ES2401280A2 - Continuous process for the preparation of a liquid solution of calcium tiosulfate (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Continuous process for the preparation of a liquid solution of calcium thiosulfate. An efficient process for the continuous preparation of calcium thiosulfate (cas2 o3) is described from calcium sulphide, by oxidation. The process involves oxidizing a calcium polysulfide intermediate in a series of reactors to produce calcium thiosulfate as a clear liquid at high concentration with minimal byproducts. The process produces the complete destruction of polythionates, which allows the calcium thiosulfate produced to be useful in certain leaching processes for precious metals. The invention also makes it possible to recycle the water from the process of the leaching process for its use as a reactive raw material in the process for the production of calcium thiosulfate. (Machine-translation by Google Translate, not legally binding)

Description

Continuous process for the preparation of a liquid solution of calcium thiosulfate

RELATED APPLICATION DATA

This application is a continuation in part of the US patent application No. 12 / 764,843 filed on April 21, 2010, the totality of which is incorporated herein.

FIELD OF THE INVENTION

The present invention is directed to the production of a high purity calcium thiosulfate solution with high yield, and minimal solid by-products and soluble contaminants, such as polythionates, using tank reactors with continuous agitation. The resulting calcium thiosulfate is particularly suitable for leaching precious metals. It is also a suitable plant nutrient.

BACKGROUND OF THE INVENTION

The thiosulfate ion, S2O32-, is a structural analogue of the SO42- ion in which an oxygen atom is replaced by an S atom. However, the two sulfur atoms in S2O32- are not equivalent. One of the atoms of S is a sulfur-type sulfur atom that gives thiosulfate its reducing properties and complexing capabilities.

Thiosulfates are used for tanning leather, paper and textile manufacturing, flue gas desulfurization, cement additives, dechlorination, ozone and hydrogen peroxide extinction, coating stabilizers, as an agricultural fertilizer, as a leaching agent in mining , etc.

Due to these complexing capabilities with metals, thiosulfate compounds have been used in commercial applications such as photography, waste treatment and water treatment applications.

The thiosulfates easily oxidize to dithiotes, trionates, tetrathionates and finally sulfates:

2S2O32- + 3O2 - 252O62-

S2O62- + O2 - 25042

7S2O32- + 3 / 2O2 - 253O62- + 2S4O62

2S3O62- + 6O2 - 65042-

S4O62- + 5O2 - 45042-

Due to this transformation, thiosulfates are used as fertilizers in combination with cations such as ammonium, potassium, magnesium and calcium. Ammonium thiosulfates, alkali metals and alkaline earth metals are soluble in water. Water solubilities of thiosulfates decrease from ammonium thiosulfates to alkali metals to alkaline earth metals.

Calcium is an essential plant nutrient. The availability of calcium is essential in plant biochemistry, and has recently been learned, in the efficacy of urea nitrogen fertilizer applied to the surface. The need for soluble calcium for high value crops is different than the role of important land modifications such as lime or plaster. Both soluble calcium and these modifications of the soil are extremely important in soil fertility and plant nutrition and complement each other.

In the mining industry, gold leaching with thiosulfate is preferred over conventional leaching with cyanide due to the dangerous nature of cyanide. Calcium thiosulfate is an alternative replacement for lime / cyanide suspension in gold leaching.

"Calcium sulphide" is a term commonly used for a mixture of calcium thiosulfate and calcium polysulphide resulting from the reaction of lime and sulfur. US Patent No. 1,685,895 describes the formation of a solution of calcium sulphide from lime in lumps, sulfur in lumps and hot water.

J.W. Swaine, Jr. et al., In U.S. Patent No. 4,105,754 describe the production of calcium thiosulfate by a metathesis reaction of ammonium thiosulfate and calcium hydroxide or calcium oxide. This approach requires the constant removal of ammonia by bubbling with air below the boiling point of the mixture and capturing the gas.

Japanese Patent No. 6,039 issued in 1973 describes the preparation of calcium and magnesium thiosulfate treating sulfur and the corresponding sulfite in an alkaline solution. Only high yields are obtained with magnesium thiosulfate. This patent also describes the formation of calcium thiosulfate from a salt exchange process between magnesium thiosulfate and calcium hydroxide.

Sodium thiosulfate and calcium chloride were used to produce calcium thiosulfate in Spanish patent No. 245,171. The byproduct of this approach is a large amount of sodium chloride in the calcium thiosulfate product.

Lee et al., In US Patent No. 4,976,937 describe the formation of a calcium polysulphide / calcium thiosulfate mixture from a calcium sulphide mixture at 6-100 ° C to be used for the removal of sulfur dioxide from combustion gases.

Vonkennel and Kimming in US Patent No. 2,198,642 describe the production of a stable solution of calcium thiosulfate from calcium chloride and sodium thiosulfate.

Russian patent No. RU 216101 C2 describes the preparation of sodium thiosulfate and calcium thiosulfate from sulfur and a solution of sodium or calcium in stoichiometric amounts under autoclave conditions with an oxidant.

Hojjatie et al., In US Patent No. 6,984,368 B2 describe the preparation of liquid calcium thiosulfate fertilizer solution from lime, sulfur and oxygen. The patent describes the preparation of calcium thiosulfate in batch preparation.

Leaching gold with thiosulfate has been proven technically feasible. For example, see U.S. Patent No. 4,070,182, U.S. Patent No. 4,269,662 and U.S. Patent No. 4,369,061, which describe the use of ammonium thiosulfate in gold leaching. The application of copper-ammonium thiosulfate in the gold leaching process is described in US Patent No. 4,654,078. Choi, et al., In U.S. Patent No.

7,572,317 describe the use of ammonium, sodium and calcium thiosulfate in gold leaching.

BRIEF SUMMARY OF THE INVENTION

The present invention is a continuous process for the preparation of calcium thiosulfate and is an improvement over prior art processes. The invention forms minimal by-products, improves the process equipment for faster oxygen absorption, and subsequently shortens the reaction time, while producing the calcium thiosulfate product in a continuous manner using a multi-reactor installation of tanks with continuous agitation. (CSTR).

The present invention further relates to a continuous process for the preparation of calcium thiosulfate by the oxidation of calcium polysulphide (calcium sulphide) at particular pressures, using certain molar ratios of lime and sulfur, and at certain temperatures and oxidation durations, to produce a liquid solution of high concentration calcium thiosulfate in a suspension that has minimal solid and minimal by-products or no undesirable polythionate. The solid by-products produced in the process of the invention are less than about 2% by weight of the liquid solution, and consist of insoluble calcium salts such as sulphite, sulfate and carbonate, unreacted sulfur and a small amount of the thiosulfate product. retained calcium

The aforementioned solution of calcium thiosulfate and by-products can then be treated with an acid to reach a certain pH, to avoid decomposition of the product. The suspension can then be treated with an appropriate flocculant to separate the suspension from the desired liquid and provide ease of filtration.

Accordingly, it is an objective of the present invention to provide a method for the production of high purity calcium thiosulfate by an oxidation reaction of calcium sulfate, where economical raw materials such as calcium oxide or calcium hydroxide are used, Sulfur, water and oxygen.

It is yet another objective of the present invention to produce calcium thiosulfate by oxidation of calcium sulfate where difficult and potentially expensive processing and separation steps are avoided.

It is another objective of the present invention to produce high purity thiosulfate in a high concentration of approximately 22-29%, without additional concentration.

It is yet another objective of the present invention to produce calcium thiosulfate with minimal residual contamination of by-products.

It is yet another objective of the present invention to minimize solid by-products at their minimum level of up to about 2% by weight of the calcium thiosulfate solution.

It is yet another objective of the present invention to produce calcium thiosulfate with very low levels (ppm level) of polythionates, which are soluble oxidation by-products. The calcium thiosulfate product produced by this method is suitable for certain precious metal leaching applications.

It is yet another objective of the present invention to produce calcium thiosulfate in a continuous process using a series of tank reactors with continuous agitation (CSTR).

It is yet another object of the present invention to provide a method for easy separation of the calcium thiosulfate product from solid by-products originating from calcium oxide or calcium hydroxide.

It is yet another object of the present invention to provide a method for producing stable calcium thiosulfate product at near neutral pH.

It is yet another objective of the present invention to eliminate any potential bacterial growth in the final product by the addition of an appropriate chemical such as alkali metal metabisulfite salts.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described in more detail, with reference to preferred embodiments, given by way of examples, and illustrated in the accompanying figures in which:

 Figure 1 is a flow chart of a process, which includes a schematic illustration of a process for the continuous production of calcium thiosulfate according to a preferred embodiment of the present invention.

