WO1998058877A1

WO1998058877A1 - A process for converting phosphogypsum wastes

Info

Publication number: WO1998058877A1
Application number: PCT/PL1998/000027
Authority: WO
Inventors: Dobros$m(D)awa KOSICKA
Original assignee: Zielinski, Marek
Priority date: 1997-06-23
Filing date: 1998-06-15
Publication date: 1998-12-30
Also published as: PL186189B1; PL320762A1; AU7679098A; MA24576A1

Abstract

The present invention relates to a process for converting phosphogypsum wastes into a form suitable for use especially as raw materials in the production of building materials as well as filling additives for paints and enamels or synthetic resins, by heating with continuous stirring, in a wasteless procedure, while the heating is carried out from ambient temperature with a temperature gradient of 0.05 - 0.33 deg/s until the content of SO4<-2> groups is reduced below 0.23 % by weight. The invention makes it possible to utilise significant quantities of phosphogypsum wastes which otherwise create a serious ecological problem. At the same time, the invention may allow one to save considerable amounts of natural resources.

Description

A PROCESS FOR CONVERTING PHOSPHOGYPSUM WASTES

The subject of the present invention relates to a process for converting phosphogypsum uastes into a form especially suitable for use in the production of building materials and as fillers in paints and enamels. The problem of the continously increasing quantities of phosphogypsum uastes resulting from the production of phosphoric acid is well knoun. Satisfying the demand for phosphoric acid uhich is a necessary substrate in the production of fertilizers αr phosphate compounds, used in cosmetic and detergents industry, one cannot avoid large quantities of uastes, knoun as phosphogypsum or phosphoric gypsum, uhich shou a very acid reaction, pH 2 - 2.5, uheπ a uet method is used or quite an acid reaction, pH 3 - 3.5 in the case of the semihydrate method. These uaste regardless of the manufacturing process, using phosphorites as must available rau materials, contain mainly 55 - 77 % by ueight of calcium sulphate, and 20 - 37 % by ueight of total uater (crystaline and unbounded) and compounds of phosphor, fluorine, chlorine, lanthaπides series, rare - earth elements, and others, depending on the mining location. Such a variety of chemical compounds, even if some of them occur in small quantities, and the clearly acid reaction, despite the apparent analogy to gypsum, as the uaste name may indicate, practically make it impossible to utilise these uastes in large quantities uithout any previous treatment. At the same time, grouing quantities of those uastes uorlduide create increasingly serious ecological hazard. There are knoun attemps to utilise phosphogypsum uastes by using them as binding materials in quantities up to 2 % of uet mix in the production of building materials as described in Polish Patent Application P.277085. Also knoun from Polish Patent No 152954 tent Application P.277085. Also known from Polish Patent No 152954 is the use of small quantities of untreated phosphogypsum uaste as iπeralizer in ceramic materials.

From Polish Patent No 158532 a process is knoun for preparing uaterproof gypsum - based aggregates and other building materials, uherein the phosphogypsum uaste, is used as one of the ingrediens of the slurry in a quantity limited to 20 % by ueight. It is obvious that such a level of utilisation can only be treated as a partial solution to this uaste problem. There are also knoun processes for recovering rare - earth elements from phosphogypsum uastes as described in Polish Patent No 129444 and 153042. These processes, however, are of no practical importance as a as the reduction of the huge uaste volume is concerned. According to the Polish Patent Application No P.290377 anhydrite binders are prepared by pelletising rau phosphogypsum uastes uhich are then burned at temperatures of 743 °C, 843 °C and 997 °C for one hour, and after cooling to room temperature they are desintegrated.. According to Polish Patent No 153543, gypsum can be produced from phosphogypsum uaste by the addition of small quantities, belou 1 % by ueight, of uaste copperas, after uhich it is roasted uith burnt lime and barium chloride at a temperature of 450 - - 500 °C. Also knoun from the literature are processes for the production of binding materials and insulating building materials using phosphogypsum uastes, uhere thesa wastes are first dry or wet treated to neutralise phosphoric salts, sodium and fluoride compounds contained therein, so that the dehydration and crystallisation of gypsum proceed without disturbance. In the uet methods the uater consumption is up to 10 tons per 1 ton of phosphogypsum uastes, uhile in the dry methods, the use of large quantities of additives, such as burnt lime or limestone powder is necessary. The disadvanta- ge of these processes is, as shown in the literature, that the building materials containing the phosphogypsum wastes processed as above are characterised by a high degree of absorption.

From Polish Patent No 108675 is known a process for converting phosphogypsum waste into chalk and ammonium sulphate by a gaseous treatment with ammonia or carbon dioxide, while the process is conducted in the presence of at least 10 % by weight of calcium carbonate in order to maintain the pH value within the limits 7.5 - 8.

From Polish Patent No 119292 is known a process for whiten- ing phosphogypsum wastes with ozone in order to obtain a whiteness degree of phosphogypsum up to 75 %, so that it is suitable for use in paper bleaching processes.

It is a significant disadvantage of these knoun methods that they consume much time, energy and very often large quantities of uater, creating at the same time other uastes harmful for the environment.

Also knoun from Polish Patents Nos. 170069, P.299472 and P.299473 and Polish Patent Application P.303058 is the preparation of the ceramic - like materials uith very good mechanical qualities and a great chemical resistance. In these processes phosphogypsum uastes are used as inorganic additives, after being heated for at least 1 hour at a temperature of 167 - 177 °C.

