US3499793A - Process for descaling devices used in concentrating sodium aluminate solutions - Google Patents

Description

United States Patent T Int. Cl. C23g 1/14 US. Cl. 134-2 6 Claims ABSTRACT OF THE DISCLOSURE A process for the elimination of deposits formed on surfaces in contact with sodium aluminate solutions, such as exchange surfaces of evaporators used in the Bayer process, by circulating a milk of lime on the scaled surfaces.

In the production of aluminum hydroxide from bauxite by the Bayer process, bauxite is treated with aqueous solutions of sodium hydroxide to form impure liquors of sodium aluminate which are concentrated in various types of evaporators, such as creeping, down stream and forced flow devices. All of these devices have exchange surfaces; and, in particular, tube bundles, each tube having one face in contact with the liquor to be concentrated and the other face in contact with a source of heat, generally steam, or with enclosures subect to a depression concurring to the boiling.

The sodium aluminate solutions contained some dis solved impurities from the treated bauxite that are likely to precipitate under certain determinable temperatures and concentration conditions. When the impurities precipitate, they form deposits on the surfaces of the exchangers, collectors and pumps. These deposits will reduce the output of these various devices and, in some cases, completely prevent them from operating.

These deposits are formed, first, of SiO -A1 O Na O complexes of variable formulas which are known generally as sodium aluminum silicates. Secondly, crystallized sodium fluorides are formed; and, finally, complexes of Al O Na O-oxides of elements such as vanadium, phosphorus, and arsenic in variable proportions are formed. These complexes are generally known as vanadiferous salts.

To eliminate these deposits from the surfaces of the devices, various mechanical or chemical descaling processes are presently utilized. When the deposit is sodium fluoride, the mechanical methods are, generally, costly, time consuming, diflicult, and often ineffective, and the known chemical methods present serious disadvantages. When acidic solutions are utilized, for example, diluted sulfuric acid, the formation of hydrofluoric acid resulting from the reaction between the acid and the sodium fluoride corrodes cast iron and steel. Sodium hydroxide solutions will disolve sodium aluminum silicate deposits, but it will not affect the sodium fluoride deposits and subsequent fragilization of the steel may result. Finally, hot water may act satisfactorily against vanadiferous salts, but it reacts only slightly with sodium fluoride which is compacted and only slightly soluble; and water will not react at all against sodium aluminum silicate.

Our invention provides a process for eliminating the deposits which overcome these limitations and disadvantages. Our method comprises circulating milk of lime on the scaled surfaces. The milk of lime may have any desired concentration of Ca(OH) provided that it can circulate through the device to be scaled. We have found 3,499,793 Patented Mar. 10, 1970 Ca(OH) concentrations of between 2.0 and 100 grams/ liter to be suitable.

The process is extremely simple to carry out. It is sufficient, for instance, to circulate the milk of lime through a closed circuit which includes the surface to be scaled. This circuit can be the same circuit followed by the liquors of sodium aluminate that are to be concentrated. It is not necessary to raise the milk of lime solution to its boiling point; however, it is preferable to operate the process above room temperature, for example, between 50 C. and C.

The activity of the milk of lime is easily ascertained from the increase in NaOH in the descaling liquor. The sodium-calcium exchange in the deposits results in the appearance of sodium hydroxide in the descaling liquor according to the following formula:

Our inventive process has many advantages. Since the only reagent used is Ca(OH) the elimination of the various deposits is completed without corroding the metallic surfaces involved, especially if those surfaces con tain ferrous metals. Furthermore, the process is exceedingly efficient in that not only does it provide a dissolving action, but it provides a disintegrating action due to the sodium-calcium cation exchange. The descaling results in a mud that is carried along by the caustic liquor. This mud does not, however, interfere with the further action of the lime in the circulating suspension.

The following example illustrates the effectiveness of our inventive process. A creeping evaporator with a steam-sodium aluminate counter-flow was utilized. The evaporator had four evaporation bundles, each bundle consisting of 300 steel tubes having a 49 mm. inner diameter and a length of 7 meters. After three months of operation, it was found that the first and second bundles had a uniform sodium aluminum silicate deposit of a thickness of approximately 2 mm. In the third bundle, approximately one-half of the tubes were found to be almost completely obstructed with a sodium fluoride deposit and the balance had a deposit of approximately 10 to 20 mm. All of the tubes in the fourth bundle were found to have been covered with a 10 mm. thick deposit of sodium fluoride and vanadiferous salts.

The process of this invention was followed utilizing a lime suspension containing 20 g./l. Ca(OH) The suspension was circulated through the evaporator at a temperature of 50 C. for 12 hours at a rate of 60 cubic meters per hour. At the end of the 12 hour period, it was found that the deposits in the first and second bundles were less compact and had a tendency to chip off when scrapped. The deposits in the third bundle that had almost completely obstructed the tubes had been cleared for few centimeters at each end, and the balance of the tubes in the third bundle had an average decrease in deposit thickness of 4 to 5 mm. The deposits in the tubes of the fourth bundle had practically disappeared.

After five identical descaling operations, each operation following a period of four weeks of normal and continuous operation of the evaporator, it was found that no deposits existed in the first, second and fourth bundle of tubes. In the third bundle, only 10 tubes remained obstructed, and each of those tubes had about a 20 cm. zone cleared at each of its ends.

After one year following the above procedure, that is circulating the milk of lime following a four week period of normal evaporator operation, no evidence of deposits in any of the bundles was found.

These same results were found to exist in other circuits used for the circulation of aluminate solutions.

While we have described a preferred embodiment of our invention, it may be otherwise embodied within the scope of the appended claims.

We claim:

1. The process of removing deposits from the surfaces of apparatus used in concentrating solutions of sodium aluminate, said deposits formed by the impurities present in the sodium aluminate solution of the Bayer process, said process comprising circulating a milk of lime on the scaled surfaces.

2. The process claimed in claim 1 in which the milk of lime has a concentration of Ca(OI-I) between 20 and 100 grams per liter.

3. The process claimed in claim 1 in which the milk of lime is circulated at a temperature above room temperature and below its boiling temperature.

4. The process claimed in claim 3 in which the temperature of the milk of lime is between 50 and 90 C.

5. The process of claim 1 in which the milk of lime is circulated for a period of 12 hours at a rate of 60 cubic meters per hour.

6. The process of removing at least one of the deposits selected from the group consisting of complexes, sodium fluoride, and Al O -Na O oxides of vanadium, phosphorous, and arsenic complexes from scaled heat exchange surfaces comprising circulating a milk of lime on the scaled surfaces.

References Cited UNITED STATES PATENTS BARRY S. RICHMAN, Assistant Examiner US. Cl. X.R.