Treatment of brine to remove metals and silicon therefrom

Details Treatment of brine to remove metals and silicon therefrom Treatment of brine to remove metals and silicon therefrom

2001-06-25

2003-09-30

Bos, Steven (Department: 1754)

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

C210S713000, C210S715000, C210S723000, C423S158000

Reexamination Certificate

active

06627085

ABSTRACT:

FIELD OF THE INVENTION
The herein disclosed invention finds applicability in the field of brine purification processes.
BACKGROUND OF THE INVENTION
In traditional brine treatment methods, heated brine is mixed with sufficient soda ash and cell liquor to yield 500-700 ppm excesses of both NaOH and Na
2
CO
3
. Magnesium and calcium precipitate out as Mg(OH)
2
and Ca CO
3
, which are then settled out in a clarifier. The clarified brine is filtered and neutralized with HCl for use in electrolytic cells. The treated brine is optimized for magnesium and calcium; and to contain less than 6 ppm silicon. New technologies, however, require brine containing 1 ppm or less of silicon. The herein disclosed invention is designed to produce a brine with 1 ppm or less of silicon.
Prior Art Patents
Noll et al (U.S. Pat. No. 2,307,466) teaches a method designed to lower silica to acceptable levels in water. The process uses magnesium oxide alone or in combination with sodium hydroxide to rid the water of silica. The method taught by Noll et al is distinct from that of the instant invention in that silica is reduced in water and not in brine. Further, the process of Noll et al employs magnesium oxide, while the process of the herein disclosed invention employs sodium hydroxide and sodium carbonate to form a precipitate of magnesium hydroxide and magnesium carbonate.
Goetz (U.S. Pat. No. 2,401,924) teaches a method of removing silica from water using insoluble magnesium compounds. There is no teaching by Goetz to remove silica from brine employing the method of the herein disclosed invention.
Dille et al (U.S. Pat. No. 2,963,355) teaches the removal of silica from the fluid in the interior of a heating tube. There is no disclosure in this patent of a brine treating process.
Patil et al (U.S. Pat. No. 4,038,365) teaches a process for purifying brine comprising adding to said brine sodium carbonate and sodium hydroxide to cause the precipitation of calcium carbonate. There is no recognition in this patent of the need to remove silica to a low level, thereby allowing efficient industrial use.
Featherstone (U.S. Pat. No. 4,765,913) teaches the method of reducing silica in geothermal brine by pH adjustment by the addition of bases which react with heavy metals in the brine to form finely divided insoluble compounds which function as seed crystals to cause supersaturated amounts of silica to be precipitated from the brine.
None of the prior art patents cited show the inventive method of treating brine to reduce silicon content to acceptably low levels.
Definitions as Used in This Invention
The term caustic refers to sodium hydroxide.
The term brine refers to a salt solution at least containing silicon, magnesium and calcium.
The term ppm refers to parts per million.
A typical pretreated brine of this invention contains 8-12 ppm Si; 600-900 ppm Ca; and 50-70 ppm Mg.
While the herein disclosed invention has been characterized as using sodium hydroxide and sodium carbonate. It is readily apparent that a metal cation such as potassium or like metal ions could substitute for sodium.
OBJECTS OF THIS INVENTION
A main object of this invention is to produce a brine of low silicon content.
A further object of this invention is to produce a low silicon brine useful in modem technology, such as electrolytic cells.
A significant object of this invention is to produce a process for efficiently removing silicon from brine.
An additional object of this invention is to provide an improved continuous process for the removal of metals and silicon from a brine stream.
These and other objects of the present invention will become apparent from a reading of the following specification taken in conjunction with the enclosed drawings.
SUMMARY OF THE INVENTION
The traditional prior art treatment of brine uses caustic and carbonate addition to precipitate most of the metals, mainly magnesium and calcium. Silicon co-precipitates with the other salts, and after settling in a clarifier, the precipitate is removed as sludge. Using high caustic and carbonate excesses, this treatment effectively removes metals, and the silicon is reduced from 12 ppm to 5 ppm in the treated brine stream. New technology requires brine that contains 1 ppm silicon or less.
The inventors have discovered a novel treatment process that uses low caustic excess to both remove the metal contaminants, and reduce silicon concentration to 1 ppm or less. A further improvement is achieved by combining brine sludge recycle to remove metals and silicon at an increased range of caustic excesses. The invention can employ a continuous or batch process with the following features.
a. A means of feeding caustic and carbonate to a reaction tank which contains raw brine and is equipped with a mixer or some means of agitation.
b. A feedback control mechanism to maintain consistent 50-100 ppm caustic excess in the reactor.
c. Feeding the material from the reaction tank to a clarifier tank in which the solids are settled and removed as sludge.
d. Alternately, the sludge is recycled from the clarifier to the reaction tank. In this case, the caustic excess may be higher.
e. Treated brine is fed from the clarifier through a filter.
The invention involves a process for the removal of metal and silicon contaminants from a brine stream. Previous processes efficiently removed metals, but not silicon. The process of this invention removes silicon to much lower levels than previous processes. Metals such as magnesium and calcium are also removed. The process takes advantage of the discovery that silicon is best removed at low caustic excesses. The invention thus uses less caustic than previous methods. Previous methods relied upon high caustic and carbonate excesses, which then required substantial neutralization with HCl after treatment. With lower caustic excess, less HCl is needed for neutralization. The herein disclosed process may be combined with magnesium-laden sludge recycle to allow a greater effective range of caustic excesses. A greater range is useful if caustic excess control is difficult.
Conventional Brine Treatment Process
Raw sodium chloride brine intended for chloralkali electrolytic cells is solution mined from underground salt deposits, and contains high levels of hardness. A typical treatment method involves dosing the brine with enough sodium hydroxide (NaOH) and sodium carbonate (Na CO
3
) to produce high excesses (200-500 ppm) of both chemicals. This treatment causes the calcium and magnesium to precipitate as calcium carbonate and magnesium hydroxide, which are both mostly insoluble in water as well as salt water. Silicon adsorbs to the precipitates, primarily to the magnesium hydroxide, and is removed with other precipitates in a clarifier. The clarified and filtered brine is at a pH of about 11-12, which is neutralized as required with hydrochloric acid before entering the chloralkali cells. New chloralkali membrane technology is required by industrial specifications to employ brine with lower silicon than the previous typical brine treatment method. Thus, there is a great benefit to be derived from the silicon removal process of this invention.
Brine Treatment of This Invention
A novel modification of the standard treatment method has been developed to reduce silicon to the desired specification of 1 ppm or less. It has been found that if NaOH addition was reduced to maintain an excess level of 0-70 ppm, the silicon could be reduced to less than 1 ppm. The actual amount of NaOH added and resulting pH varied somewhat based on Na
2
CO
3
levels, however, in the presence of 400 ppm Na
2
CO
3
excess, 0-70 ppm NaOH excess corresponded to a pH range of 9.0-9.9. If NaOH excesses were kept to 0-50 ppm (pH 9.0-9.8), then silicon was reduced to less than 0.5 ppm. Unfortunately, it was found that magnesium precipitation suffered when NaOH excesses were less than 30 ppm. Thus, a relatively narrow operative range was discovered for treating raw brine.
Example of Brine Treatment Method
Sufficient NaOH and Na
2
CO
3
are mixed into the brine to maintain a Na
2
CO
3

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