Wood preservative compositions and a process for their production

A copper arsenate composition suitable for incorporation in a copper/chrome/arsenic wood preservative composition is made by reacting copper metal with a source of 5-valent arsenic such as arsenic acid under the oxidizing influence of hydrogen peroxide. The quantity of arsenic acid used may be made consistent with the arsenic to copper ratio required in the final wood preservative composition which may be made up by including a suitable source of chromium with the copper arsenate reaction product.

BACKGROUND OF THE INVENTION 1.Field of the Invention 
This invention relates to wood preservative compositions containing copper 
arsenate and to a method for their production. 
2. Brief Description of the Prior Art 
Compositions containing, as active ingredients, compounds of copper, of 
chromium and of arsenic, hereafter called copper/chrome/arsenic 
compositions, are in widespread use as wood preservatives. Such 
compositions are required to be water-soluble and are generally made 
available as pastes, as slurries of solid and solution, or as concentrated 
solutions, each for dissolution and/or dilution in water before use. 
The British Standards Institution has published British Standard No. 4072 
relating to the composition of copper/chrome/arsenic compositions and 
their use in the treatment of wood. This Standard specifies two 
formulations identified as Type 1 and Type 2 having overall weight 
compositions as follows: 
______________________________________ 
Type 1 
Type 2 
______________________________________ 
Copper sulphate - expressed as 
32.6% 35.0% 
CuSO.sub.4 5H.sub.2 O 
Dichromate - expressed as 
41.0% 45.0% 
K.sub.2 Cr.sub.2 O.sub.7 or Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O 
Arsenic - expressed as As.sub.2 O.sub.5.2H.sub.2 O 
26.4% 20.0% 
______________________________________ 
Similar compositions have been Standardised by the American Wood Preservers 
Association as Types A, B and C having overall compsitions as follows: 
______________________________________ 
Type A Type B Type C 
______________________________________ 
Copper sulphate - expressed as 
18.1 19.6 18.5 
CuO 
Sodium dichromate - expressed 
65.5 35.3 47.5 
as CrO.sub.3 
Arsenic - expressed as As.sub.2 O.sub.5 
16.4 45.1 34.0 
______________________________________ 
The above Standards illustrate the range of copper:chromium:arsenic ratios 
commonly used for wood preservative purposes. The ingredients used to 
attain such ratios are varied somewhat subject to constraints arising from 
the avoidance or reduction of the content of undesired impurities, for 
example the sulphate and/or nitrate anions, in the compositions or arising 
from the need for water solubility in the composition at least at the 
final treatment concentration. 
U.S. Pat. No. 4,103,000 describes the production of copper/chrome/arsenic 
wood preservative compositions corresponding to Types A, B and C 
identified above from copper arsenate and chromic acid and also describes 
a process for the production of slurries of copper arsenate suitable for 
such use. The last mentioned process comprises reacting copper metal with 
arsenic acid at a temperature of 100.degree. C. in the presence of an 
oxidising agent and of an acid catalyst selected from the group consistng 
of nitric acid, hydrochloric acid and sulphuric acid. Oxygen is sparged 
into the reaction mixture, preferably under pressure, as the oxidising 
agent, although if nitric acid is selected as the catalyst oxygen need not 
essentially be used. 
SUMMARY OF THE INVENTION 
The present invention provides the production of copper arsenate by a new 
or improved process and the conversion of the copper arsenate so produced 
into copper/chrome/arsenic wood preservative compositions. 
A basis of the present invention is the selection of hydrogen peroxide as 
an oxidising agent together with the realisation that if this oxidising 
agent is utilised within suitably controlled process parameters a catalyst 
for the production of copper arsenate may be omitted. 
According to one aspect thereof the present invention therefore provides a 
process for the production of copper arsenate suitable for conversion into 
a copper/chrome/arsenic wood preservative composition by reacting copper 
metal with a source of 5-valent arsenic in an aqueous phse in the presence 
of an oxidiing agent characterised in that the oxidising agent consists 
essentially of hydrogen peroxide. In the practice of this invention 
contamination of wood preservative compositions made from copper arsenate 
so produced by acid anions derived from the catalysts described in U.S. 