 Figure 2 is a graphical representation of the oxidation reduction potential for the oxidation of calcium polysulphide (CaSx) to calcium thiosulfate (CaS2O3).

Figure 3 is a graphical representation of the batch oxidation of CaSx to CaS2O3 as a function of reaction time.

 Figure 4 is a graphical representation of the residence time of CaS2O3 in a system using two CSTRs in series.

 Figure 5 is a graphical representation of the residence time distribution in a two series CSTR system, showing the impact of improved mixing efficiency through a preferred orientation and position of the liquid feed injection point.

 Figure 6 is a graphical representation illustrating the effect of temperature, pressure and agitation on the rate of the calcium thiosulfate oxidation reaction.

 Figure 7 is a graphic representation of the effectiveness of the oxidation reaction as a function of temperature and pressure.

 Figure 8 is an HPLC chromatogram for thiosulfate, tritionate and tetrathionate.

 Figure 9 is an HPLC chromatogram of the "process water" components that can be used for the preparation of calcium thiosulfate.

 Figure 10 is an HPLC chromatogram of calcium thiosulfate made with process water that has been enriched or fortified by the addition of trionate and tetrathionate.

 Figure 11 is a graphic representation of the effect of pH on the efficacy of the flocculant in a calcium thiosulfate suspension produced according to the invention.

 Figure 12 is a graphical representation of the pH stability of calcium thiosulfate over time.

 Figure 13 is a graphical representation of the relationship between the specific gravity of a solution of calcium thiosulfate and its CaS2O3 assay or its corresponding calcium content.

 Figure 14 is a graphical representation of the relationship between the assay of a calcium thiosulfate solution and the percentage by weight of calcium in the calcium thiosulfate solution.

 Figure 15 is a graphical representation of the specific gravity of calcium thiosulfate at varying temperatures.

DETAILED DESCRIPTION OF THE INVENTION

The invention involves a continuous process for the production of calcium thiosulfate (CaS2O3) from calcium polysulphide (CaSx) that allows large volumes of production with minimal peak consumption of cooling water, lower product transfer flows and less problems of foaming in case of high pressure operation. The present invention is a significant and unexpected improvement over the state-of-the-art batch processes for the production of calcium thiosulfate.

The process involves the use of a series of tank reactors with continuous agitation (CSTR) under such conditions that high purity calcium thiosulfate is produced continuously with minimal dissolved by-products, and little or no polythionate. This calcium thiosulfate product is suitable for leaching precious metals due to its minimal polythionate content, as well as other uses.

The term CSTR as used herein is intended to encompass any vessel or tank within which calcium polysulphide can be oxidized to calcium thiosulfate. Preferred CSTRs are highly efficient mixers. Examples of CSTR include, but are not limited to the following: tanks equipped with thrusters or other mixer type agitators and series mixing equipment with high shear and high impact such as bubble columns, packed columns, tray columns, columns spray, jet loops, pipes / tubes and tanks with cavitation technology.

In a preferred embodiment of the invention, the CSTRs used for the oxidation reaction are tanks equipped with propellants or blades for stirring the reactive materials in the tank. In an even more preferred embodiment, the tanks are equipped with thrusters that have three stirring blades arranged vertically in the thruster. While tanks with a single propeller having three stirring blades are preferred, the invention also encompasses tanks having more than one propeller, as well as propellants having less than or more than three stirring blades each.

If oxygen is used as the oxidizing agent, as it is in a preferred embodiment of the invention, then the CSTR must be able to be pressurized and depressurized.

Preferably, the CSTRs are equipped with heating and cooling means, such as heating and / or cooling coatings. CSTRs used in the present invention are also provided with means to transfer materials, such as pipes and / or tubes and pumps, to facilitate the transfer of materials (such as reagents, starting materials, gases, liquids, reaction products , etc.) inside and outside the CSTR, including from a CSTR to a subsequent CSTR in series.

Preferably, the CSTRs used in this invention are equipped with devices for controlling temperature, pH and pressure, as well as other conditions such as a device for measuring oxidation reduction potential (ORP) and also for sampling CSTR content. Controlling the pressure and other conditions, and sampling the content, is desirable to determine the progress of the oxidation of calcium polysulphide to calcium thiosulfate. The process of the invention provides a way of oxidizing the entire calcium polysulphide to calcium thiosulfate, while preventing overheating of the calcium thiosulfate that would cause it to decompose. Once all or substantially all of the CaSx in the reactor has been oxidized to calcium thiosulfate, the solution is transferred to storage or other equipment for further processing, as discussed herein.

The following are methods to evaluate samples to determine the progress of oxidation.

Control of oxidation reduction potential (ORP). CaSx has a certain ORP value, and as it oxidizes to calcium thiosulfate, the ORP changes. Once the ORP value stops changing, the oxidation is complete.

Pressure control During the process, the pressure in the reactor will decrease as CaSx oxidizes. Once all CaSx has been oxidized, the pressure in the reactor will drop and remain stable, that is, it will stop decreasing.

Color change control. Calcium thiosulfate is a colorless, transparent solution. If all CaSx has not been oxidized, the solution will have color. For example, CaSx has a red color, but as it oxidizes to calcium thiosulfate, the solution changes from red to orange to yellow, becoming lighter in color as the production of calcium thiosulfate increases.

Lead acetate paper The presence of H2S indicates that the oxidation reaction is not complete (ie, not all of the CaSx has been oxidized to calcium thiosulfate), because it means that not all of the sulfide has yet been converted to thiosulfate. Therefore, a quick and easy way to monitor the progress and conclusion of the oxidation reaction and conversion of polysulfide to thiosulfate is the presence or absence of a color change in lead acetate paper exposed to the sample. If polysulfide exists, lead acetate paper turns black.

Other methods and devices can be used to check the presence of H2S in addition to or instead of lead acetate paper.

The invention further involves the use of specific process conditions, which include operating pressures, operating temperatures, stirring speeds for mixing the reagents and molar ratios of feed of the raw materials. Using these specified conditions, the process of the present invention provides a superior product to the state of the art batch processes, which produces a high purity calcium thiosulfate product, high concentration with minimal insoluble by-products and few oxidation by-products. soluble, such as polythionates. The resulting product does not need to be concentrated further by evaporation or other means, which results in significant savings in time to prepare a salable product, as well as monetary savings because less materials and treatment processes are needed to concentrate and / or filter the product before that is in a usable and salable state.

The process involves the oxidation of a suspension of calcium sulphide (also referred to herein as calcium polysulphide or CaSx) with pressurized oxygen, preferably pure oxygen. In an alternative embodiment, sulfur dioxide can be used to oxidize the calcium sulphide suspension, with alterations appropriate to the process. For example, oxidation using sulfur dioxide can be achieved at atmospheric pressure.

The raw materials used in this invention are economical calcium oxide, sulfur and oxygen, which are all used in the formation of the desired product. No secondary byproduct is formed. Residual solids are formed from impurities in the commercial calcium oxide starting material. In the process according to the invention, the residual solids in the calcium thiosulfate are minimal, and are typically in the amount of less than 2% by weight of the amount of the calcium thiosulfate in the solution.

The process described herein also prevents undesirable oxidation of the calcium thiosulfate product to calcium sulfate. In general, filtration of colloidal suspensions of mixtures of calcium sulfate, calcium sulphite, calcium carbonate and sulfur is slow and difficult. Under the conditions described herein according to the present invention, very little of these by-products is formed and the by-products that are formed are quickly and easily separated by adjusting the pH and by using a certain flocculant.

 Figure 1 illustrates a scheme for an exemplary non-limiting embodiment of the process of the invention.

There are two main steps in the calcium thiosulfate production process according to the invention: production of calcium polysulphide (CaSx) and oxidation of CaSx to calcium thiosulfate (CaS2O3).

The process of this invention comprises three general sections: a lime quench section, a reaction section and a filtration section.

