HH62/2/LINLIT/25

Transcription

[Page] 24

POLLUTION FROM IRONSTONE MINING.

There are two classes of water from ironstone
workings:-
(1) Water pumped from the mine, which comes
up either almost perfectly clear, as at
Addie's No. 3 Pit in Lanarkshire, or with
a muddy like appearance, as at Crofthead
Pit, where it has already, before pumping,
been subjected to conditions favouring
precipitation of some of its iron which
appears in suspension.
(2) Refuse bing soakage, which is not very large
in quantity, but extremely rich in iron
salts, and very acid in reaction.

The first class (1), or pumped water, is highly
charged with ferrous salts. On exposure to air and
motion, iron is precipitated partly as a carbonate and
partly as a hydrated oxide, the water assumes an
ochry hue, and the irony deposit coats the stones and
bottom of the river all along its course. The water
of the other class (2), or bing soakage, is clear, and
of a rich mahogany tint, and holds its iron almost
wholly in solution, and mainly as ferric salts.

I made a number of experiments with these waters
to find some easy method of purifying them.

The pumped water, clear on coming out of the pit,
became opaque in an hour. On standing 48 hours, a
quantity of its iron was deposited, but it still re-
mained somewhat opaque, and distinctly irony. On
standing a week, the water became perfectly clear,
but iron in solution was in considerable quantity.
On standing two weeks, there still remained a large
amount of iron in solution.

[Page] 25

The bing water underwent no change on standing,
as was to be expected.

By adding 2 drachms of freshly slaked lime to 1
gallon of the pumped water which had stood for 48
hours, and shaking, a complete precipitation was
effected in less than 10 minutes, leaving a clear water,
free of even the faintest trace of iron. A larger
quantity than 2 drachms was required 24 hours earlier.

The addition of 10 drachms to 1 gallon of the bing
soakage, and shaking, effected as complete purification
and decolourisation in the same time as 2 drachms did
with the other.

A point of great practical importance is this, that
unless lime is added in sufficient quantity to cause
marked alkalinity, the precipitate is slow in falling;
but if it be added in sufficient quantity, the water in
a few minutes is rendered absolutely free of iron.

Another point of equal importance is the condition
in which the lime is when added. If freshly burned
limestone be used, even in a comparatively fine state
of division, a quantity equal to three times the weight
of the freshly slaked lime is required to give as satis-
factory results in the same space of time. The reason
of this is that the presence of too much water pre-
vents the lime assuming that peculiar condition
known as slaked lime, in which the stone falls to an
almost impalpable powder, exposing an enormous
surface to act quickly in producing alkalinity. Care,
therefore, in properly slaking is a desideratum.
Agitating at the moment when added is, of course,
desirable. In working with large quantities of water,
perhaps a good plan might be to add milk of lime,
which is simply properly slaked lime well stirred
with water; or to have recourse to some method of