WO1990005112A1

WO1990005112A1 - Quicklime hydration using calcium hypochlorite mother liquor

Info

Publication number: WO1990005112A1
Application number: PCT/US1989/004600
Authority: WO
Inventors: Gregg E. Neuendorf
Original assignee: Olin Corporation
Priority date: 1988-10-31
Filing date: 1989-10-13
Publication date: 1990-05-17
Also published as: AU4491389A

Abstract

This invention relates to the manufacture of calcium hypochlorite. More particularly, this invention relates to an improved process for calcium hypochlorite having reduced volumes of water added and effluents for disposal. Calcium hypochlorite is a commercial bleaching and sanitizing agent used particularly in the disinfection of water in swimming pools and spas. A process for producing a calcium hypochlorite compound which comprises: a) mixing a first aqueous hypochlorite liquor with quicklime to form a mixture of fine particles of hydrated lime and coarse particles of tailings, and steam, b) separating the fine particles of hydrated lime from the coarse particles of tailings, c) slurrying the fine particles of hydrated lime in a second aqueous hypochlorite liquor, and d) reacting the slurry of hydrated lime particles with a chlorine-containing compound to produce a calcium hypochlorite compound.

Description

QUICKLIME HYDRATION USING CALCIUM HYPOCHLORITE MOTHER LIQUOR

This invention relates to the manufacture of calcium hypochlorite. More particularly, this invention relates to an improved process for calcium hypochlorite having reduced volumes of water added and effluents for disposal. Calcium hypochlorite is a commercial bleaching and sanitizing agent used particularly in the disinfection of water in swimming pools and spas.

Hydrated lime, Ca(OH)₂ or CaO.H_0, is a basic raw material used in commercial processes for the production of calcium hypochlorite. Hydrated lime is prepared by the addition of water to quicklime (CaO) to hydrate the lime and water is thus supplied to the calcium hypochlorite process. However, where it is desired to minimize the addition of water and to limit or eliminate the treatment and disposal of liquid effluents, the addition of the water of hydration is undesirable.

The use of hypochlorite-containing liquors to form the lime slurries used in producing calcium hypochlorite has been widely practiced. Hydrated lime is admixed with the hypochlorite solutions and the water of hydration present is added to the process. Now it has been found that the addition of water of hydration to the calcium hypochlorite process can be eliminated, the volume of effluent to be disposed of can be reduced, and raw material costs reduced in a process for producing a calcium hypochlorite compound which comprises: a) mixing a first aqueous hypochlorite liquor with quicklime to form a mixture of fine particles of hydrated lime and coarse particles of tailings, and steam, b) separating the fine particles of hydrated lime from the coarse particles of tailings, c) slurrying the fine particles of hydrated lime in a second aqueous hypochlorite liquor, and d) reacting the slurry of hydrated lime particles with a chlorine-containing-compound to produce a calcium hypochlorite compound.

More in detail, in the novel process of the present invention quicklime or unslaked lime is employed _as one reactant. Quicklime, CaO, is available from a large number of sources throughout the world and contains varying amounts and kinds of impurities. Calcium hypochlorite products suitable for use in the sanitizing and disinfecting of water bodies such as swimming pools or spas can be produced from quicklime sources having an active lime content of at least 90 percent, and preferably from about 94 to about 98 percent. The active lime content is defined as the amount of CaO in the quicklime. Impurities found in quicklime sources include insoluble materials such as iron compounds, silica, aluminum salts, magnesium salts, unburned limestone, etc. In the process of the present invention, quicklime is admixed with an aqueous hypochlorite solution to form a mixture of fine hydrated lime particles and coarse particles of tailings. During the hydration reaction, the heat evolved vaporizes a portion of the water as steam. This steam can be recovered as an energy source usable, for example, in other steps of the process.

Hydrated lime [slaked lime, Ca(OH)₂] is produced as a fine powder admixed with the coarse particles of tailings. The tailings include a portion of the insoluble impurities originally present in the quicklime as well as small amounts of active lime.