In the shutdown section, a heavy amount of dried lime and a measured amount of water are mixed to produce a lime suspension. Dry lime consists mainly of calcium oxide, also known as quicklime, and reacts with water to produce calcium hydroxide, also known as slaked lime. The reaction is exothermic:

CaO + H2O - Ca (0H) 2

The lime suspension is then transferred to reactor 1 shown in Figure 1 for the formation of calcium sulphide, and oxidation in subsequent reactors 2, 3 and 4, according to the following steps:

Production of calcium sulphide and thiosulfate

3Ca (OH) 2 + 4S - 2Ca5 + Ca52O3 + 3H2O

Sulfide dissolves additional sulfur to form polysulfide (CaSx)

2CaS + CaS2O3 + (2x - 2) S - 2Ca5x + CaS2O3

Global CaSx reaction

3Ca (OH) 2 + (2x + 2) S - 2Ca5x + CaS2O3 + 3H2O

Oxidation reaction of CaSx to calcium thiosulfate

2CaSx + 3O2 (x = 2) - 2Ca52O3

Global calcium thiosulfate reaction

3Ca (OH) 2 + 6S + 3O2 - 2Ca52O3 + 3H2O

The reaction section of the process is carried out through the CSTR operation, which produces high purity and high concentration calcium thiosulfate at about pH 11. Acidification of the product with an appropriate acid lowers the pH to a more desirable 7 , 5-8.5. The resulting calcium thiosulfate product has a concentration of approximately 22-29% and is stable for months.

DESCRIPTION OF THE CONTINUOUS CALCIUM TIOSULFATE PROCESS

Step 1: Continuous production of calcium polysulfide (CaSx)

In this continuous process, the raw materials (suspension of lime, sulfur and water for dilution) are fed into the CaSx reactor (calcium polysulfide) based on the required production load and the consumption ratios of the raw materials as defined by chemical reactions (See Figure 1, CSTR - reactor 1). The feed of raw materials is balanced by the output of the CaSx solution by level or flow control. This step is preferably performed in a CSTR.

The term "dilution water" is any water suitable for diluting the lime and sulfur suspension added in this first phase. For example, it can be used as dilution water, "tap" water, well water, deionized (distilled) water or even recycled water from the gold leaching process, which has increased levels of polythionates. The kinetics of the CaSx reaction is such that the reaction will be almost complete during the residence time in the reactor. Any unreacted raw material will subsequently react in the CaSx storage tanks or in the calcium thiosulfate reactor.

With respect to Figure 1, sulfur is added to the lime suspension in reactor 1. The mixture is stirred, and an exothermic reaction takes place between the sulfur and lime to produce a suspension of calcium sulphide (CaSx). The following reaction takes place in the CaSx reactor (reactor 1):

Production of calcium polysulphide:

3Ca (OH) 2 + (2x + 2) S - 2Ca5x + CaS2O3 + 3H2O

Although the reaction is exothermic, heat must be supplied to the reactor to heat the raw materials and maintain the reaction at the required operating temperature of about 90-98 ° C or about 195-208 ° F. It is preferable to carry out this reaction as quickly as possible, since some calcium thiosulfate will be formed in this step, and the calcium thiosulfate decomposes to calcium sulphite (CaSO3) at approximately 100 ° C. The shorter the time of this reaction, the less time the calcium thiosulfate is exposed to decomposition temperatures. Calcium thiosulfate that decomposes to calcium sulphite (CaSO3) cannot be recycled in the process and will undesirably increase the solid by-product.

The optimal calcium sulphide solution will contain enough calcium to correspond to approximately 25-29.5% calcium thiosulfate - this is approximately 6.6-7.8% Ca ++. As indicated in the above reaction formula, the calcium sulphide solution will contain some calcium thiosulfate. However, calcium thiosulfate will decompose at temperatures near boiling (100 ° C), forming undesirable solid (insoluble) by-products. Therefore, while it may be desirable to have an increased concentration of calcium thiosulfate in the calcium sulphide solution, this should be weighed against the risk that the calcium thiosulfate that is present during the CaSx production reaction will decompose to form undesirable solid byproducts. The operators of the process according to the invention will reach a compromise between the level of calcium thiosulfate in calcium sulphide and the potential for by-products.

The number "x" for the polysulphide part of calcium sulfide (that is, the "x" in CaSx) should be as close to 2 as possible, because the equation for the oxidation of calcium polysulphide is: CaSx + O2 - Ca52O3 + (x-2) S. Theoretically, if x = 2, residual sulfur (which forms undesirable by-products) will not exist. However, the higher the concentration of thiosulfate in the calcium sulphide solution, the greater the number x for the remaining polysulfides. Finally, the lower the number x in the remaining polysulfides, the lower the solubility of the solution. Each contributing factor here should be analyzed to determine what are the priorities, for example, increased concentration of thiosulfate and / or the solubility of calcium sulfide reaction products. The underlying concern is the stability of calcium thiosulfate at temperatures near boiling.

The calcium sulfide synthesis part of the process of the invention should be carried out in the shortest possible time, to increase the production speed, and to decrease the decomposition of the product that will be produced at elevated temperatures over time. To determine the point at which the synthesis of calcium sulphide must cease and oxidation must begin (and the materials in the reactor 1 shown in Figure 1 must be transferred to the reactor 2), the point at which the concentration should be determined of Ca ++ is maximum. The final calcium sulphide intermediate is a suspension. A sample of the calcium sulphide suspension is then filtered, and the sample is then titrated with EDTA to quickly and easily control the concentration of Ca ++. When the concentration of Ca ++ stabilizes, the calcium sulphide suspension should be transferred to the next reactor for the oxidation of CaSx.

The optimum temperature was approximately 90-92 ° C. It was determined that calcium thiosulfate, in pure solution, decomposed at 97 ° C. The high production of H2S at temperatures above 9294 ° C is also noted. In addition, problematic foam formation occurs during the synthesis of calcium sulphide carried out near the boiling point, but this is not apparent at slightly lower temperatures. The shutdown of CaO alone increases the temperature of the initial raw materials to 50-60 ° C. Samples were taken periodically from each suspension that reacts to track the progress of the Ca ++ concentration. The concentration of Ca ++ as CaSx is stabilized from 135 to 190 minutes at approximately 90-92 ° C.

If lime (CaO) or lime off [Ca (OH) 2] is used as a source of calcium, a suspension of lime or lime is quenched in water is first provided and then sulfur is added to the suspension. Either a pre-existing suspension of lime is used or a suspension is formed by switching off, which is mixing the lime with water to form lime. In a preferred embodiment, the lime is from about 96% to about 99% pure. Lime of lower purity can be used, but at the cost of higher solid by-products at the end of the reaction, as well as a slower reaction kinetics. It has also been found that these by-products and inerts consume reactor volume and decrease overall production capacity.

Preferably, the mixture of sulfur and lime suspension is heated. In a preferred embodiment, it is heated to at least about 70 ° C (about 158 ° F). More preferably, it is heated to a temperature in the range of from about 85 to about 99 ° C (about 185-210 ° C). Even more preferably, it is heated to a temperature in the range of from about 90 to about 92 ° C (about 195-198 ° F).

Sulfur is preferably combined with calcium hydroxide at a molar ratio of sulfur to calcium hydroxide from about 1: 1 to about 6: 1. More preferably, the molar ratio is about 3.4: 1 to about 3.8: 1. Even more preferably, the ratio is approximately 3.6: 1.

In a preferred embodiment, the molar ratio of sulfur to calcium hydroxide to water is at least about 2: 6: 30.

In another preferred embodiment, the molar ratio of sulfur to calcium hydroxide to water is about 3.6 to about 4.9: 1: 25.5.

If the ratio of sulfur to calcium hydroxide to water used is around 3.6: 1: 25.5, then the reaction takes about 2-6 hours to complete.

In an exemplary embodiment, the calcium polysulphide solution was produced in a molar ratio from about 3.6 to 4 moles of sulfur per mole of calcium hydroxide, and then the amount of calcium hydroxide required to obtain the ratio Stoichiometric 2: 1 is added before or during the oxidation step.

Step 2: Continuous production of calcium thiosulfate (CaS2O3) in CSTR

According to a preferred embodiment of the invention represented in Figure 1, the oxidation of CaSx to calcium thiosulfate is done in two CSTRs in series (reactor 2 and reactor 3). The calcium polysulphide solution is transferred from reactor 1 to reactor 2. Alternative embodiments of the invention can use more than two CSTRs in series to perform this step. The reaction that takes place in both CSTRs is the following oxidation reaction:

2CaSx + 3O2 (x = 2) - 2Ca52O3

The CSTRs (reactors 2 and 3 shown in Figure 1) are used to stir the mixtures therein, to facilitate the oxidation of the CaSx solutions (and CaSx and calcium thiosulfate) maximizing the contact between the oxygen introduced into, and the liquid in, the reactor. In a preferred embodiment, either or both reactors 2 and 3 stir the mixtures therein at a speed of about 100-1200 rpm at about 70 ° C. In another embodiment, stirring occurs at a speed of about 600-1000 rpm at about 70 ° C. In yet another embodiment, stirring occurs at a speed of about 600-900 rpm at about 70 ° C. In yet another embodiment, stirring occurs at a speed of about 600-800 rpm at about 70 ° C.

