Improving Global Quality of Life

9.7.2 Welding in the minerals processing industry – Alumina case study
General process description
Alumina refining in general consists of the Bayer process, followed by calcination. In the Bayer process,
caustic soda is used to dissolve (digest) the aluminium oxides and hydroxides in the bauxite ore at an elevated
temperature of between 150 and 290 °C, the liquor is then separated from the sand and mud residue and
precipitated, at reduced temperature of about 70 °C, as pure aluminium hydroxide. This is then calcined
to convert the hydroxide to aluminium oxide, at a temperature of about 1,000 °C. A large number of heat
exchangers are used to heat the liquor and slurry using both steam and process vapour, and to recover heat.
General vessel description
The alumina refining process uses a large number of holding tanks, thickeners and washers at atmospheric
pressure, with tank diameters ranging up to 45 m. Precipitation tanks, also open to atmosphere, are typically
30 m high by 12 m diameter. Digestion vessels, and flash vessels used to flash down to atmospheric pressure,
can have design pressures of up to 5 MPa. Heat exchangers typically operate at similar pressures.
The materials of construction for most tanks, vessels and pipes are various grades of mild steel, except for
the calcination plant where high nickel superalloys are used, and the acid cleaning system (used to remove
sodalite scale from heat exchanger tubes) which is made from different grades of stainless steel. Vessel wall
thicknesses are usually 20 mm or less, but can range up to 40 mm at the bottom of high tanks.
Common welding procedures employed
Mild steel tanks, vessels and pipe spools are fabricated using conventional welding techniques and procedures.
There are no special requirements apart from compliance with the normal quality and equipment codes,
such as those for pressure vessels, piping, and tankage. There is one exception that post weld stress relief is
required to guard against caustic embrittlement, a form of stress corrosion cracking, which can occur under
certain process conditions.
Repair welding on site is usually done with MMAW and GMAW. There is also considerable interest in
productivity improvement techniques that could be employed to reduce turnaround time in vessel and tank
repair, eg FCAW. Some bauxite ore bodies contain large amounts of quartz, rendering the process slurries
highly abrasive. In these cases there is a need for selective hardfacing of some process equipment, and it is
common practice to use hardfacing consumable category 2560 for these. Tungsten carbide in a NiSiB matrix
(with or without Cr) is also used.
Issues, Needs and Challenges
With regard to caustic embrittlement, there are issues with the quality of field thermal stress relief, and with
the logistics as the codes prohibit the use of spot stress relief. If reliable alternative stress relief methods
were available, they would find application. There are issues if welding in the presence of caustic residue, as
this leads to cracking. There is a general interest in low stress welding procedures in the industry. As more
duplex stainless steels are becoming commercially viable, such as 2906, 2003 and 2101, there will be a need
for robust welding procedures for these alloys as there is potential application for these alloys in the alumina
industry due to their resistance to caustic. There are occasional issues with preferential weld corrosion.
122 Improving Global Quality of Life Through Optimum Use and Innovation of Welding and Joining Technologies