Hydrated lime particles are separated from the coarse particles of tailing using any solid-solid separation means including mechanical air separators and screening.

The hydrated lime particles recovered have low concentrations of available chlorine and varying amounts of impurities present. The hydrate lime particles are reacted with an aqueous solution of hypochlorous acid, an alkali metal hypochlorite and/or chlorine as the chlorinating agent to produce as the calcium hypochlorite compound dibasic calcium hypochlorite, hemibasic calcium hypochlorite or neutral calcium hypochlorite dihydrate. Where the concentration of impurities is believed to be higher than desired, in one embodiment of the process of the invention, the hydrated lime particles are reacted with the chlorinating agent to produce a slurry containing crystals of a basic calcium hypochlorite compound such as dibasic calcium hypochlorite, hemibasic calcium hypochlorite, or mixtures thereof, and insoluble impurities. The crystals of basic calcium hypochlorite compound are separated from the insoluble impurities in any suitable separation means. The basic calcium hypochlorite crystals are further reacted with a chlorinating agent to produce neutral calcium hypochlorite crystals.

In another embodiment of the process of the present invention, the hydrated lime is admixed with an aqueous alkali metal hydroxide solution and the mixture chlorinated to produce a slurry of calcium hypochlorite crystals and alkali metal choride crystals. This reaction may be represented by the following equation in which sodium hydroxide is the alkali metal hydroxide: 4NaOH + 2Ca(OH)₂ + 4C1₂ >

2Ca(OCl)₂ + 4NaCl + 4H₂0 The calcium hypochlorite crystals are separated from the salt crystals by known procedures such as elutriation, flotation, decantation, and centrifugation among others. The coarse particles of tailings formed in the hydration of quicklime retain lime values which can be recovered, for example, by reacting the particles with hypochlorous acid or chlorine. Preferably, the coarse particles are reduced in size, for example, by crushing prior to the reaction. During the reaction, a slurry of insoluble impurities in a calcium hypochlorite solution is produced, and after separation of the insoluble impurities, the calcium hypochlorite solution is employed in the process. When chlorine is the reactant, salt crystals are formed which are separated with the insoluble impurities. Where the hydrated lime -6-

Admixing excess amounts of quicklime with hypochlorite solutions results in the formation of high temperatures which boil off a portion of the water present and results the formation of solid masses of lime and tailings.

Suitable hypochlorite liquors include those such as dibasic calcium hypochlorite mother liquors or paste liquors in calcium hypochlorite processes illustrated by U.S. Patent No. 4,399,117, issued August 16, 1983 to W. J. Sakowski; U.S. Patent No. 4,468,377, issued August 28, 1984 to W. J. Sakowski et al; U.S. Patent No. 3,767,775, issued October 23, 1973 to S. Tatara et al; U.S. Patent No. 4,258,024, issued March 24, 1981 to J. 0. Hoffer et al; U.S. Patent No. 4,428,919, issued January 31, 1984 to D. A. Stermole, among others.

Preferably, the hypochlorite liquor is an effluent having low concentrations of hypochlorite ion which would normally require treatment to eliminate the hypochlorite ion and disposal in a public waterway. The process of the present invention results in significant reductions in the volume of effluents as the hydration reaction converts excess water into steam, a reusable energy source. The hydrated lime produced contains low concentrations of available chlorine, for example, from about 0.5 to about 4 percent by weight, which are recovered from the effluent. In addition, lime values contained in the coarse particles of tailings, which are normally discarded in the commercial processes for hydrating lime can be recovered. Further, the process eliminates one source of water to a process in which the disposal of aqueous effluents has become increasingly costly. -5-

particles have been reacted to form a slurry of crystals of a basic calcium hypochlorite and insoluble impurities, the insoluble impurities may be mixed with the particles of tailings and the mixture reacted with a chlorinating agent to recover lime values.