In an alternative embodiment of the invention, either or both reactors 2 and 3 stir the mixtures therein at a speed of about 900-1500 rpm at about 90 ° C. In another embodiment, stirring occurs at a speed of about 1200-1500 rpm at about 90 ° C. In yet another embodiment, stirring occurs at a speed of about 1500 rpm at about 90 ° C.

The first of the two CSTRs in series (reactor 2) oxidizes approximately 60-90% of the CaSx in solution that was produced in reactor 1. In a preferred embodiment, reactor 2 is pressurized with oxygen and mixed with a High efficiency mixer to maximize contact between oxygen gas and calcium thiosulfate solution.

In each of reactors 2 and 3, the oxidation of calcium polysulphide to calcium thiosulfate is achieved with pressure. These reactors are preferably purged so that the air in them is replaced with oxygen gas, and the pressure inside the reactors increases. The reactors are pressurized at about 15-80 psig, more preferably at about 40-80 psig and the contents are heated to about 70-95 ° C, more preferably up to about 70-75 ° C.

The partially oxidized calcium thiosulfate solution produced in reactor 2 is continuously pumped from reactor 2 to the second serial CSTR (reactor 3), where it oxidizes up to about 70-95% of the complete oxidation during its residence time in this reactor. A continuous flow of almost completely oxidized calcium thiosulfate is discharged into storage (not shown in Figure 1) or into the next phase of the process. Since the reaction is exothermic, all CSTR oxidation reactors (reactors 2 and 3) must be maintained at the desired oxidation temperature (approximately 60-80 ° C). In a preferred embodiment, oxidation occurs in a reactor maintained at approximately 70 ° C. The operating temperature can be increased to accelerate the chemical reaction and minimize the reaction time, but it should not increase beyond about 98 ° C, or more preferably beyond about 94 ° C, to avoid causing calcium thiosulfate to degrade, so which produces the undesirable formation of polythionates.

The inventors determined that the use of lime of greater purity produced a reduced amount of solid by-products. Preferably, the lime is from about 96% to about 99% pure. The amount of solid by-products decreased to less than 2% by weight of the total product using lighter purity. The reaction time also decreased using lighter purity.

Examples of lime with different levels of purity are as follows: source 1:> 99% by weight of Ca (OH) 2; source 2: 97.40% by weight of Ca (OH) 2; source 3: 94.20% by weight of Ca (OH) 2; and source 4: 95.80% by weight of Ca (OH) 2. Lime of higher purity reacts more quickly and forms less solid by-products during the reaction.

The oxygen used for the purpose of oxidizing can be supplied by atmospheric air or by a source of enriched oxygen supply. While atmospheric air is an option, an enriched oxygen supply is preferred, because the higher the oxygen concentration, the faster the reaction will occur. The oxygen is distributed to the oxidation reactor at the desired pressure and at the volume required to support the oxidation reaction. The main factors that determine the rate of oxidation and the time to complete the oxidation reaction are the oxygen concentration, the contact area of the calcium sulfide suspension with the oxygen, and the reaction temperature. The objective is to complete the reaction in a reasonable amount of time consistent with the production requirements and avoid prolonged reaction times that can produce increased amounts of decomposition and oxidation products to form calcium sulfate.

The oxygen supplied by the air at atmospheric pressure has a low concentration, which produces longer reaction times that are not preferred for industrial production. The availability of oxygen for the reaction can be increased by compressing the air at higher pressures, which maximizes the contact between oxygen and the CaSx / CaS2O3 solution. Increasing the air pressure to five atmospheres or around 60 psig increases the oxygen available for the reaction to approximately the same level as using pure oxygen in atmospheric conditions. When air is used, inert gases must be allowed to escape or purge periodically. Alternatively, preferably pure oxygen is used, in part because it can be used at lower pressures and with minimum requirements to purge inert gases.

In CSTR reactors, the preferred pressures are in the range of 15-80 psig, and more preferably 60-80 psig. Even more preferably, the pressure is 80 psig.

An alternative to the use of oxygen or air as the oxidant is to use sulfur dioxide, which can be used without pressurizing CSTR oxidation reactors, that is, it can be used at atmospheric pressure.

In a preferred embodiment, the CSTRs are each equipped with a propeller, each propellant has three stirring blades arranged vertically relative to each other. The highest stirring blade (first blade) is just below or on a surface adjacent to the CaSx / CaS2O3 liquid solution in the CSTR, and the lowest stirring blade (the second blade) is located adjacent to the bottom of the CSTR . The oxidizing agent, preferably oxygen, is preferably introduced into the CaSx / CaS2O3 solution in the CSTR at a point adjacent to the medium stirring blade (the third blade), at a point between the highest and lowest stirring blades , also referred to herein as feed injection point.

One or more mechanisms are preferably supplied to the CSTR reactors to allow continuous purging of the vapor phase to prevent inert formation in the steam space and to reduce foaming.

Step 3: Completion step up to 100% oxidation

Because some product fractions will always lack residence time in the system for complete oxidation, a completion step will be required if the CaSx / calcium thiosulfate solution produced in the

Oxidation reactors (for example, reactors 2 and 3 shown in Figure 1) will be completely oxidized, as preferred. A representation of a termination reactor (reactor 4) is illustrated in Figure 1. The reactor 4 may consist of a single reactor, such as a cavitation reactor, or it may consist of a series of reactors, as discussed below. Two possible solutions are proposed to completely oxidize the solution:

one.

It has been shown that a cavitation reactor that operates at oxygen pressures of up to about 3000 psig completes the oxidation in a significantly reduced time. At low oxygen pressures, the cavitation reactor can be used to terminate approximately 1% to 5% final oxidation. At oxygen pressures of up to about 3000 psig, the cavitation reactor can oxidize approximately 50-100% of the CaSx to calcium thiosulfate at faster rates than a conventional CSTR.

2.

A series of semi-continuous reactors that terminate the solution of CaSx / calcium thiosulfate in an automated batch sequence, providing an additional oxidation step in a batch reactor. Oxidation can be controlled using a fixed oxidation time or using in-line measurements that indicate complete oxidation of the product, such as calibrated ORP measurements.

Oxidation of the calcium polysulphide intermediate (CaSx) by oxygen to the calcium thiosulfate product was performed using the oxidation reduction potential (ORP) to determine the progress of the oxidation process. This was done to ensure maximum conversion for the highest yield, avoid excessive oxidation and avoid the formation of soluble by-products such as polythionates and insoluble by-products such as sulfate. The stoichiometric reactions followed were:

 3Ca (OH) 2 + 6.2S - 2Ca52.1 + CaS2O3 + 3H2O (1)

 2CaS2,1 + CaS2O3 + 3H2O + 3O2 - 3CaS2O3 + 3H2O + 0.2S (2)

The oxidation reduction potential (ORP) values were measured using an ORP electrode. Verification of electrode function was confirmed using an ORP standard. ORP values were measured under similar reaction conditions (T, P and stirring speed) during four sets of reactions. The results are shown in figure 2.

The data indicates that, as CaSx oxidation progresses, the redox potential increases. In all four reactions, there is a point where the change in potential is significant and Mx My approaches zero. 5e warns of a point of equivalence in all reactions. The measurement of the ORP during the oxidation of CaSx to calcium thiosulfate determined exactly the termination of the oxidation process. A sharp increase in the ORP of the CaSx suspension is noted when the oxidation is complete.

OXIDATION OF CALCIUM TIOSULFATE: KINETICS OF REACTION FOR A BATCH PROCESS

The inventors determined that the total time per batch or reaction (until 100% termination) to oxidize calcium thiosulfate depends on different parameters, such as stirring, reaction temperature, reaction pressure, etc. The impact of these parameters will be further explained in this document. However, regardless of these parameters, the oxidation rate can be expressed as a function of the relative reaction time (as a fraction of 0-100% of total batch time).

 Figure 3 shows a non-linear relationship, with a high oxidation rate at the beginning, which is braked towards the end of the batch. This explains the higher oxygen consumption and higher cooling requirements (reaction heat) at the beginning of the batch, compared to the end of the batch.

CALCIUM TIOSULFATE OXIDATION: IN A CSTR

An important consideration in maintaining a good oxidation rate is to provide efficient gas / liquid contact, which provides adequate contact area and contact time for the gas carrying oxygen and the calcium sulphide liquid suspension to react. Contacting is important because the reaction takes place mainly at the oxygen-liquid suspension gas interface. If this interface area is not adequate, the reaction will be slow, which produces a large number of undesirable by-products.

