When it is desired to make a high-grade crystal for granulated sugar, the clarification must be more complete, and a water-white thick liquor should result, without subsequent treatment by bleaching agents and other chemicals, except the neutralizing of the slightly yellowish tint, which will be mentioned later.
Reagents Used in Clarification.
There is a great variety of reagents at the command of the sugar manufacturer, each of which has certain merits over others, and all are valuable in their place when properly used. It will therefore be the duty of the operator to select those which best meet his individual conditions.
It is the purpose of this article to give a brief survey of the more common reagents which, under certain conditions, may be used to advantage in these Islands.
Lime.—This is perhaps one of the most common and most widely used of all the reagents. Since the object in view is to increase the purity of the juice, it is obvious that the purest rock obtainable should be used in the preparation of the lime. Another reason why a good lime should be employed, is that one of the main impurities of the lime rock is magnesium, which, when mixed with cane juice, becomes very troublesome in the incrusting of the evaporator tubes, thus greatly lowering the coefficient of heat transmission.
Much of the lime on the market in the Philippines has been made without any attempt to select pure clean limestone or shells. This is not suitable for putting into cane juice, and will result in a great deal of trouble whenever used in modern evaporating plants. There is, however, an abundant supply of limestone found in various parts of the Philippines, which analyses show to be almost free from impurities, and which will make a most excellent lime for clarifying purposes if burned properly. At present there is no modern plant for burning this rock on a large scale and consequently much of the work is done in a very crude and unsatisfactory manner. Most of the lime for clarification, in modern sugar factories, is imported, and constitutes a very heavy expense. If a lime kiln were installed in conjunction with some of our sugar factories, fresh and well-burned lime might be made as needed. The carbon dioxide could be used in the juice clarification, as is done in Java, and thus a good grade of plantation sugar could easily be manufactured. Any excess of burned lime might very readily be sold to other factories, which now use only high-priced imported lime.
The lime used should be of the unslaked type, and should be protected from the air until a short time before using. The process of preparing this consists of heating lime rock to a very high temperature, in a kiln for that purpose, whereby the limestone is broken into two component parts, expressed by the following chemical equation: CaCO3 (limestone) heated to high temperature—>CaO (calcium oxide) + CO2 (carbon dioxide). This calcium oxide, commonly known as “quick lime,” is the substance desired in clarification. It should be slaked by being placed in water just before it is desired for use. This milk of lime should not be used until after the high temperature caused by the violent chemical action has subsided. On account of the heat involved and the high alkalinity in local portions, it is never safe to apply crude lime to the juice without previously slaking it in water, nor is it advisable to use a quantity of juice to mix this lime, as is quite often practiced in these Islands, since in this case there may be a loss of sucrose, with a resulting dark-colored product, which will impair the color of the clarified juice. The following chemical equation will express the reaction when this lime is slaked: CaO (calcium oxide) + H2O (water)—>Ca(OH)2 (calcium hydroxide).
This calcium hydroxide is a substance which is very caustic, and care must be exercised in handling it. Like all bases, it has a great affinity for acid, and consequently its first action is to neutralize part of the acids present. It then coagulates albumins and albuminoids, which form a part of the impurities, and throws down insoluble salts of sulphates, carbonates and phosphates, and of the bases iron and aluminum. These act as mechanical precipitants, assisting in bringing down other impurities. The compounds of calcium are practically insoluble in cold cane juices, and may be readily filtered, or settled, and the supernatant liquor drawn off. In the addition of lime, as well as in the application of other reagents, much care must be observed that the proper amount is added. If too little is used, there will be poor clarification and settling of the precipitate, while if too much is used, so that alkalinity is reached, and the juice heated to a high temperature, there will be a darkening of the juice caused by the decomposition of the reducing sugars by the calcium, and the formation of dark-colored compounds, which are very hard to remove. If the juice is limed to three-tenths or four-tenths cubic centimeter acidity against N/10 NaOH, using phenolphthalein as an indicator, there will be little or no chance of trouble. With the above dangers in view, it is not safe to employ the haphazard methods of liming usually practiced here, but the milk of lime should always be made of stated density and a measured or weighed amount should be supplied to each clarifier of juice, corresponding to prevailing conditions.
Sulphur dioxide.—Where a better grade of sugar than 96° test is desired, it is often advisable to subject the juice to further treatment, one reason for which is to increase the acidity so that a larger amount of lime may be added to effect the clarification. In addition to this the sulphur acts to some extent directly as a clarifying agent, by precipitating some of the impurities. It also acts as a bleaching agent by extracting the oxygen from the impurities and lastly it acts as a disinfectant. It is formed by burning crude sulphur in a stove made for that purpose. S (sulphur) + O (oxygen heat)—>SO2 (sulphur dioxide).