To hydrate the quicklime in the novel process of the present invention, a hypochlorite liquor is admixed with the quicklime as described above. Any suitable hypochlorite liquor may be used and these are normally obtained during a separation step of a calcium hypochlorite crystal or paste from a calcium hypochlorite liquor; the separation of an alkali metal chloride from an alkali metal hypochlorite or the alkaline liquor formed in a gas scrubber. Frequently, the hypochlorite liquor will include both calcium and alkali metal ions.

The aqueous hypochlorite liquor is added to the quicklime in substantially equal amounts by weight to provide sufficient water to hydrate the quicklime to hydrated lime. For example, suitable ratios of quicklime to hypochlorite liquor include those in the range of from about 1:0.7 to about 1:1.2, and preferably from about 1:0.8 to about 1:1.1 by weight.

Attempts to produce a slurry of hydrated lime by mixing quicklime with excess amounts of calcium hypochlorites result in curdling and the formation of coarse particles in which calcium hypochlorite, the hydrated lime and tailings are combined and from which the calcium hypochlorite and hydrated lime cannot be easily recovered. -8-

EX PLE 1

Dibasic calcium hypochlorite mother liquor (160 parts) containing 4 percent by weight of Ca(OCl)₂ was charged to a reactor. Crushed commercial quicklime (200 parts; 96 percent CaO) was added and the reaction mixture stirred. During the hydration reaction about 40 parts of steam was evolved. Following completion of the hydration reaction, the reaction mixture was allowed to cool. The hydrated lime reaction mixture was dried and screened through 200 mesh screens and hydrated lime particles separated from coarse tailings. Hydrated lime (280 parts) was recovered.

EXAMPLE 2

Hydrated lime (20 parts) produced by the process of Example 1, was mixed in 100 parts of dibasic calcium hypochlorite mother liquor to provide a lime slurry. The lime slurry was continuously pumped into a crystallizer. Also added continuously to the crystallizer was a calcium hypochlorite solution [10 percent by weight of Ca(OCl)₂] which reacted with the lime to produce dibasic calcium hypochlorite crystals. A slurry of dibasic calcium hypochlorite crystals was continuously recovered from the crystallizer and filtered. Dibasic calcium hypochlorite crystals were mixed with a concentrated sodium hypochlorite solution and water. This slurry was continuously pumped to a reactor while adding chlorine to produce a neutral -7-

The novel process of the present invention is illustrated by the following examples without any intention of being limited thereby. All parts and percentages are by weight unless otherwise indicated.

calcium hypochlorite dihydrate paste. The neutral calcium hypochlorite dihydrate paste recovered from the reactor was filtered to separate a neutral calcium hypochlorite dihydrate cake and a paste filtrate. The neutral calcium hypochlorite dihydrate cake was dried to produce hydrated calcium hypochlorite containing 7 percent water as the product.

Comparative Example A

Crushed calcium oxide was mixed with an 11 percent calcium hypochlorite solution in a weight ratio of 1:10. The temperature rose 20°C in three minutes indicating the slaking reaction had proceeded normally. The solids were nearly all hard grit which settled rapidly. Analysis of the system showed 91 percent of the Ca(OH)₂ was in the grits.

Comparative Example B

Crushed calcium oxide was mixed with dibasic calcium hypochlorite mother liquor containing 4 percent calcium hypochlorite in a weight ratio of 1:10. The temperature rose 22°C in four minutes. The settled grits contained 94 percent of the slurry's Ca(OH)₂.

EXAMPLE 3

Coarse tailings (20 parts) formed in the process of Example 1, were mixed in 260 parts dibasic calcium hypochlorite mother liquor to produce a lime slurry. The lime slurry was chlorinated to react with the hydroxide and form hypochlorite. The chlorinated slurry was filtered to remove the impurities contained in the tailings. A second lime slurry was produced by adding hydrated lime produced in Example 1 to dibasic calcium hypochlorite mother liquor. The lime slurry, filtered chlorinated lime slurry, and paste filtrate recovered from the process of Example 2, were continuously added to a crystallizer in a ratio of about 7:9:6, respectively. A slurry of dibasic calcium hypochlorite crystals was continuously recovered from the crystallizer and filtered. Dibasic calcium hypochlorite crystals were mixed with a concentrated sodium hypochlorite solution and water in a ratio of about 1.0:1.0:0.7, respectively. This slurry was continuously pumped into a reactor while adding chlorine to the reactor to produce neutral calcium hypochlorite dihydrate paste. The paste recovered from the reactor was filtered to separate a neutral calcium hypochlorite dihydrate cake and a paste filtrate. The cake was dried to produce hydrated calcium hypochlorite containing 7 percent water.

