MXNL06000058A - Production of ferrous sulfate and calcium chloride. - Google Patents

Abstract

Described is a process for producing hepta-hydrated ferrous sulfate and a calcium chloride solution from pickled residue liquors containing ferrous chloride, hydrochloric acid and water, which consists in: treating the pickled residue solution with a lime slurry within an agitated reactor so that the ferrous chloride is transformed into ferrous hydroxide and a calcium chloride solution which are separated by filtration. The ferrous hydroxide is treated within an agitated reactor which includes a cooling jacket for transforming the ferrous hydroxide into hepta-hydrated ferrous sulfate.

Description

PRODUCTION OF FERROUS SULFATE AND CALCIUM CHLORIDE The present invention relates to a process for the production of ferrous sulfate and calcium chloride, from residual steel pickling liquors containing: Ferrous chloride, water and hydrochloric acid, using calcium oxide and sulfuric acid as raw materials.

KNOWN BACKGROUND ON THE STATE OF THE TECHNIQUE OF THE INVENTION The carbon steel sheets, after an annealing step and before being submitted to the processes of cold rolling or galvanizing, have to be washed inside special tanks, with dilute solutions of sulfuric acid or hydrochloric acid, a process called pickling, with the purpose of eliminating the metallic oxides contained in the surface of the steel and which were formed in the annealing stage of the carbon steel sheets.

When sulfuric acid is used as the washing or pickling agent, this acid, in addition to removing the surface oxides and transforming them into ferrous sulphate, reacts with iron part of the carbon steel sheet and forms ferrous sulfate and hydrogen, which escapes the atmosphere. During this operation, a large part of the sulfuric acid is depleted and the pickling solution loses its strength, so it has to be replaced.

The spent sulfuric acid solution contains high concentrations of ferrous sulphate and low concentrations of sulfuric acid, products that give it a highly polluting character, so for its adequate disposal, successive steps are contemplated: neutralization of residual acid, concentration and crystallization by cooling, of a large part of the ferrous sulfate contained therein, which after drying has a commercial value as ferrous hepta hydrate sulfate. This ferrous sulphate production process is a good solution to the ecological problem of disposal of the residual pickling liquors of carbon steel sheets, but it has a high energy consumption, since it is necessary to evaporate water in the concentration system / crystallization of ferrous sulfate.

When a solution of hydrochloric acid is used in the process of pickling of carbon steel sheets, this process has important advantages, since it is possible to recover, by distillation, part of the remaining hydrochloric acid, once the pickling liquor has lost its strength, that is to say when it has been saturated with ferrous chloride, product of the reaction between hydrochloric acid and iron and when the concentration of hydrochloric acid has already been reduced to levels where the pickling process is no longer efficient.

The typical composition of a residual pickling liquor, when a solution of hydrochloric acid has been used as an oxidizing agent, may be as follows: ferrous chloride content 38.0%, free hydrochloric acid content 4.0% and water content, the remaining 58.0% and once a large part of the hydrochloric acid has been recovered by distillation, there remains a solution of ferrous chloride properly saturated, with a content of Ferrous Chloride is close to 46% and the content of hydrochloric acid is close to 2.0 %.

The ferrous chloride in solution, after the distillation of a large part of the hydrochloric acid, can be easily oxidized with the oxygen contained in the atmospheric air transforming itself into ferric chloride, which is a product of high commercial value and widely used as a coagulant, in the processes of wastewater treatment.

Other processes have been developed that contemplate a greater recovery of hydrochloric acid from a residual solution of the pickling process that contains ferrous chloride and that consist of treating said residual solution with sulfuric acid, this with the purpose of transforming all the ferrous chloride, in ferrous sulfate and thus regenerate the hydrochloric acid that reacted with the steel, according to reaction # 1 FeCI2 + H2S04? FeS04 + 2 HCl We are referring to, among others, the process described in US Pat. No. 5,417,955 of David W. Connoly, where the residual pickling solution, containing ferrous chloride and hydrochloric acid, is treated with hot sulfuric acid, inside a reactor, where It constantly recirculates a current of air, which draws water and hydrochloric acid.

