Data on Indigenous Foundry sands and bonding clays

Abstract

Sands for foundry moulding purposes can be broadly divided into the following groups: 1.Naturally occurring moulding sands containing 8 to 20 % of clay grade matter. 2.Crude silica sands containing 1 to 3 % of clay grade matter and sometimes even up to 5% in the unwashed state. 3.Washed and dried silica sands containing more than 96% silica, the whole of the silica being derived from the presence of quartz. 4.Special sands such as zircon, olivine. Group I constitutes the main bulk of sands used for non-ferrous and iron castings. These are usually prepared by crushing and milling soft yellow sand-stones, carboniferous rotten rocks etc., where most of the clay minerals are usually present as aggregates interstitial to the quartz grains and these break down and get themselves uniformly distributed over the grains during milling sand. It is rather difficult to lay a precise specification for moulding sands in general, as the requirements of these sands vary widely from foundry to foundry, depending upon the nature of the castings produced. However, there are some basic characteristics which can be fairly expected from sands of this group and which may be profitably kept in view for assessing the usefulness for foundry moulding purposes. As for example, it is preferable to have as low an amount as possible of non-bonding minerals like quartz, muscovite, siderite, calcite etc., which tend to lower the bonding capacity of the clay minerals. The best practical capacity of clay minerals associated with natural moulding sand will be to determine the moulding characteristics particularly the strength values at different moisture contents. Besides, refractories, grain size , shape and the distribution are other factors which will determine the usefulness of the sand for iron and non-ferrous foundry work.