Pat. No. 4,103,000 may be avoided. 
DETAILED DESCRIPTION OF THE INVENTION 
The copper metal utilised in the practice of this invention may be in any 
convenient particulate form which provides a sufficient surface area for 
reaction. The copper may be in powdered form although larger particles, 
for example turnings or even scrap copper, may alternatively be used. 
Preferably the copper is in reactive form and it may, advantageously, be 
mildly etched, for example with a dilute mineral acid/hydrogen peroxide 
mixture to ensure this. 
The source of 5-valent arsenic is preferably arsenic acid. Sodium arsenate 
may alternatively be used although the consequent inclusion of sodiuim 
ions into the composition may not be preferred. Arsenic trioxide may also, 
alternatively, be used. Suitably, the arsenic compound is used in the form 
of an aqueous solution or suspension, preferably an aqueous solution of at 
lest 50% by weight concentration but particularly peferably of at least 
65% by weight concentration for example, suitably, from about 75% to about 
80% by weight concentration and preferably not more than about 95% by 
weight concentration. 
The hydrogen peroxide utilised in the practice of this invention is 
suitably used as an aqueous solution having a concentration of about 30% 
by weight to about 85% by weight although it is preferred to use hydrogen 
peroxide having a concentration of from about 50% to about 75% by weight. 
The overall stoichiometry of the reaction between copper metal and arsenic 
cid in the presence of hydrogen peroxide may be stated by the formula 
EQU 3Cu+2H.sub.3 AsO.sub.4 +3H.sub.2 O.sub.2 .fwdarw.Cu.sub.3 (AsO.sub.4).sub.2 
+6H.sub.2 O 
Reference hereafter to stoichiometry is with reference to the quantities 
expressed in the above formula. The arsenic:copper ratio stated in the 
above equation corresponds to that required in the copper/chrome/arsenic 
composition Type A identified above. As it is seen from the other 
Standards quoted above, compositions containing more arsenic inrelation to 
copper may be required for certain timber preservation applications. 
Preferably, according to this invention, the entire content of arsenic 
required in the final wood preservative composition, or at least the 
majority of it, for example at least 75% of it, or more suitably at least 
90% of it, is supplied as the source of 5-valent arsenic specified above. 
The quantity of the source of 5-valent arsenic relative to the quantity of 
copper is preferably therefore from about 90% stoichiometric to about 500% 
stoichiometric or more i.e. up to about 1000% stoichiometryic if required. 
It has been found to be beneficial to conduct the reaction in an aqueous 
liquid medium having a substantial volume in relation to the quantity of 
the copper metal and the inclusion of the entire or substantially entire 
arsenic content of the desired wood preservative compostion in the 
reaction medium as above taught assists in this by increasing the volume 
of the reaction mixture without entailing undue dilution with water which 
would act to decrease reaction efficiency and also assists in pH control. 
Preferably the initial pH of the reaction medium is less than 1.0 
particularly preferably less than 0.8. 
It is a feature of the present invention that the hydrogen peroxide may 
advantageously be used in an excess over the stoichiometric quantity in 
order to increase the reaction efficiency. The excess is preferably at 
least 20% and particulary preferably at least 40% although it may be found 
to be beneficial to utilise a molar excess of hydrogen peroxide of from, 
for example, up to 200%. 
The reaction according to the present invention is preferably carried out 
by establishing a body of an aqueous phase containing the source of 
5-valent arsenic and the copper metal and lastly introducing the hydrogen 
peroxide. Preferably the addition of the hydrogen peroxide is carried out 
slowly, for example over a period of at least 4 hours for example from 4 
hours to 24 hours. This apparently reduces the proportion of hydrogen 
peroxide lost through decomposition and increases the reaction efficiency. 
The reaction is preferably conducted under slightly elevated temperature 
conditions, for example at from 35.degree. C., particularly preferably 
from 45.degree. C. to below reflux temperature, preferably below 
100.degree. C. and particularly preferably below 95.degree. C., to 
minimise decomposition of the hydrogen peroxide. It is preferred not to 
allow reflux to occur since this tends to reduce the efficiency of the 
process. It is further preferred not to utilise an oxygen or air sparge 
since this also tends to reduce the efficiency of the process. 
