Improvements in the Manufacture of Pearl Hardening.
Year: 1891

page_text: Date of Application, 6th May, 1891-Accepted, 13th June, 1891
COMPLETE SPECIFICATION.
Improvements in the Manufacture of Pearl Hardening.
I, JAMES BEVERIDGE of 4 Kent Road Gravesend in the County of Kent,
Technical Chemist. do hereby declare the nature of this invention and in what
manner the same is to be performed, to be particularly described and ascertained in
and by the following statement :-
My invention relates to the manufacture of crystallized sulphate of lime
technically known as "pearl hardening" and extensively used in the manufacture
of paper.
In the Specification of my former application for Letters Patent dated
8th August 1890 No. 12400 I described how sulphate of soda may be advantageously
employed in manufacturing pearl hardening.
My present invention relates to another mode of manufacturing pearl hardening
in which I employ sulphate of magnesia.
The sulphate of magnesia from which the ferruginous and other impurities have
as far as practicable been eliminated is added to an aqueous solution of bisulphite
of lime or a solution of sulphite of lime in aqueous sulphurous acid in such quantity
that nearly the whole of the sulphate of magnesia will be converted into sulphate of
lime having the formula CaSO4 + 2 H2O, while bisulphite of magnesia, which is
extensively used in the manufacture of wood pulp and for other purposes is obtained
as a by-product. After separating the sulphate of lime from the bisulphite of
magnesia I treat the former with aqueous sulphurous acid or other mineral acid or
acids in order to remove any normal sulphite of lime and subsequently wash with
water.
In carrying my invention into practice I first prepare an aqueous solution of
sulphate of magnesia, and this can be effected in the well-known manner by passing
diluted oil of vitriol either hot or cold through a series of tanks or other vessels
containing a native carbonate of magnesia such as magnesite, such tanks being so
arranged that the liquor flows freely from one to the other. The acid acts on the
carbonate of magnesia forming a solution of sulphate of magnesia containing more
or less free acid, and in order to neutralize the last traces of acid in the said liquor
flowing from the last tank of the series and also to precipitate or remove by
precipitation any iron which exists in solution and otherwise to free the said liquor
from all sedimentary matter I add to it a suitable alkali or alkaline earth in slight
excess, or in other words till the free acid is neutralized and other precipitable
matters have been precipitated. I find that carbonates of soda, lime or magnesia,
or hydrates of any of these bases, are very suitable compounds for this purpose.
The neutralized or slightly alkaline liquor after being diluted with water is allowed
to remain at rest in suitable vessels till the precipitated or suspended matter has
subsided, or in lieu of this settling process the somewhat muddy liquor may be
filtered in any of the well known ways. The clear sulphate of magnesia liquor is
now practically free from iron and other impurities.
In carrying my invention into practice I add to the purified sulphate of magnesia
liquor an aqueous solution of bisulphite of lime procured by any of the well known
methods in such quantity that nearly the whole of the sulphate of magnesia will be
converted into sulphate of lime having the formula CaSO, + 2 H2O while the
bisulphite of lime is converted into bisulphite of magnesia. The reaction which
takes place may be represented by the following equation viz.:-
CaSO,SO, + MgSO4 + 2 H2O = MgSO₂SO₂ + CaSO + 2 H₂O.
4
In practice I find that for every equivalent or 56 parts of dry lime (CaO) in
chemical union with sulphurous acid in the bisulphite liquor it is advisable to add

page_text: bisulphite of lime liquor the mixture is agitated' with a revolving agitator and if
the temperature of the charge does not register 100° Fahrenheit I prefer to heat it
to this temperature (although it is not absolutely necessary to do so) by injecting
steam into it. After the lapse of an hour or so a sample of the mixture is with-
drawn and filtered and the sulphate of magnesia and bisulphite of lime estimated in
a measured quantity of the filtrate in order to ascertain whether or not the right
proportion of bisulphite to sulphate liquor has been used. The amount of sulphate
of magnesia thus formed in the whole charge should be about from 5 to 10 per cent.
of the original amount operated upon. If the test shows a considerable variation
from this percentage a proportionate addition of the sulphate of magnesia solution
or of the bisulphite of lime liquor should be made and the agitation of the mixture
be then renewed for a while.
I have obtained good results when 100 gallons of purified sulphate of magnesia
liquor having a strength of 21 degrees of Twaddell's hydrometer at a temperature
of 60° Fahrenheit and containing about 10 lb. more or less of dry sulphate of
magnesia (MgSO) per gallon have been added to about 280 gallons more or less
of bisulphite of lime liquor having a strength of 10 degrees of Twaddell's hydro-
meter at 60° Fahrenheit and having substantially the following composition, viz.:
0.531 lb. of bisulphite of lime (CaSO₂SO,) per gallon together with a slight excess
of sulphurous acid (SO2) over and above that required to form bisulphite (CaSO₂SO2)
with the lime (CaO) usually occurring in bisulphite of lime liquors.
The mixture of crystallized sulphate of lime or "pearl hardening" and bisulphite
after the agitation has ceased, is allowed to remain at perfect rest till the white
insoluble sulphate has subsided. The clear supernatant liquor consisting substan-
tially of bisulphite of magnesia is syphoned off and filtered if necessary; it may be
used for a variety of purposes in paper mills such for example as for the manufacture
of wood pulp by a well known process.
The white magma remaining in the vessel is next washed by decantation with
successive quantities of fresh water to remove soluble bisulphites and then treated
with a weak and preferably warm aqueous solution of sulphurous acid of about
1° or 2° Twaddell's hydrometer in order to remove normal sulphite of lime (CaSO,)
which may have been precipitated along with the sulphate of lime. Or in lieu of
such treatment with warm aqueous sulphurous acid, hot water slightly acidified with
sulphuric, hydrochloric or nitric acid may be employed. The sulphate of lime is
now run off through a tap or plug hole in the bottom of the vessel on to a filter and
finally washed with water free from ferruginous and insoluble impurities until the
washings are free from sulphurous acid, or compounds thereof or other mineral acid.
The sulphate of lime or "pearl hardening" thus obtained is practically free from
iron compounds and is of a good white color. It is allowed to drain and further
dried in a hydro-extractor or by suction applied beneath the filter-bed, or it may be
mixed with water and pumped or otherwise forced into a receptacle in the beater
room of a paper mill for use when required.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that I do
not claim as part of this invention anything that is claimed in my Specification
No. 12400 of 1890 but what I claim is —
The manufacture of pearl hardening by adding together in approximately the
proportions hereinbefore specified a purified aqueous solution of sulphate of magnesia
and an aqueous solution of bisulphite of lime or a solution of sulphite of lime in an
aqueous solution of sulphurous acid and submitting the mixture to agitation with or
without the application of heat substantially as hereinbefore described.
J. C. MEWBURN,
55 & 56, Chancery Lane, London, Patent Agent.
J. C. MEWBURN,
55 & 56, Chancery Lane, London, Patent Agent.