A large part of the entrained hydrochloric acid is condensed in an absorber cooled with water and the rest of said uncondensed acid returns to the reactor together with the recirculating air.

In order to make this distillation of hydrochloric acid efficient for its recovery, the mass is maintained inside the reactor at a temperature close to 109 ° C, using external energy and enough water is added to condense the hydrochloric acid to a concentration of 21% and with a water content of 79%, which is precisely the composition of the water / hydrochloric acid azeotrope that distills at 109 ° C. Recirculation of air within a closed system Reactor / condenser, prevents generation of acid emissions hydrochloric to the atmosphere, for the benefit of the environment.

The reference process contemplates an additional step of oxidation of ferrous sulphate to ferric sulphate, with oxygen from the air, inside a second oxidizing reactor, for which in the first reactor an excess of sulfuric acid is used, which allows a more efficient separation of crystals of insoluble ferric sulfate and excess sulfuric acid.

The ferric sulfate formed is separated by crystallization and transferred to a dryer and the acid mass is taken back to the reactor generating ferrous sulphate for refusal In the process described in a previous patent, (US PATENT 4,382,916 by Brazier K. Beecher), the residual pickling liquor is heated and transferred to a first evaporator crystallizer that works at about 115 ° C, equipment that has the purpose Evaporate all the remaining hydrochloric acid and a large part of the water until a large part of the ferrous chloride crystallizes in the form of a dihydrate.

The ferrous chloride crystals formed in the first crystallizer are separated by filtration or centrifugation of the mother liquor and are transferred to a second crystallization reactor, where they are reacted, as in the Connaly process with sulfuric acid to transform them into sulphate. ferrous and hydrochloric acid. (reaction # 1) The mass contained in this second evaporator crystallizer, recirculates permanently to through a heat exchanger operated with steam, for the purpose of evaporating water and all the hydrochloric acid formed in reaction # 1 and crystallize all the ferrous sulfate, also formed in reaction # 1, in the form of a dihydrate crystalline (Fe S04, 2H20), which is separated from the mother liquor by filtration or centrifugation. The hydrochloric acid formed by reaction in the second crystallizer is sent to a rectification column, with the purpose of producing a concentrated hydrochloric acid The hydrochloric acid evaporated in the first crystallizer is very diluted and this is mixed with the concentrated acid left from the rectification column, with the purpose of achieving the concentration of acid required, according to its final use.

The process of Brazier K Beecher, contemplates a step of recrystallization of ferrous sulphate with the purpose of improving its purity.

There are other patents and other processes where the ferrous chloride contained in the residual pickling liquor is reacted with the sulfuric acid to regenerate the hydrochloric acid and produce ferrous sulfate or ferric sulfate and hydrochloric acid, which differ the two already mentioned, in the technological route used to distill and recover the hydrochloric acid formed.

In almost all of these mentioned processes, there are mechanisms to recover part of the energy consumed in the distillation of hydrochloric acid, since the cost of the energy used to perform this operation is very high and affects in a very sensitive way, the cost of production.

The inclusion of heat recovery equipment in the process increases the cost of the investment, since it involves handling hot hydrochloric acid, which due to its high corrosive power, requires highly specialized and high-priced materials.

On the other hand, calcium chloride is generally obtained as a by-product, both in the manufacture of sodium carbonate, in the so-called SOLVAY process, and in many other processes, such as in the production of phosphoric acid or calcium phosphate, when the phosphate rock is reacted with hydrochloric acid DETAILED DESCRIPTION OF THE PROCESS a) .- Unlike known processes, in the proposed process, the ferrous chloride contained in the residual pickling liquor is reacted with a lime slurry, prepared on the basis of water and calcium oxide, controlling the PH of the mass between 2.0 and 9.0, but preferably in 6.5.

