Method for producing a filter material containing iron for the treatment of water

Abstract

A method for producing an iron-containing filter material for water treatment includes the steps of reacting a trivalent iron compound and a base inside a vessel until the trivalent iron is completely neutralized, to obtain an iron hydroxide and a salt consisting of the anion of the trivalent iron compound and the cation of the base; feeding the iron hydroxide and the salt into ceramic membranes to wash the iron hydroxide from the salt in cross-flow; feeding the iron hydroxide suspension to a membrane filter press where part of the water is removed, to obtain a panel having a moisture content of less than 77% by weight; inserting the panel into containers; and positioning the containers inside a refrigeration chamber operating at atmospheric pressure and at temperatures less than 0° C. for a time between 24 and 240 hours.

Claims

1. A method for producing an iron-containing filter material for water treatment, comprising: reacting a trivalent iron compound and a base inside a vessel until the trivalent iron is completely neutralized, to obtain an iron hydroxide and a salt consisting of an anion of the trivalent iron compound and a cation of the base; feeding the iron hydroxide and the salt into ceramic membranes and wash the iron hydroxide from the salt with water in cross-flow, thereby forming an iron hydroxide suspension; feeding the iron hydroxide suspension to a membrane filter press where part of the water is removed, to obtain a panel having a moisture content of less than 77% by weight; inserting the panel into containers; and positioning the containers inside a refrigeration chamber operating at atmospheric pressure and at a temperature of less than 0° C. for a time between 24 and 240 hours.

2. The method as claimed in claim 1, wherein the step of positioning the containers inside a refrigeration chamber comprises positioning the containers inside a refrigeration chamber operating at −5° C. for 12 hours, then at −7.5° C. for 12 hours, then at −10° C. for 12 hours, at −15° C. for 12 hours, at −20° C. for 12 hours, at −25° C. for 12 hours, and at −30° C. for 12 hours.

3. The method as claimed in claim 1, wherein the step of positioning the containers inside a refrigeration chamber comprises positioning the containers inside a refrigeration chamber operating at atmospheric pressure and at a temperature of less than 0° C. for a time between 24 and 48 hours.

4. The method as claimed in claim 1, wherein the trivalent iron compound is chosen from the group consisting of ferric chloride, ferric nitrate, and ferric sulphate.

5. The method as claimed in claim 1, wherein the base is selected from the group consisting of sodium hydroxide, potassium hydroxide, and ammonia.

6. The method as claimed in claim 1, wherein the trivalent iron is obtained by starting from a divalent iron compound which is oxidized to trivalent iron during a reaction, by use of an oxidant blown into a reaction environment.

7. The method as claimed in claim 1, wherein filtration in the membrane filter press is carried out to obtain the panel having a moisture content of less than 72% by weight.

Description

(1) As can be seen from the drawing, in the process according to the invention a trivalent iron compound, for example ferric chloride, is reacted with a base, for example sodium hydroxide. The reaction product is an iron hydroxide in colloidal form, together with a salt composed of the cation of the base and of the anion of the trivalent iron compound.

(2) For example, all pairs obtainable by choosing the iron compound from ferric chloride, ferric nitrate and ferric sulphate, and the base from sodium hydroxide, potassium hydroxide and ammonia, are suitable.

(3) A divalent iron compound, such as ferrous sulphate, can also be used as the iron starting compound, and oxidizing the divalent iron to trivalent iron during the reaction, by using an oxidant such as air or oxygen blown into the reaction environment.

(4) In particular, the process consists of causing sodium hydroxide originating from a vessel 4 to react inside a vessel 2 containing ferric chloride until the ferric chloride is completely neutralized, to obtain iron hydroxide and sodium chloride.

(5) The iron hydroxide and sodium chloride are then fed into ceramic membranes 6 for washing the iron hydroxide from the sodium chloride in cross-flow.

(6) The iron hydroxide suspension is then fed to a membrane filter press 8 where part of the water is removed, until a panel 10 is obtained having a moisture content of less than 77% by weight, preferably less than 72% by weight.

(7) The panel 10 is then inserted into containers 12 which are positioned inside a climatic chamber 14 refrigerated to a temperature of less than 0° C. for a time between 24 and 240 hours, preferably between 24 and 48 hours, depending on the container dimensions, while operating at atmospheric pressure.

(8) The product is then left to thaw to ambient temperature, at which it is present in the form of mechanically stable grains 16, with a particle size between 0.1 and 3 mm, the percentage of material below and above these limits being less than 10% by weight.

(9) The filter material obtained by the process according to the invention presents numerous advantages, including: an arsenic absorption capacity at pH 7 of around 30 mg/g, there is little removal from the bed in a filter in which the material has been inserted, such that with a back-wash velocity of 25 m/min, only a material fraction of less than about 1% is removed, the water passing through the filter undergoes low pressure drop, which is found to be less than 0.1 bar for a filter bed of height 1 metre.

(10) A practical example is described below.

(11) 1000 kg of water were fed into a first reactor fitted with an agitator. 63 kg of ferric chloride in a 40% solution were added.

(12) 250 kg of water were fed into a second reactor. 65 kg of sodium hydroxide in a 30% solution were added thereto.

(13) The dilute sodium hydroxide solution of the second reactor was slowly added by a metering pump to the dilute ferric chloride solution contained in the reactor.

(14) The addition continued under agitation while constantly monitoring the pH, until pH 7 was reached.

(15) During the reaction the temperature increased by about 2° C.

(16) After the reaction the iron hydroxide dispersion obtained in this manner was washed in cross-flow with an ultrafiltration apparatus consisting of silicon carbide membranes, while continuing to replace the saline water permeated from the membranes with pure water, until the sodium chloride formed by the reaction was virtually eliminated.

(17) The washed suspension of iron hydroxide was filtered with a membrane filter press until a pressure of about 12 bar was reached. The panel obtained had a solids content of about 28%.

(18) The panel, reduced to fragments of about five centimeters in size, was placed in a plastic container with thin walls of about 1 mm, which was rested on a polystyrene panel and covered on its top with a second polystyrene panel, simply rested on it.

(19) The container was inserted into a climatic chamber maintained at a temperature of −8.5° C. (±1° C.) and the temperature at the centre of the container, at the heart of the material, was constantly monitored.

(20) After 41 hours, the temperature at the container centre had reached −8.2° C.

(21) The temperature can be maintained constant at less than 0° C. in the aforedescribed manner, or can be varied in accordance with a predetermined programme.

(22) For example, in a second experiment the climatic chamber was made to operate initially at a temperature of −5° C. for 12 hours, then at −7.5° C. for 12 hours, then at −10° C. for 12 hours, then at −15° C. for 12 hours, then at −20° C. for 12 hours, then at −25° C. for 12 hours, then at −30° C. for 12 hours. The total residence time in the climatic chamber was 84 hours, and at the end of the cycle the temperature at the container centre had reached −21° C.

(23) The container was removed from the climatic chamber and the product left to thaw to ambient temperature.

(24) The aforesaid ambient temperature was between 8° C. and 15° C. The thawing process lasted 240 hours.

(25) After thawing, the granulate obtained had the aforestated characteristics.