Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2001-02-28
2002-11-19
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
Reexamination Certificate
active
06482385
ABSTRACT:
This invention relates to a process for producing a persalt and more particularly to the production of sodium percarbonate and sodium percarbonate so produced. It also relates to sodium percarbonate itself, and to compositions containing it.
Sodium percarbonate as used herein is the name commonly employed in industry for sodium carbonate peroxyhydrate having the formula Na
2
CO
3
.1.5H
2
O
2
and is often referred to as PCS. Although PCS has a wide range of uses employing its oxidant and disinfectant qualities, its currently most widespread use is as a bleach in washing compositions and especially the compact and super-compact compositions that have been promoted by the detergent industry since the 1980s, or in dish washing compositions or as a bleach additive or in hard-surface cleansing compositions.
Sodium percarbonate can be produced by a variety of process routes, normally employing a reaction between hydrogen peroxide and a source of sodium and carbonate. In some variations, often called collectively as “wet processes”, the process comprises forming an aqueous solution of sodium carbonate, mixing it with an aqueous solution of hydrogen peroxide under reaction conditions including the concentrations of the sodium carbonate and hydrogen peroxide and temperature selected such that sodium percarbonate (PCS) crystallises out of solution. The solid PCS product is then separated from the solution, often by filtration or centrifugation, is possibly subjected to further surface treatments or granulation, and dried. The separated solution, which is usually referred to as mother liquor, is discarded, forms the basis for a surface treatment or is recycled. Although a wet process could be carried out in a batch-wise fashion, it is usually carried out continuously.
In order to recover a greater fraction of product from the solution in a wet process, the practice has been commonly adopted for many years of conducting the reaction in the presence of an otherwise unreactive sodium salt such as specifically sodium chloride or adding the salt to promote further crystallisation before the solid PCS is separated. This practice, typically referred to as “salting out”, also drives the reaction further towards completion, converting a higher fraction of the reactants into the desired product and as a further consequence enables greater production to be obtained from a reactor of a given size. The mother liquor after separation from solid PCS contains residual concentrations of sodium and carbonate ions and hydrogen peroxide, and it will be recognised that these concentrations would be lowered as a result of the presence or introduction of the salting-out agent.
Like any process that it is desired to operate commercially, it is desirable, in principle, to minimise reagent losses, for example by recycling at least a fraction of the mother liquor to dissolve further sodium carbonate in a make-up tank before transfer into the reaction vessel. This has the additional advantage of avoiding or minimising the proportion of mother liquor which is discharged into the environment. With increasingly stringent controls being introduced in many countries on what can legally be discharged into water courses or the aquifer, the cost has increased of pre-treatment of alkaline waste liquors like mother liquors before discharge. However, there are inherent practical difficulties associated with recycling mother liquor. In particular, because it contains residual hydrogen peroxide, there is risk that on introduction of sodium carbonate into the make-up tank, further crystallisation of sodium percarbonate will be induced, producing a dispersion of solid fines that subsequently act as seeds in the reactor, so that the resultant product has less desired properties. In particular, a product which thereby contains a high proportion of small particles is rendered significantly less desirable for incorporation in compact and super-compact washing compositions. Ausimont spa in EP-A-748764 has drawn attention to the problem of premature crystallisation and solves the problem by treating the mother liquor before it is recycled to the soda ash dissolution vessel to deliberately decompose hydrogen peroxide. However, such a solution inevitably impairs process economics by increasing the waste of an important reactant.
Even if the hydrogen peroxide is not decomposed as a deliberate action in the mother liquor, there is a potential for its decomposition during the step of dissolution of sodium carbonate into recycled mother liquor, particularly where conditions are chosen to promote the rate and extent of dissolution, such as the elevated temperatures commonly employed in current continuous processes that employ sodium chloride as salting-out agent. The extent of loss of hydrogen peroxide under otherwise identical conditions in the dissolution tank is naturally related to its concentration in the mother liquor and consequently the problem would be exacerbated if little or no salting out agent is employed in the crystalliser, for the reasons identified above.
The use of a salting-out agent remains common practice in wet processes, in view of the significant advantages identified above. However, its presence in the precipitated PCS reduces the content of hydrogen peroxide which is available for release from the solid. Additionally and importantly, it has been suggested that the most effective salting out agent and the one which has been employed commercially, sodium chloride, can adversely affect the stability of PCS, but there is contrary teaching on this point. For example, Kemira in WO 9405594 has suggested that the presence of sodium chloride actually improves the stability of PCS.
Although, in theory, a wet process can be operated without addition of a salting-out agent, the absence of the salting out agent results in a lower conversion of reagents to PCS and a lower recovery of PCS from the liquor, so that the recycled mother liquor contains a higher concentration of hydrogen peroxide, thereby increasing the likelihood of premature crystallisation on recycle as well as the extent of hydrogen peroxide losses during recycle.
Expressed most starkly, the problem facing a persalt manufacturer who might wish to use a wet process to make PCS is that the wet process in common practice employs a salting out agent for effective operation of the process, whereas the product obtained has potentially less desirable properties.
It is an object of certain aspects of the present invention to provide a wet process for producing sodium percarbonate which ameliorates or overcomes one or more of the problems or disadvantages of operating without the benefit of the presence of a substantial concentration of salting-out agent.
It is a second object of further aspects of the present invention to provide a wet process for producing sodium percarbonate having properties rendering it particularly suitable for incorporation in washing compositions.
According to one aspect of the present invention there is provided a continuous process for the manufacture of sodium percarbonate in which hydrogen peroxide, sodium carbonate and an aqueous medium are introduced into a reaction vessel in which the hydrogen peroxide and sodium carbonate react with formation of sodium percarbonate which precipitates out of solution, the precipitated sodium percarbonate is separated from the mother liquor and recovered as product and at least a fraction of the mother liquor is recycled to the reaction vessel characterised in that
in step a) particulate sodium carbonate and recycled mother liquor are fed continuously into a dissolution tank maintained within a temperature range of from 15 to 50C., the relative feed rates and temperature being controlled such that the concentration of sodium carbonate in solution does not exceed 95% of a saturated solution;
in step b) sodium carbonate solution is continuously withdrawn from the dissolution tank;
in step c) the sodium carbonate solution withdrawn from the dissolution tank in step b), additionally or alternatively particulate so
Bigini Stefano
Deli Pier-Luigi
Droste Uwe
Horne Graham R.
James Alun P.
Ildebrando Christina
Larson & Taylor PLC
Silverman Stanley S.
Solvay Interox (Societe Anonyme)
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