Claims

WHAT IS CLAIMED IS:

1. A process for producing a calcium hypochlorite compound which is characterized by: a) mixing a first aqueous hypochlorite liquor with quicklime to form a mixture of fine particles of hydrated lime and coarse particles of tailings, and steam, b) separating the fine particles of hydrated lime from the coarse particles of tailings, c) slurrying the fine particles of hydrated lime in a second aqueous hypochlorite liquor, and d) reacting the slurry of hydrated lime particles with a chlorine-containing compound to produce a calcium hypochlorite compound.

2. The process of claim 1 characterized in that the quicklime has an active lime concentration of at least 90 percent.

3. The process of claim 2 characterized in that the hypochlorite in the aqueous hypochlorite liquor is calcium hypochlorite, an alkali metal hypochlorite, or mixtures thereof.

4. The process of claim 3 characterized in that the weight ratio of quicklime to first aqueous hypochlorite liquor is 1:0.7 to 1:1.2.

5. The process of claim 1 characterized in that, prior to. step b) , the reaction mixture is dried.

6. The process of claim 1 characterized in that the coarse tailings particles are crushed and reacted with a chlorinating agent.

7. The process of claim 1 characterized in that the chlorine-containing compound is hypochlorous acid or chlorine.

8. The process of claim 1 characterized in that the coarse particles of tailings are crushed and reacted with a chlorinating agent to produce a dilute lime solution containing insoluble impurities.

9. The process of claim 1 characterized in that the calcium hypochlorite compound is a basic calcium hypochlorite compound selected from the group consisting of dibasic calcium hypochlorite, hemibasic calcium hypochlorite, and mixtures thereof.

10. The process of claim 9 characterized in that the basic calcium hypochlorite compound is dibasic calcium hypochlorite.

11. The process of claim 1 characterized in that the first aqueous hypochlorite liquor is a dibasic calcium hypochlorite solution.

12. The process of claim 1 characterized in that the second aqueous hypochlorite liquor is an alkali metal hypochlorite solution.

13. The process of claim 10 characterized in that the basic calcium hypochlorite compound is chlorinated to produce a slurry of neutral calcium hypochlorite dihydrate crystals.

14. A process for producing a calcium hypochlorite compound which is characterized by: a) mixing a first aqueous hypochlorite liquor with quicklime in substantially equal amounts to form steam and a mixture of fine particles of hydrated lime and coarse particles of tailings, b) separating the fine particles of hydrated lime from the coarse particles of lime tailings c) slurrying a first portion of the fine particles of hydrated lime in a second aqueous hypochlorite liquor, d) reacting the slurry of fine particles of hydrated lime with a chlorinating agent to produce a slurry of dibasic calcium hypochlorite crystals, e) separating the dibasic calcium hypochlorite crystals from a third hypochlorite liquor, f) crushing the coarse particles of tailings, g) reacting the crushed particles of tailings with a fourth dilute hypochlorite liquor to form a slurry of insolubles in a hypochlorite solution, h) separating the insoluble impurities from the hypochlorite solution, i) feeding the hypochlorite solution to step d), and j) separating dibasic calcium hypochlorite crystals.

15. A process for hydrating quicklime is characterized by mixing an aqueous hypochlorite liquor with quicklime in a ratio of from about 1:0.7 to about 1:1.2 by weight to form a mixture of fine particles of hydrated lime and coarse particles of tailings, and steam, and recovering the fine particles of hydrated lime.