The method of achieving efficient contact between the hydrogen 
peroxide-containing liquid reaction medium and the copper metal will 
depend to a great extent on the physical form of the copper. When the 
copper is finely divided or is in relatively small pieces it is preferred 
to agitate the liquid medium sufficienlty to suspend the copper, by for 
example, mechanical stirring. If the copper is scrap copper which may be 
in the form of bales comprising quite large pieces, for example plumbers 
scrap, it may be desired to pass the liquid medium over the surface of the 
scrap copper in the form of a bed or a packed column. Advantageously the 
scrap copper is first shredded. The contact between the hydrogen peroxide 
containing liquid reaction medium may advantageously be continued after 
the addition of the hydrogen peroxide to the medium has been completed for 
example for at least 15 minutes and possibly up to 3 hours or more. 
The overall reaction between the copper metal, the source of 5-valent 
arsenic and the hydrogen peroxide is found to be exothermic. On a 
laboratory scale this was found to be insufficient to maintin the desired 
temperature, particularly at the upper and of the reaction temperature 
range envisaged. On a commercial scale, however, external heating may be 
unnecessary and cooling may even be required. 
The direct product of the above reaction is a slurry of copper arsenate. 
This slurry product may be utilised in the production of 
copper/chrome/arsenic wood preservative compositions in the same manner as 
taught in the disclosure of U.S. Pat. No. 4,103,000 namely the production 
of such compositions by the mere addition of the required quantity of 
chromic acid and, if it has not been included as excess rectant, any 
required further quantity of a source of 5-valent arsenic. Alternativley, 
the product of this invention may be utilised in the manufacture of 
copper/chrome/arsenic wood preservative compositions by mixing it with 
sodium dichromate and a sufficient quantity of an added acid such as a 
strong mineral acid, to ensure that a water-soluble end-product is 
obtained in the manner described in our copending United Kingdom Patent 
Application No. 8601570.

The invention will now be illustrated by reference to the following 
specific examples thereof. 
EXAMPLES 1 TO 9 
A series of experiments were conducted which varied specific process 
conditions from a standard process. According to the standard process 
arsenic acid (80% wt concn) and new copper wire which had been etched with 
a dilute acid/hydrogen peroxide mixture and water washed (0.91 mm 
diameter, 10 mm lengths) were placed in a reaction vessel fitted with 
stirring means and thermostatically controlled heating means and 50% wt 
concentration aqueous hydrogen peroxide was added slowly over a period of 
4 hours with stirring at 500 revolutions per minute (rpm). The reactants 
were used in a stoichiometry relative to the copper, the hydrogen peroxide 
being in a 150% stoichiometry arsenic acid in a 190% stoichiometry. This 
gives a copper arsenate product having a copper to arsenic ratio as 
specified in the Type C copper/chrome/arsenic Standard referred to above. 
The reaction temperature was maintained at 90.degree. C. After the 
addition of the hydrogen peroxide had been completed stirring was 
continued for a further 2 hours. The copper arsenate precipitate was 
filtered off and disolved to allow the determination of the weight of any 
residual copper and the calculated reaction efficiency was based on the 
weight of copper found to have reacted. The Standard process gave an 
efficiency, so calculated, of 89.1%. 
______________________________________ 
Example Reaction 
No Condition varied from Standard 
Efficiency % 
______________________________________ 
1 None 89.1 
2 H.sub.2 O.sub.2 stoichiometry 100% 
77.6 
3 Reaction temperature 50.degree. C. 
62.9 
4 Reaction temperature 50.degree. C. 
41.3 
Addition + stirring times 
2 + 1 hours 
5 Reaction temperature 50.degree. C. 
85.5 
and addition + stirring times 
8 + 2 hours 
6 Arsenic acid 900% stoichiometry 
93.3 
7 Arsenic acid 100% stoichiometry 
40.6 
but diluted to 25% wt concentration 
8 Air sparge added to supplement 
18.1 
agitation and arsenic acid 272% 
stoichiometry 
9 Speed of stirrer reduced to 
54.9 
250 rpm (no sparge) 
______________________________________