To provide sufficient residence time for the oxidation reaction, a series of CSTRs are used in the present invention. Using a series of CSTR keeps the product longer in the system compared to a reactor.

 Figure 4 graphically illustrates the results of a simulation of two CSTRs in series. The graph shows the fraction of product that remains in the system, after it was loaded into the first reactor (corresponding to reactor 2 in Figure 1) at time 0 and assuming perfect mixing. It is shown that some of the product from the first reactor is immediately lost (reactor 2). The reactor 2 loses the product exponentially. As a consequence, the second reactor (corresponding to reactor 3 in Figure 1) is slowly fed with the product loaded through injection at a point in time 0. At the same time, it also loses product, which produces the curve for the second reactor The addition of both curves ("global") shows the fraction of product that remains in the total system (ie, the first and second reactors together) over time.

This explains that part of the product loaded to the system barely has residence time to be oxidized, while another fraction of the product may be in the system for a very long time. Both incomplete oxidation and excessive oxidation could have a detrimental effect on the final product.

This also explains why some fraction of the product will never oxidize completely, because it did not have enough residence time in the reactor. As a consequence, an end step will always be required to completely complete the reaction up to 100% oxidation.

As shown in Figure 1, a series of CSTRs are used for the continuous oxidation of CaSx to calcium thiosulfate. The CSTRs used in the invention operate at particular pressures and temperatures, using oxygen or air. Oxygen is preferred. In an alternative embodiment, instead of oxygen, sulfur dioxide can be used as the oxidizing agent at atmospheric pressure.

The CSTR reactors of the invention have feed injection points located at certain points that minimize the feed material avoiding passing through the system without being reacted. This maximizes the contact between oxygen and liquid in the reactor, which subsequently increases the residence time in the reactor, using high efficiency mixers that maximize gas-liquid dispersion as well as solid suspension. In a preferred embodiment of the invention, the CSTR has a rotating agitator or mixer, comprising a central rotating shaft having two or more stirring blades or propellers arranged vertically relative to each other on the axis. In an even more preferred reaction form, the agitator or mixer has three or more blades arranged vertically relative to each other on the shaft, and the injection point of the oxygen feed is located adjacent to the middle blade.

A continuous process will typically require a completion step to complete the 100% oxidation reaction, due to the distribution of residence time in the system. Figure 5 shows the residence time distribution in a two-series CSTR system - perfect mix against the original reactor design and the improved reactor design at a flow of 55 gpm for a reactor volume of 8500 gallons. "Original" refers to a reactor with a liquid injection point at the top of the reactor according to an embodiment of the invention. "Enhanced" refers to a preferred embodiment of the invention, wherein the reactor has an agitator with three stirring blades (propeller) spaced vertically spaced apart from each other, and where the liquid injection point is located adjacent to the medium stirring shovel. The preferred embodiment has a longer residence time in the reactor, and is closer to the theoretical residence time than the so-called "original" embodiment. The term "perfect blend" refers to the theoretical residence time.

CALCIUM TIOSULFATE OXIDATION: REACTION EFFECTIVENESS INCREASED TO HIGHER OPERATING PRESSURES

An important factor in the oxidation of intermediate calcium polysulphide to calcium thiosulfate product at high concentration is the oxidation reaction pressure. The effect of pressure on the concentration of the product is shown in Figure 6.

The operating pressure can be increased to maximize contact between oxygen and the CaSx / calcium thiosulfate solution. In an installation of a laboratory-scale equipment of a process and system according to the invention, increasing the pressure from 15-20 psig to 80 psig decreased the overall oxidation time from 70 to 75%. Subsequently, this allows oxidation to take place at lower temperatures while maintaining the efficiencies of the reaction. This improvement is valid for processes in discontinuous reactors as well as for continuous processes. The results of different laboratory tests are shown in table 1.

It should be noted that the incremental improvement in efficiency decreases with increasing pressure. A compromise has to be reached between the operating pressure and greater reaction efficiency on the one hand and equipment cost on the other hand.

Rpm stirrer

Temp. reaction ºC O2 psig pressure Reaction time until complete oxidation minutes

750

 70 twenty 243

Four. Five

 117

60

 84

80

 64

80

 60

Table 1: Improved reaction times at higher operating pressures

CALCIUM TIOSULFATE OXIDATION: CSTR OXIDATION SIMULATION

By combining the distribution of residence time in Figure 4 with the speed of

In batch oxidation in Figure 3, the oxidation efficiency of the configuration of two CSTRs in series can be predicted. Therefore, the fraction of product left by CSTRs after a certain period of residence (derived from Figure 4) must be multiplied with the oxidation level (0-100%) of the product at this specific residence time (derived from Figure 3). The accumulation of the result will indicate the level of oxidation expected at the exit of the second CSTR, depending on the normal reaction time of the batch. The result is shown in table 2.

10 The more efficient the reactor can be made, considering the preferred conditions disclosed in this document (mixture, temperature and pressure), the higher the level of oxidation of the product in the discharge of the second CSTR:

Efficiency of the reactor as oxidation time per batch [min]

% oxidized after 2 CSTR at 55 gpm

120

 93.6

150

 91.2

200

 86.7

250

 82.3

Table 2: Expected oxidation levels at the discharge of the second CSTR, as a function of reactor efficiency 15 (measured as reaction time per batch in a similar reactor design)

With a very efficient reactor design (normal oxidation time in a similar 120 minute batch reaction), an oxidation level of 93% is expected at the output of a series of two CSTRs. In a less efficient design (normal oxidation time in a similar 250 minute batch reaction), the expected level of oxidation at the exit of the second CSTR will be only 82%.

20 CALCIUM TIOSULFATE OXIDATION: OPTIMAL REACTION TEMPERATURE AND POLYTIONATES

In a batch laboratory reactor, different oxidation tests were performed at different oxidation temperatures. Higher oxidation temperatures showed faster reaction times (increasing the operating temperature from 70 ° C to 90 ° C decreased reaction times in laboratory equipment to less than 30% of initial reaction time). The effect of temperature on the time of

The reaction is shown in Figure 7.

However, increased oxidation rates also produced increased levels of polythionates in the final product. This is due to an increased oxidation of the thiosulfate molecule.

3 CaS2O3 + 2 O2 + H2O - 2 Ca53O6 + Ca (OH) 2

4 CaS2O3 + O2 + 2 H2O - 2 Ca54O6 + 2 Ca (OH) 2

In the different tests, it was confirmed that polythionate levels increased with increasing oxidation. That explains why higher levels of polythionates were seen when oxidized for extended times.

or at high temperatures.

The polythionates were controlled using HPLC. Figure 8 shows a typical HPLC chromatogram for thiosulfate (2), trionate (3) and tetrathionate (4). Trionate was produced by oxidation of thiosulfate. However, not all thiosulfate was oxidized, and as a result the trionate contained some impurity of the starting thiosulfate (the impurity is peak 2 in the chromatogram).

For the specific application of gold leaching using calcium thiosulfate, high levels of polythionates in calcium thiosulfate should be avoided. This is because the polythionates are heavily charged in the resin used in the gold leaching process. Therefore, they reduce the gold and copper charge of the resin, as discussed in U.S. Patent No. 6,632,264 B2 and U.S. Patent No. 6,344,068 B1. The present invention avoids and / or minimizes the production of polythionates, maintaining the reaction temperature during the oxidation of CaSx at approximately 70 ° C (160 ° F). The reaction temperature can be increased to higher levels (up to about 90 ° C or 195 ° F) to accelerate the reaction, but only if the polythionate levels remain within acceptable limits.

In one embodiment of the invention, higher levels of polythionates may be permissible at the outlet of the reactor, because the polythionates are unstable and will decompose to thiosulfate and sulphite at elevated pH and elevated temperature (US Patent No. 6,632. 264 B2):

2 S4O62- + 6 OH-- 3 52O32- + 2 SO32- + 3 H2O

2 S3O62- + 6 OH-- 52O32- + 4 SO32- + 3 H2O

The mechanism of the tetrathionate decomposition reaction is described in the following equations, in which the tetrathionate first decomposes to thiosulfate and trionate, and the latter further decomposes to thiosulfate and sulphite:

4 S4O62- + 6 OH-- 5 52O32- + 2 S3O62- + 3 H2O

2 S3O62- + 6 OH-- 52O32- + 4 SO32- + 3 H2O

If pH, temperature and residence time are high enough, polythionate levels will drop due to previous decomposition reactions. This is a commitment that must be reached during the operation and / or design of the thiosulfate plant.

