Plasma Spraying
- Pyrotechnics
- Diamond Tools
- Refractory Industry
- Brake and Friction linings
- Soldering and Brazing Industry
- Metal Blasting
- Flux Cored Wire

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