Scale forms when the concentration of a dissolved mineral exceeds its solubility limit and the mineral precipitates. Scale is and can be a problem in equipment used in many types of industrial operations utilizing an aqueous system.
As used herein the term "aqueous system" is meant to include any system containing water, including but not limited to cooling water, boiler water, desalination, gas scrubbers, blast furnaces, sewage sludge thermal conditioning equipment, reverse osmosis evaporators, paper processing, mining circuits and the like wherein such systems are operated under harsh conditions of temperature and pH.
The term "harsh conditions" as used herein is intended to be definitive of an aqueous system wherein the temperature is in the range of from about 100.degree. C. to about 200.degree. C. and the pH is in the range of from about 10 to about 14.
Typical equipment used in industrial aqueous operations that require scale inhibition includes, but is not limited to, boilers, evaporators, heat exchangers, other heat transfer equipment, pipes and any other equipment that comes into contact with the aqueous system.
For purposes of this application, such industrial operations are illustrated by the kraft process for the production of wood pulp.
Wood pulp is the basic raw material used in the manufacture of almost all grades of paper and various types of packing products such as drums and cartons.
In order to produce pulp from wood, it is necessary to separate the cellulose fibers from the various organic compounds, mainly lignin, which bind them together. Various mechanical and chemical methods are used to effect this separation, but the most widely used technique is known as the kraft or sulphate process, since it produces pulp which gives high strength and good aging properties to paper products.
In the kraft process, a cooking liquor (white liquor) of sodium hydroxide and sodium sulphide is used to extract the lignin from wood. The process of extraction is carried out in digesters, either batch or continuous. The pH in the digester is generally between about 11 and about 14.
The liquor temperature is maintained between about 150.degree. to about 175.degree. C. A period of from about 2 to about 3 hours is usually required for complete digestion. The pulp is then washed before being sent for further treatment such as bleaching prior to its further use.
The economics of the kraft process depend on the recovery of the cooking liquor. In this recovery process, the digestion chemicals contained in the used cooking liquor (black liquor) are recovered via evaporators, furnaces and a causticizer for reuse in preparing new cooking liquor. Before the black liquor can be used as a feed it is necessary that the black liquor be concentrated, usually to 45% by weight or higher. This concentration is carried out in a multiple-effect evaporator, where live steam is introduced to the first unit (where the liquor is at its highest solids concentration) and flows to the final unit. Such evaporators can be described as one long heat transfer surface where the purpose is to boil off water by providing significant contact between the black liquor and steam heated surfaces. However, a common problem which is experienced in such evaporators is the formation of substantial amounts of deposits which tend to stick to the interior walls or tubes of the evaporator. The primary source of liquor scaling in the evaporator system is insoluble calcium carbonate.
The cooking liquor (white liquor) produced from this process contains sodium hydroxide, sodium sulphide and sodium carbonate due to incomplete reaction in the causticizer, as well as soluble calcium and precipitated calcium carbonate.
In the kraft process, calcium is extracted from the wood, and because of the high pH, temperature and presence of carbonate in the cooking liquor this calcium precipitates as calcium carbonate. The most visible form of the scale is in the cooking liquor heaters which maintain desired digester process conditions and often have to be cleaned about every 2-4 weeks.
Scale formation can also occur on the liquor separator screens which in turn leads to a restriction of liquor flow which reduces plant production and eventually necessitates plant shutdown for cleaning.
Because of the tendency for calcium carbonate scaling in the aqueous systems of the kraft process due to the conditions experienced therein, there becomes a real need for the addition of antiscalants to various locations within the process.
In addition to the scaling problem experienced in the digester, scaling is also a problem in the equipment used for carrying out the bleaching of the wood pulp. Pulp produced by the kraft process is normally bleached in a multistage sequence to obtain the desired brightness and strength. Different bleaching agents are used for this purpose. Chlorine or chlorine dioxide is the most reactive bleaching agent to the lignin remaining in the pulp. Although conditions in the bleach plant are less severe than those found in a kraft digester, the driving force for scale formation is significant. For example, calcium concentrations can climb to over 100 ppm, the pH of the stream entering the bleaching stages is greater than about 11 and the temperatures are still elevated.
From the foregoing, it can be seen that while conditions vary in severity with various stages of the kraft process i.e. the digester, bleach plant and evaporator, such aqueous systems as are contained within the various stages are all under harsh conditions of temperature and pH and as such experience the problem of scale formation during operation.
Thus, it becomes readily apparent that there is a real need in the kraft process for the production of wood pulp which comprises the steps of digesting wood chips in a digesting zone, bleaching of the resulting wood pulp in a bleaching zone and the concentrating of the separated liquor from the digesting zone in an evaporation zone for the providing of an improvement thereto which comprises the addition of a polymeric antiscalant which will inhibit the formation of scale.
Accordingly, it is an object of the present invention to provide a novel process for the inhibition of calcium carbonate scale in aqueous systems which otherwise experience such scale formation due the harsh conditions occurring in the system.
Another object of the present invention is to provide a process for improving the kraft process for the production of wood pulp wherein the formation of calcium carbonate scale is inhibited during plant operations.
A still further object of this invention is to provide novel polymeric antiscalants for use in the treatment of industrial aqueous systems.
Other aspects, objects and the several advantages of this invention will become apparent in light of the following specification and appended claims.