DESTRUCTION OF POLITIONATOS IN WATER OF THE RECOVERED PROCESS

A major advantage of the calcium thiosulfate process according to the present invention is the complete destruction of polythionates in the CaSx process. This is specifically useful in leaching processes for precious metals, since there will be no polythionate formation, which makes it possible to recycle water from the leaching process for use in the thiosulfate production process according to the invention. Process water is the solution that remains after using a thiosulfate solution to leach gold from gold ore and that the gold-thiosulfate complex has been removed from it. A series of tests were performed to track the polythionate content of calcium thiosulfate production.

In the first experiment, a sample of the process water mentioned above was used for the preparation of calcium thiosulfate. Lime, water (from process water) and sulfur were heated using appropriate molar ratios and temperature. The resulting calcium polysulphide was oxidized at appropriate temperature and pressure. HPLC analyzes of this solution showed the presence of 2 mg / l of trionate and 9 mg / l of tetrathionate.

In the second experiment, the process water solution was fortified with 3.63 g / l of trionate and 0.62 g / l of tetrathionate. An HPLC chromatogram of this "process water" solution (thiosulfate (2), trionate (3) and tetrathionate (4)) is shown in Figure 9. This process water was used in the production of a calcium thiosulfate solution using the conditions described herein for the production of calcium thiosulfate. Analyzes of this solution of calcium thiosulfate by HPLC showed the same amounts of trionate or tetrathionate compared to the previous experiment, which did not use water from the fortified process, indicating the destruction of polythionates during the process. This demonstrates that the final levels of polythionates in the calcium thiosulfate produced according to this invention are independent of the polythionate levels of the incoming feed streams. The lack of polythionate peaks in Figure 10 shows the destruction of polythionates.

FILTRATION OF THE CALCIUM TIOSULFATE SOLUTION

Colloidal suspensions of calcium salts, such as sulfate, sulphite and suspended sulfur, are generally difficult to filter. Flocculants and coagulants have been used together with filter aids for difficult to filter suspensions. There is no comprehensive quantitative theory to predict the behavior of these materials that can be used for their selection. This should finally be determined experimentally. Different anionic and non-anionic flocculants were used for the filtration efficiency of the resulting completely oxidized calcium thiosulfate solution. The calcium thiosulfate solution produced according to this invention has much smaller amounts of the calcium salts mentioned above, but may still have some. By

Thus, calcium thiosulfate can be called a suspension due to the presence of sulfate and sulphite salts and sulfur in the solution before the application of the filtering and filtration aid.

Filtration studies were carried out using diatomaceous earth for the precoating that was

mixed directly in the calcium thiosulfate suspension. The amount of diatomaceous earth was 0.125% of

the suspension. Since the application of flocculants was studied in a temperature range, the objective was

10 verify that the temperature of the suspension did not compromise the efficacy of the flocculant. The flocculant dose was varied at each temperature until the appearance of the flocculant was consistent. The temperature rise to 55 ° C increased the filtration rate.

The flocculant dose from underdosing to overdosing was also investigated. In general, the best efficacy was achieved when approximately 50-70 ug / gm flocculant to suspension was used. The size 15 of the flocculant did not seem to compromise the filtration rate. Anionic flocculants performed much better

than the non-anionic.

pH of the suspension

Filtration speed (gm / min) % filtrate (by total weight of the suspension) % solid cake (by total weight of the suspension)

Control

 Flocculant  Control Flocculant  Control  Flocculant

10.5

 4.49 10.41 90.5  91.8 9.0 8.7

10.0

 6.14 9.01 90.2  91.8 8.8 8.1

9.0

 9.87 15.54 91.8  92.5 8.1 8.6

8.5

 16.43 20.22 92.9  94.3 7.1 7.6

8.0

 17.71 19.18 93.2  96.1 7.5 7.0

7.5

 22.33 14.88 94.7  94.7 7.3 7.3

7.0

 15.14 17.70 94.4  97.1 6.0 5.9

6.0

 17.81 22.34 96.4  97.3 4.9 5.5

Table 3. Effects of pH adjustment on calcium thiosulfate filtration

The effect of pH on flocculant efficacy and ease of filtration of the suspension was also studied. It was observed that anionic flocculants lost their effectiveness at pH> 11. The evaluation of the suspensions of

20 calcium thiosulfate treated with an anionic flocculant, AE874, and untreated suspensions were performed at pH values ranging from 6.0 to 10.5. Evaluation parameters included filtration rate, relative settling after consistent time and% solid cake and% filtering compared to the original weight of the suspension. The data is evaluated in table 3 and figure 11.

The data indicates that the filtration rate improves for both untreated suspensions and for

25 suspensions treated with flocculant as the pH is reduced. However, the filtration rate in flocculant-treated solutions still exceeds that of untreated mixtures. The optimum pH is indicated between 8-8.5. (For the control suspension at pH = 10.5, speed = 4.5 gm / min; the control at pH = 8.5, speed = 16.4 gm / min and flocculant treated suspension at pH = 8.5, speed = 20.2 gm / min). The data also indicates that as the pH is reduced, the amount of filtrate increases and the amount of filter cake solids decreases, relative to the amount of the

30 suspension treated.

ADJUSTMENT WITH PRODUCT ACID CALCIUM TIOSULFATE

Different acids were tested for pH adjustment including mineral acids and acetic acid. He

Calcium thiosulfate has a very low buffering capacity and requires a very small amount of acid to

Change your pH In general, strong mineral acids tend to break down calcium thiosulfate, which makes

35 easy to exceed the pH. Acetic acid is recommended; however, a disadvantage is that a larger amount of acetic acid is required to adjust the pH compared to mineral acids.

Calcium thiosulfate at neutral or near pH 7.5-8.5 pH is very stable and has a long shelf life. Be

monitored the pH stability of a sample of calcium thiosulfate for several months. As noted

Graphically in Figure 12, acid-adjusted solutions are stable, while the pH of the solution

40 unadjusted control fell. After 217 days of storage, the pH was 10.27. After 265 days of storage, the pH was 8.22, while the pH of an adjusted sample remained stable. With respect to Figure 12, the "control" curve is calcium thiosulfate that was not adjusted with acid, and the other curves show an adjustment with calcium thiosulfate acid at different acid concentrations. The curve indicated "old CaS2O3" in the calcium thiosulfate solution in which the pH was adjusted with an amount of 2 N acetic acid that was insufficient to bring the solution to the specified pH range after treatment.

DETERMINATION OF PHYSICAL AND CHEMICAL PROPERTIES

The following describes that the calcium thiosulfate produced according to the present invention has physical and chemical properties similar to those identical to calcium thiosulfate produced by prior art processes. Characteristics of calcium thiosulfate were studied to develop a concentration determination based on specific gravity. In addition, the relationship between the concentration of calcium thiosulfate and the% calcium in calcium thiosulfate was determined. The results are shown in Figure 13 and Figure 14.

The specific gravity of 24% calcium thiosulfate with varying temperatures was investigated. The specific gravity of 24% calcium thiosulfate (23.95%) measured at temperatures ranging from 6.0 ° C to 39.5 ° C (from 42.8 ° F to 103.1 ° F) is shown in Figure 15.

The long-term stability of calcium thiosulfate was addressed. The boiling point was determined in solutions from concentrations of 10% to saturation (34%). The boiling point temperature increased with increasing calcium thiosulfate concentration. The boiling point temperature varied from 97-103 ° C. The boiling point for the 24% product was ~ 99.5 ° C. When calcium thiosulfate solutions of all concentrations were brought to boiling temperatures, decomposition occurred. Stability studies were performed at 40 ° C for only one week. All calcium sulfate concentrations maintained stability during this period at 40 ° C in closed bottles.

As is typical with solutions, it was noted that the drop in the freezing point increases with increasing concentration of calcium thiosulfate. The freezing point varies from 0 ° C in the 10% solution, to -8 ° C in the 34% solution. The freezing point for 24% thiosulfate was ~ -4 ° C. It does not precipitate at a reduced temperature, it freezes. All solutions remained stable after freezing and thawing with the exception of the 34% saturated solution. He kept crystals after defrosting. The concentration of Ca ++ fell by 1%. It was frozen to solidify a 24% solution for approximately two weeks to provide data that support the claim that calcium thiosulfate maintains stability despite freezing. Defrosted product test: pH = 7.7, sg = 1,250,% Ca ++ = 6.31%,% calcium thiosulfate = 23.65. The thawed solution contained a small amount of fine white solids. Calcium thiosulfate is extremely stable when stored indoors. The test of a sample solution after many months did not change.