The content of calcium hydroxide in the slurry is controlled, between 2% and 52%, but preferably with a lime slurry between 20% and 30% of calcium hydroxide, to which certain impurities have previously been decanted, such as uncalcined calcium carbonate and some sandy compounds contained in the calcium oxide used to produce the slurry and which would eventually impurify the ferrous hydroxide formed ..

The reaction is carried out inside a stirred reactor and through which a current of nitrogen gas circulates continuously, with the purpose of transforming all the ferrous chloride contained in the pickling liquor, into insoluble ferrous hydroxide and soluble calcium chloride, in accordance with Reaction # 2, controlling the PH of the mass between 1.5 and 9.5, but preferably between 5.5 and 7.5.

FeCl2 + Ca (OH) 2? CaCl2 + Fe (OH) 2 The purpose of the continuous recirculation of nitrogen gas inside the reactor is to prevent the ferrous chloride from oxidizing to ferric chloride In this same step, the free hydrochloric acid contained in the pickling liquor reacts with part of the lime contained in the slurry, to transform said free hydrochloric acid into soluble calcium chloride and water, according to Reaction # 3. 2HCl + Ca (OH) 2? CaCl2 + 2H20 b) .- Step followed by the mass reacted in step a) .-, is transferred to a pressure filter with facilities for washing the cake with water, where the dough is filtered and washed, in order to separate the insoluble ferrous hydroxide, from the calcium chloride solution formed in reactions # 2 and # 3. c) .- The filtered solution obtained in step b) .-, is concentrated inside an evaporator, until a calcium chloride solution is obtained between 25% and 45%, but preferably at a concentration of 30% calcium chloride .

The calcium chloride solution of 30% obtained in step c) .-, is in itself, a final product that has a high commercial value, since it is widely used as: Antifreeze of streets and roads in cold climates, as an agent anti dust on unpaved roads and that has a wide market. In the USA alone, close to one million tons per year of calcium chloride 100% base are consumed.

The calcium chloride solution also has an important market as a fertilizer when mixed with urea.

The solution of calcium chloride at 30% concentration, is a valuable raw material, since it can also be treated with sulfuric acid, with the purpose of transforming all the calcium chloride contained in the solution, in hydrochloric acid and calcium sulfate, which can be separated by decanting, centrifugation or filtration and washing according to the Reaction # 4 CaCl2 + H2SO4 + 2H20? 2HCI + CaS04.2H20 d) .- The wet ferrous hydroxide, separated in the pressure filter of step b) .-, is reacted with concentrated sulfuric acid, inside a closed reactor type of double arrow blades (Pug Mili) and equipped with jacket of cooling, where cold water circulates, with the purpose of transforming it into ferrous hepta sulfate hydrated according to reaction # 5.

Fe (OH) 2 + H2SO4 + 5 H20? FeS04.7H20 Reaction # 5 is exothermic, so initially ferrous double hydrated sulphate is formed, so it is necessary to cool the dough to transform it into ferrous sulphate hydrated hepta, using as water of crystallization, the water contained as moisture in the ferrous hydroxide cake separated in the filter of step b) .- .. e) .- Finally, the ferrous sulfate hepta hydrate produced is bagged, to prevent the ferrous sulfate from oxidizing to ferric sulfate, if subjected to prolonged exposure to atmospheric air.

