Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1998-12-01
2003-02-04
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S427000, C423S206200, C023S300000, C023S30200R
Reexamination Certificate
active
06514475
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to a process for the production of anhydrous soda, sodium carbonate anhydrate, with a bulk density of at least 800, preferably at least 1000, more in particular at least 1300 kg/m
3
, by crystallisation.
Soda is an important chemical, which is produced in large amounts. It is used, among others, in production of glass. There are generally two processes for this production, the so-called Solvay process and the trona process.
The former is based on the production of solid sodium bicarbonate, which is calcined into light soda anhydrate (crystal water free soda). This calcination is the chemical conversion of bicarbonate to carbonate, carbon dioxide and water.
The latter process is based on the mining of trona, mainly consisting of the double salt of sodium carbonate and sodium bicarbonate, with two water molecules, and varying amounts of contaminants. Trona is converted to soda, dissolved and purified. Subsequently soda is crystallised from the clear solution as the monohydrate, followed by drying to give light soda anhydrate. A review of the various aspects of the refining of trona is given in the paper by Aitala et al, Process for Selection Criteria for refining Trona to Commercial Products, presented at the First International Soda Ash Conference (ISAC), Jun. 11, 1997.
Light soda ash produced by either the trona-route or the Solvay process has a low bulk density. This results in high transport costs and gives rise to dusting during transportation and handling. Depending on the process used, this light soda ash has a bulk density of 550 (Solvay) or 800 (Trona) kg/m
3
.
A heat densification process can be used to increase the density of the product to about 1000 kg/m
3
. During this process hot water is added to the light product to produce monohydrate, which is immediately evaporated in a calciner to form anhydrate again. This requires a large amount of energy.
Sodium carbonate can be crystallised as various hydrates. In aqueous solutions, the anhydrous form (Na
2
CO
3
) is only stable at temperatures above 109° C., which is higher than the boiling point at atmospheric pressure of a saturated soda ash solution (105° C.). This means that evaporative crystallisation can only produce anhydrous soda at elevated pressures i.e. when the boiling temperature is higher than the transition temperature (the transition temperature is hardly influenced by pressure).
Because evaporative crystallisation in pressurised vessels is expensive, in the trona process the soda is often crystallised by evaporation of water at lower temperatures: Na
2
CO
3
. 1H
2
O or monohydrate is produced. After a drying step at 150 to 200° C., whereby the crystal water is removed, the end-product is porous and usually has a bulk density of 800 kg/m
3
.
The conventional evaporative crystallisation, drying and heat densification steps require a high energy input. The resulting product, after heat densification is a dense soda, having a density of about 1000 kg/m
3
.
From earlier work it is known to produce crystal water free soda, i.e. sodium carbonate anhydrate, by direct evaporative crystallisation at atmospheric pressure from an aqueous solution. The driving force for the crystallisation in this process is the evaporation of water. In this process the boiling point of the saturated sodium carbonate solution is increased to a temperature above the transition temperature above which sodium carbonate monohydrate is unstable, by adding an amount of an organic solvent. Upon evaporative crystallisation sodium carbonate anhydrate can be recovered directly from the crystallising mixture, in a dense form. This sodium carbonate anhydrate has a bulk density of far above 1000 kg/m
3
, generally in the range of 1300 to 1600 kg/m
3
.
This system is limited to crystallisation by evaporation of the solvent, i.e. the water, which means that the economical application of the process is limited to those systems wherein all sodium carbonate is fully dissolved.
In view of the high energy requirements of the is conventional process (evaporating water in the monohydrate evaporative crystallization; removing the crystal water to produce anhydrate; heat densification treatment), it would be an important improvement if one or more of these steps could be deleted, without decrease of the bulk density of anhydrous soda.
Another, very profitable improvement would be if there could be provided a process for converting light soda ash to soda ash having a higher bulk density, without evaporation of water being necessary, i.e. a process to replace the heat densification process. Finally, it would also be very important to provide a process wherein soda ash of high bulk density is produced more or less directly from bicarbonate containing feedstocks, such as trona, or pure bicarbonate.
SUMMARY OF THE INVENTION
The present invention is based on the principle that when recrystallising soda from a dispersion thereof in an aqueous medium, it is possible to produce soda, free of crystal water, of high to very high bulk density, provided that the ‘water-activity’ of the systems is modified in such a way that the temperature of the crystallising system is below the boiling point of the system and above the transition temperature of the monohydrate-anhydrate transition. The crystallisation of soda anhydrate occurs here via a solution mediated conversion of one solid phase into another solid phase.
The invention in its broadest form is accordingly directed to a process for the production of sodium carbonate-anhydrate having a bulk density of at least 800 kg/m
3
, said process comprising:
providing a suspension of solid sodium carbonate and/or solid sodium bicarbonate and/or solid double salts at least comprising one of sodium carbonate and sodium bicarbonate, in a mixture containing water and an organic, water miscible or partly water miscible solvent, which solvent influences the transition temperature below which sodium carbonate monohydrate is stable, whereby the type and the amount of solvent is selected in such way that the said transition temperature is below the boiling point of the said mixture of water and an organic, water miscible or partly water miscible solvent,
in case sodium bicarbonate is present, converting the bicarbonate into carbonate,
crystallising sodium carbonate anhydrate from said mixture at a temperature above the said transition temperature and below the said boiling point, and
recovering the sodium carbonate anhydrate.
The term ‘solvent’ as used herein indicates an organic water miscible or partly water miscible organic liquid, which influences the water activity, which will generally result in a decrease of the transition temperature of the anhydrate-monohydrate transition. This transition temperature is defined as the temperature in the phase diagram of sodium carbonate, above which the anhydrate crystal form of sodium carbonate is stable. The term ‘solvent’ includes, unless otherwise indicated also combinations of two or more solvents.
The starting materials for the process of the present invention are the various forms in which sodium carbonate and bicarbonate occur or are produced. More in particular, as will become evident from the further discussion, the various crystal forms of sodium carbonate are important starting materials. Also the various double salts of sodium carbonate and sodium bicarbonate, optionally containing crystal water, such as trona may be used.
The various embodiments of the invention are all based on solution mediated recrystallisation or conversion, contrary to the evaporative crystallisation that is conventionally used for commercial soda production, and where the evaporation of water from the process is essential to cause crystallisation. The process of the invention accordingly has the distinct advantage of avoiding the evaporation step and thus of much lower energy requirements, thereby reducing the production costs. Further the invention allows it to produce soda of much higher bulk density, up to 50% higher, thereby reducing storage and tra
de Graauw Johannes
Oosterhof Harald
van Rosmalen Gerda Maria
Witkamp Geert-Jan
Bos Steven
Fitch Even Tabin & Flannery
Technische Universiteit Delft
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