Examples

The following steps 1-5 are involved in an exemplary embodiment of the process of the invention:

Step 1: Lime Shutdown

308 grams of water are placed in a stirred reactor equipped with a thermometer and 404 grams of commercial CaO are loaded into the reactor. The exothermic mixture is stirred for 30-40 minutes for complete shutdown.

Step 2: Preparation of calcium polysulfide

The previously quenched lime [Ca (OH) 2] is heated to 194 ° F and 443 grams of molten sulfur are added with stirring. The heating and stirring continue for 3-4 hours until all the sulfur dissolves.

Three experimental samples of calcium polysulfide were prepared as described above, each using a different amount of quenched lime [Ca (OH) 2]. It was analyzed every

sample, and the results are shown in table 4.

Exp. #

Ca (OH) 2], g S, g Expected CCa,% by weight * CCa in calcium sulfide solution,% by weight Relative amount of solids,% Ca recovery,% CCaS2O3,% by weight **

one

52.0 81.02 7.11 7.26  1.0 99.0 27.45

2

59.7 93.02 7.76 7.99  1.1 97.2 27.0

3

70.0 109.07 8.56 8.17  8.9 70.8 ***

Table 4: preparation of the maximum concentration of the CaSx solution

* The calculation is based on the recovery of 100% Ca ++ of lime in the calcium sulphide solution

** Actual CCaS2O3 in the final calcium thiosulfate solution, produced from the corresponding 5-calcium sulphide solution

*** Experiment # 3 was not completed due to the low calcium recovery in this step

Step 3: Preparation of calcium thiosulfate

The resulting product calcium sulfide (calcium polysulfide) from step 2 is transferred to a stirred reactor that can be pressurized at 4-8 atmospheres and equipped with an air inlet and outlet, a thermometer and a cooling system. Moderate agitation is applied to the mixture to provide a uniform liquid-gas interface without vortexing. All air is purged out of the system by performing three purges consisting of pressurizing the reactor to 10-15 psig, using oxygen, followed by depressurization of the system. The mixture is heated at 55-70 ° C. Oxidation begins by introducing oxygen to the system and maintaining the system pressure at 4-6 psig. Oxidation continues until oxygen is no longer absorbed, which is apparent from the lack of

15 pressure drop or heat rise. About 211 grams of oxygen are consumed.

Step 4: Filtration

The product resulting from step 3 is carefully adjusted to pH 7.5-8.5 in a filtration tank equipped with stirring and a pH electrode with glacial acetic acid. A filtering aid and 20-40 ppm of flocculant are added and the mixture is filtered. The resulting calcium thiosulfate product is a colorless, odorless liquid. It can

20 prepare a product with a concentration close to 30% saturation.

The following example illustrates an embodiment of the continuous process according to the invention, as demonstrated in a laboratory experiment that simulates a CSTR in which calcium thiosulfate is produced continuously without completely oxidizing the product. A subsequent CSTR would be the completion vessel, which would completely oxidize the product to calcium thiosulfate. In the experiment, 1 liter of CaSx suspension is synthesized. The

Half of the suspension of synthesized CaSx is returned to the reactor and oxidation is started in this part. Near the end of the oxidation, 50 ml of the reactor contents are removed and replaced with 50 ml of the retained CaSx suspension. Each sample collected - the intention is for intermediate samples that are close to completion, but not fully processed - is evaluated for S2O3 = by IC and S3O6 = by HPLC, visible color and pH. The data is shown in table 5.

Sample #

Color pH % by weight calcium thiosulfate mg / l of S3O6 =

one

Yellow 10.51 23.69 twenty-one

2

Yellow 10.28 23.52 173

3

Light yellow 9.80 24.01 115

4

Colorless 9.51 24.04 Four. Five

5

Colorless 9.35 24.05 7

6

Yellow 9.79 23.74 758

7

Yellow 10.11 24.77 3952

8

Yellow 9.92 23.73 1576

9

Colorless 9.35 24.18

9

Table 5. Evaluation of almost complete calcium thiosulfate

Note: Sample # 9 is the finished product. The significance of the data is that it demonstrates that the levels of S3O6 = remain low, and more importantly, that the levels of polythionates do not increase during the continuous oxidation process, which produces an end product with almost no S3O6 content = . In addition, the experiment demonstrates that any additional S3O6 = created in this process is destroyed. This procedure also confirms that calcium thiosulfate product remained stable throughout the process.

The following is an illustration of an exemplary embodiment of a large-scale production process according to the invention, where only one CSTR was used, rather than the use of the preferred embodiment of at least two CSTRs for oxidation for produce calcium thiosulfate.

Step 1: Calcium Sulfide Reaction

The formulated amount of water is added through a nozzle at the top of the reactor and stirring begins. The formulated amount of calcium oxide is added and approximately 30 minutes of downtime is allowed. The temperature of the suspension mixture is increased by approximately 22 ° C (40 ° F). The slaked lime produced in this way is transferred to the CaSx reactor. The suspension is heated to 90 ° C and the circulation pump is started.

The formulated amount of sulfur is added and the reaction temperature is maintained at approximately 90 ° C (194 ° F). The reaction mixture is allowed to react at 90 ° C for about 3 hours. At the end of the reaction, all sulfur must have reacted completely and the calcium concentration in the calcium sulphide solution must be maximum. Calcium sulfide is a slightly dense suspension at this point and will form a large mass of soft acicular crystals if allowed to cool to room temperature.

Step 2: oxidation reaction

The calcium sulphide suspension produced in step 1 is transferred to a CSTR calcium thiosulfate reactor and cooled to about 55-75 ° C.

The air in the steam space of the CSTR is purged by pressurizing the reactor with oxygen at approximately 12 psig then decreasing at atmospheric pressure. This is repeated for several purge cycles, to fill the steam space with pure oxygen. Oxidation begins by adjusting the oxygen pressure in the reactor to approximately 5 psig and opening the flow of liquid through the pump.

Sufficient cooling water flow is provided to prevent the reaction temperature from rising above the set operating temperature. The oxidation reaction continues until the oxygen flow to the reactor drops to zero. The reaction mixture does not consume more oxygen and no more heat is generated. At this point, all the polysulphide is converted to calcium thiosulfate and the mixture is very dark gray with greenish blue tint. Circulation continues for approximately another 10-15 minutes to ensure that all polysulfide has oxidized, then the product is cooled to less than about 50 ° C and the agitator and pump are turned off.

Step 3: Filtration

The product from step 2 is then transferred to the filter feed tank. The calculated amount of acetic acid is added to lower the pH of the mixture to pH 7.5-8.5. The required amount of flocculant is added and filtration is started.

Claims (50)

1. A process for preparing calcium thiosulfate comprising the following steps: (a) partially oxidize a solution of calcium polysulphide in a first reactor to produce a solution of calcium polysulphide / calcium thiosulfate (b) transfer the solution produced in step (a) to a second reactor and further oxidize the solution produced in step (a) to produce a solution of calcium thiosulfate; (c) transfer at least a portion of the calcium thiosulfate solution produced in step (b) to a third reactor and substantially oxidize completely the solution produced in step (b); Y (d) recovering the substantially oxidized calcium thiosulfate solution completely produced in step (c).

2.

The process of claim 1, wherein at least one of the steps (a), (b) and (c) further comprises stirring the solution or solutions.

3.

The process of claim 2, wherein at least one of step (a) and step (b) further comprises stirring the solution or solutions in a tank reactor with continuous stirring.

Four.

The process of claim 3, wherein step (a) and / or step (b) further comprises introducing an oxidation agent into the first reactor and / or second reactor at an approximately equidistant point of the level or surface levels of each solution or solutions in the reactor (s) and the bottom of the reactor (s).

5.

The process of claim 3, wherein the first reactor and / or the second reactor has a propeller with stirring blades immersed in the solution or solutions, and the propellant has a first stirring blade disposed on the surface of the solution or solutions , a second stirring blade disposed at the bottom of the reactor or reactors and a third stirring blade disposed between the first and second stirring blades, and

step (a) and / or step (b) further comprises introducing an oxidizing agent in the first reactor and / or second reactor adjacent to the third stirring blade.

6.

The process of claim 2, wherein the solution or solutions are stirred in one or more of the following: bubble columns, packed columns, tray columns, spray columns, mechanically agitated tanks, jet loops, pipes / tubes, stirrers, large-shear and high-impact series equipment, and cavitation reactors.

7.

The process of claim 3, further comprising stirring at a speed in the range from about 100 rpm to about 1200 rpm.

8.

