A continuous commitment to research and development, both internally and through outside research institutes, has seen a number of breakthroughs over the years in anode performance and longevity.
Tin / Calcium / Lead Anodes
Pure lead cannot support its own weight and will creep when in use. This creep will break the lead oxide layer on the anode surface causing exposure of fresh lead to oxidisation and corrosion.
Alloying of the lead increases the mechanical strength during use preventing creep and thus the cracking of the oxidised surface layer and maintains the anode shape. Lead antimony, lead tin antimony and lead silver were used for many years in copper and zinc electrowinning as alloys to provide the desired mechanical properties.
With the advent of solvent extraction, electrowinning systems to produce high purity cathode deposits, the need arose for much lower lead levels in the cathode deposit and alloys with calcium were developed to reduce the level of lead deposit.
Calcium lead alloys are relatively weak and tin is added to provide increased mechanical strength and to decrease the creep. Tin addition also reduces passivation when the power is interrupted.
Rolled alloys used for anodes are superior to cast anodes as the rolling process changes the cast grain structure to produce fine uniform directional grains and reduces the corrosion rate. The rolled anode is produced from large cast slabs which cool sufficiently slowly so that all the included air and dross generated during the pouring process are removed prior to solidification before rolling. Thus there are no oxide, air inclusions or laminations.
The rolled anode has a much finer and more uniform grain structure than cast anodes. Corrosion occurs as a fine, uniform, thin layer.
Rolled vs Cast Microstructure
Figure 1: Microstructure of cast material, approximately 110x
Figure 2: Microstructure of a rolled material, approximately 110x
Figure 3: Details of a rolled microstructure showing the fine grain size, approximately 300x.