BEST METHOD FOR CARRYING OUT THE INVENTION See figure # 1 1) .- 594.00 kilos of a residual solution of pre-cooled pickling, (Current A) containing: 269.83 kilos of ferrous chloride, 11.88 kilos of 100% hydrochloric acid base and 312.294 kilos of water, are treated in a stirred reactor ( 1), with 847,250 kilos of a slurry of lime (stream B), containing 20% calcium hydroxide free of impurities, with the purpose of transforming all the ferrous chloride contained in the residual solution into ferrous hydroxide and calcium chloride. A stream of nitrogen gas (stream C) is circulated continuously inside the reactor, in order to avoid oxidation of the ferrous hydroxide to ferric hydroxide. 2) .- The cold mass contained in the reactor (1), is continuously discharged to a filter (2) working under pressure and equipped with washing water, for the purpose of separating the wet crystals of ferrous hydroxide (stream D). ) of the calcium chloride solution formed in step # 1 (stream E). 3) .- Transfer the wet crystals of ferrous hydroxide (stream D), to a mixer reactor (3) of double arrow blades, equipped with external cooling jacket and cover where cold water circulates, with the purpose that all the Ferrous hydroxide reacts with 228,987 kilos of 98% sulfuric acid (stream F) and is transformed into 591,022 ferrous sulphate hepta hydrated (stream J).

Much of the water of hydration of the ferrous sulphate crystals is taken from the moisture contained in the wet crystals of ferrous hydroxide left from the filter (2), at the moment when the mass is cooled based on the cooling system of the own reactor (3). 4) The calcium chloride solution (stream E) from the filter (2), is transferred to an evaporator (4), with the purpose of concentrating the calcium chloride solution up to 36%

Claims (6)

CLAIMS Having described the invention, I claim as of my property the content the following clauses:

1) .- A process for the production of ferrous hepta hydrate hepta and calcium chloride using as raw materials: calcium oxide, sulfuric acid and residual pickling solutions, containing: ferrous chloride, hydrochloric acid and water by the following steps: a) .- The residual pickling solution, containing ferrous chloride and hydrochloric acid, is reacted inside a stirred reactor with a lime slurry, with the purpose of transforming all the ferrous chloride contained in the pickling solution into ferrous hydroxide and calcium chloride and all the free hydrochloric acid contained in the pickling liquor in calcium chloride and water. Inside the stirred reactor, a current of nitrogen gas circulates permanently, to avoid that the ferrous hydroxide formed in the reaction, is oxidized to ferric hydroxide b) .- The mass generated in the stirred reactor, mentioned in step a) .-, is transferred to a filter operated under pressure and equipped with a washing system, with the purpose of separating the calcium chloride solution formed, of the ferrous hydroxide crystals, with a minimum of humidity. c) .- The calcium chloride solution exiting the pressure filter is transferred to an evaporator, whose purpose is to increase the concentration of calcium chloride up to 36% d) .- The wet crystals of ferrous hydroxide exiting the pressure filter, are transferred to a double arrow blade mixing reactor, equipped with a cooling cap and jacket and reacted with 98% concentrated sulfuric acid for the purpose of transform them into ferrous hepta hydrated sulphate, which is immediately packed.

2) .- A procedure in which, according to paragraph a) .- The lime milk is prepared on the basis of calcium oxide and water, until having a concentration between 2.0% and 45% of calcium hydroxide, but preferably a slurry of lime containing 20% calcium hydroxide, which has been previously decanted, to remove impurities of higher density, such as uncalcined calcium carbonate particles and other particles of sandy type which they contain iron and aluminum compounds and they are present as impurities in calcium oxide, that is to say, they are a lime slurry already purified.

3) A procedure that according to clause a) .- The final PH of the reacted mass inside the reactor is controlled between 2.0 and 9.5, but preferably in 6.5

4) .- A procedure in which, according to clause d) .- The moisture contained in the crystals of ferrous hydroxide, provides the water of crystallization to obtain ferrous hepta hydrated sulfate.

5) .- A procedure in which, according to paragraph d), the hydrated sulfate hepta crystals are immediately packed to avoid their oxidation to ferric sulfate by the oxidizing action of the oxygen contained in the atmospheric air.

6) .- A procedure which, according to clause c) .-, The calcium chloride solution exiting the evaporator, is concentrated between 25% and 40% calcium chloride, but preferably at 38%, with the purpose to condition it to produce hydrochloric acid and insoluble calcium sulfate by reacting it with 98% sulfuric acid