The process of claim 7, further comprising stirring at a speed in the range from about 600 rpm to about 1000 rpm.

9.

The process of claim 8, further comprising stirring at a speed in the range from about 600 rpm to about 800 rpm.

10.

The process of claim 1, wherein step (a) further comprises cooling the calcium polysulphide solution to about 55 ° C to about 75 ° C before oxidizing the solution.

eleven.

The process of claim 1, wherein at least one of the steps (a), (b) and (c) further comprises oxidizing at a temperature from about 70 ° C to about 95 ° C.

12.

The process of claim 11, wherein at least one of the steps (a), (b) and (c) further comprises oxidizing at a temperature from about 70 ° C to about 80 ° C.

13.

The process of claim 12, wherein at least one of the steps (a), (b) and (c) further comprises oxidizing at a temperature from about 70 ° C to about 75 ° C.

14.

The process of claim 1, wherein at least one of the steps (a), (b) and (c) further comprises oxidizing at a pressure from about 15 psig to about 80 psig and the solution is oxidized with oxygen.

fifteen.

The process of claim 14, wherein at least one of the steps (a), (b) and (c) further comprises oxidizing at a pressure from about 40 psig to about 80 psig and the solution is oxidized with oxygen.

16.

The process of claim 1, wherein step (c) further comprises stirring in a third reactor selected from the group consisting of cavitation reactors and semi-continuous reactors.

17.

The process of claim 16, wherein step (c) further comprises oxidizing in a cavitation reactor at oxygen pressures of up to about 3000 psig.

18.

The process of claim 1, wherein step (d) further comprises adjusting the pH of the calcium thiosulfate solution at a pH range from about 7.5 to about 8.5.

19.

The process of claim 18, wherein step (d) further comprises adjusting the pH of the calcium thiosulfate solution at a pH range from about 8 to about 8.5.

twenty.

The process of claim 1, wherein step (d) further comprises filtering the oxidized calcium thiosulfate solution.

twenty-one.

The process of claim 20, wherein step (d) further comprises adding a flocculant to the calcium thiosulfate solution before filtration.

22

The process of claim 21, wherein the flocculant is an anionic flocculant.

2. 3.

The process of claim 21 wherein the flocculant is added in an amount from about 50 to about 70 ug / gm of solution.

24.

The process of claim 20, wherein step (d) further comprises increasing the temperature of the oxidized calcium thiosulfate solution before filtration.

25.

The process of claim 1, wherein step (a) further comprises preparing the calcium polysulphide solution by providing a suspension of calcium hydroxide, adding sulfur to the suspension and reacting the suspension.

26.

The process of claim 25, wherein step (a) further comprises adding sulfur to the suspension in a molar ratio of sulfur to calcium hydroxide from about 1: 1 to about 6: 1.

27.

The process of claim 26, wherein step (a) further comprises adding sulfur to the suspension in a molar ratio of sulfur to calcium hydroxide from about 3.4: 1 to about 6: 1.

28.

The process of claim 27, wherein step (a) further comprises adding sulfur to the suspension in a molar ratio of sulfur to calcium hydroxide from about 3.4: 1 to about 3.8: 1.

29.

The process of claim 28, wherein step (a) further comprises adding sulfur to the suspension in a molar ratio of sulfur to calcium hydroxide of about 3.6: 1.

30

The process of claim 29, wherein step (a) further comprises adding sulfur to the suspension in a molar ratio of sulfur to calcium hydroxide to water of at least about 2: 6: 30.

31.

The process of claim 30, wherein the sulfur is added at a molar ratio of sulfur to calcium hydroxide to water from about 3.6 to 4.9: 1: 25.5.

32

The process of claim 31, wherein step (a) further comprises adding sulfur to the suspension in a molar ratio of sulfur to calcium hydroxide to water of about 3.6: 1: 25.5.

33.

The process of claim 25, wherein step (a) further comprises reacting at a temperature of at least about 70 ° C to form the reaction mixture of calcium polysulfide.

3. 4.

The process of claim 33, wherein step (a) further comprises reacting at a temperature of about 85 ° C to about 99 ° C to form the calcium polysulfide reaction mixture.

35

The process of claim 34, wherein step (a) further comprises reacting at a temperature of about 90 ° C to about 92 ° C to form the calcium polysulphide reaction mixture.

36.

The process of claim 25, further comprising cooling the polysulfide reaction mixture to a temperature of about 55 ° C to about 75 ° C.

37.

The process of claim 1, wherein at least one of the steps (a), (b) and (c) comprises oxidizing the solution or solutions using sulfur dioxide.

38.

The process of claim 1, wherein step (d) further comprises adding a bacterial growth inhibitor to substantially completely oxidized calcium thiosulfate.

39.

The process of claim 38, wherein step (d) further comprises adding an alkali metal metabisulfite salt as a bacterial growth inhibitor to substantially completely oxidized calcium thiosulfate.

40

A process to prepare calcium thiosulfate comprising the following steps:

(a) partially oxidize a suspension of calcium polysulfide in a first tank reactor with continuous stirring at a temperature of about 70-95 ° C, at a pressure of about 15-80 psig, at a stirring speed of about 600-1200 rpm , to produce a solution of calcium polysulphide / calcium thiosulfate; (b) transfer the solution produced in step (a) to a second tank reactor with continuous stirring and further oxidize the solution produced in step (a) at a temperature of approximately 7095 ° C, at a pressure of approximately 15-80 psig , at a stirring speed of approximately 600-1200 rpm, to produce a solution of calcium thiosulfate; (c) transfer at least a portion of the calcium thiosulfate solution produced in step (b) to a third reactor and substantially oxidize completely the calcium thiosulfate solution produced in step (b) in the third reactor; Y (d) recovering the substantially oxidized calcium thiosulfate solution completely produced in step (c).

41.

The process of claim 40, wherein

step (a) comprises partially oxidizing a calcium polysulphide suspension at a temperature of about 70-75 ° C, at a pressure of about 40-80 psig, at a stirring speed of about 600-800 rpm, to produce a solution of calcium polysulphide / calcium thiosulfate; Y step (b) comprises further oxidizing the solution produced in step (a) at a temperature of about 70-95 ° C, at a pressure of about 40-80 psig, at a stirring speed of about 600-800 rpm, to produce a solution of calcium thiosulfate.

42

The process of claim 41, further comprising preparing a suspension of calcium polysulphide in a tank reactor with continuous stirring by supplying a suspension of calcium hydroxide, adding sulfur to the suspension at a molar ratio of sulfur to calcium hydroxide to water of at least about 2: 6: 30; and reacting the suspension at approximately 90 ° C-92 ° C to form a calcium polysulphide solution, and transfer the calcium polysulphide solution to the first tank reactor with continuous stirring of step (a).

43

The process of claim 42, further comprising a step of forming said calcium hydroxide suspension by combining calcium hydroxide and water.

44.

The process of claim 43, further comprising a step of forming said calcium hydroxide suspension by combining calcium hydroxide and water, wherein the calcium hydroxide is from about 96% to about 99% pure.

Four. Five.

The process of claim 40, wherein step (c) further comprises substantially oxidizing the calcium thiosulfate solution in a third reactor that is selected from the group consisting of a cavitation reactor and a series of semicontinuous termination reactors .

46.

The process of claim 40, further comprising determining the oxidative endpoint of the calcium thiosulfate solution by controlling one or more of the following:

change in oxidation potential reduction of the solution; color change on lead acetate indicator paper of the solution; color change of the solution from red to colorless; presence of hydrogen sulfide in the solution; Y pressure changes in at least one of the reactors; and stop oxidation at approximately the oxidative endpoint to minimize excessive oxidation of calcium thiosulfate and polythionate formation. 47. A continuous system for preparing calcium thiosulfate comprising: (a) a first reactor to partially oxidize a calcium polysulphide solution to form a calcium polysulphide / calcium thiosulfate solution; (b) a second reactor to further oxidize the solution produced in the first reactor to produce a solution of calcium thiosulfate; (c) a means for transferring the solution formed in the first reactor to the second reactor; (d) a third reactor to substantially oxidize completely the solution produced in the second reactor; Y (e) means for transferring the solution formed in the second reactor to the third reactor; where the first, second and third reactors are connected in series. The system of claim 47, wherein the first and second reactors are continuous stirring reactors. 49. The system of claim 48 further comprising an additional reactor in series before the first reactor to prepare a solution of calcium polysulfide. 50. The system of claim 48 comprising an additional reactor to filter the calcium thiosulfate solution produced in the third reactor. 51. The system of claim 47 wherein the fourth reactor is a semi-continuous or cavitation reactor.