It has been unexpectedly found that phosphogypsum uastes, regardless of uhat production process they result from, can be uti- used in a wasteless procedure by heating uhich converts them into raw materials suitable as components of building materials, materials for filling and stabilising empty excavations in mines, as well as fillers in paints, enamels or synthetic resins.

Process for converting phosphogypsum wastes into a form sui- table for use in the production of building materials and as fillers in paints and enamels, according to the present invention consists in heating phosphogypsum wastes on continuous stirring, with no resultant wase, preferably in a fluid bed, from ambient temperature, keeping the thermal gradient from 0.05 to 0.33 deg/s, until the

_2 content of sulphate groups SO, , below 0.23 % by ueight is obtained, as determined at ambient temperature by any knoun quantitative method.

A modification of the process for converting phosphogypsum uastes according to the invention consists in that the heating pro- cess is carried out with the thermal gradient 0.11 - 0.26 deg/s, while after reaching successively temperature rangas 107 - 117 °C, 277 - 287 °C and possibly 327 - 337 °C, as isothermal heating is carried out for 5 - 12 minutes within these ranges, while the whole process is preferably carried out in a fluized bed. The converting of phosphogypsum wastes according to the present invention by heating at elevating temperature, preferably in the fluidized bed, due to progressing dehydration and thermal decomposition, makes it possible to form assumingly more intricate and simultaneously ordered quasi - complex structures, mainly anhy- drous calcium salts and oxides, as well as rare - earth elements and traces of phosphorus. The process is carried out exclusively by a dry method in opposition to the hitherto used wet methods with multistage washing, or combined methods with multistage washing, drying and heating washed uastes at temperature even higher then 427 °C.

From the performed tests it follous that the converted phosphogypsum uastes according to the present invention, can be used even in amounts up to 30 % to replace cement in the preparation of a lo- uer strength concrete mix maintaining its functional qualities. Thanks to this, the exploitation of natural resources, used in the cement production, can be reduced and at the some time large quantities of otheruise useless and environmentally harmful phosphogypsum uastes may be utilised.

Furthermore, unlike rau phosphogypsum uastes, the converted phosphogypsum uastes, uithout any hazard to the natural environment, may be used as filling materials to stabilise explioted excavations.

Assumingly, the presence of lanthanide elements in rau phos- phogypsum uastes which wholly remain in the converted phosphogypsum wastes may advantageously affect the formation of quasi - complexes with a stable and ordered structure. Such a modification of structure is assumed to improve the useability of the converted phosphogypsum wastes as fillers for paints, enamels, synthetic resins and filling pastes where the product structure is of great importance for performance qualities.

The subject of the present invention is presented, without limitation of its applications, in the following examples.

Example 1. Raw phosphogypsum wastes containing 0.48 % by weight of

_2 SO. groups, as determinated by the gravimetric BaSO. analysis, in a quantity of 110 kg at a temperature of 25 °C, in a fluidisa- tiαn column, were heated in their fluidised state at an average heating rate of 0.14 deg/s. After about 25 minutes of heating, a 10 g sample was taken out and after it was cooled in an air - tight _2 container to the temperature 27 °C, the content of the SO, groups was determinated by the previously mentioned method. The

_2 sample contained 0.17 % by weight of SO, groups. The computed process resulted in 84.3 kg of converted phosphogypsum wastes. Example 2. To a mixer containing 2 kg of water, 10 kg of anhydrite was added batchuise, followed by the addition of 0.2 kg of sodium chloride as binding accelerant and 0.04 kg of monobasic sodium phosphate as foaming agent. After mixing, 5 kg of fine - - grained sand as filler was added and the whole mix after being remixed was casted into marked 4 x 4 x 16 cm moulds. Proceeding as above, instead of anhydrite, corresponding quantities of converted phosphogypsum wastes obtained as in example 1 were used, and then the resultant mix was casted into marked moulds as above. Proceeding also as above in another trial, all the amount of anhydrite was re- placed by corresponding quantities of raw (uncorverted) phosphogypsum wastes and the resultant mix was casted into similar marked moulds. After 28 days of storage all the samples were tested according to the standard PN-85/B-04500 for compression strength. An average compression strength of the tested samples was 11.18 MPa uhen using anhydrite, 10.83 MPa in the case of using coonverted phosphogypsum uastes and only 1.24 MPa uhen using rau, unconverted phosphogypsum uastes.

Example 3. To a solution containing 3.8 kg of uater and 0.012 kg of borax, 7 kg of industrial gypsum uas added batchuise and after mixing the resultant mix uas casted into marked moulds (4 x 4 x 16 cm). After setting, the samples uere dried at the temperature 55 °C. Proceeding as above samples were prepared where instead of industrial gypsum, the converted phosphogypsum uastes, obtained in example 1, and raw phosphogypsum wastes were used. The resultant mixes were casted into marked moulds. The setting time of the mix with converted phosphogypsum wastes uas comparable with that of the mix containing the industrial gypsum, while the setting time of the mix with raw phosphogypsum wastes was noticeable longer. The samples with conver- ted and rau phosphogypsum uastes uere dried at the temperature 55 °C, uhile the latter required a longer setting time by 10 minutes. After 28 days of stabilization, all the samples uere tested for compression strength. The average strength of the samples prepared from the industrial gypsum uas 21.7 MPa, and that of the samples made uith converted phosphogypsum uastes - 19.93 MPa, uhile the compression strength of all the samples made uith unconverted phosphogypsum uastes - belou 4 WPa.

3 Example 4. A mix uith a total volume of 10 dm containing 3 kg of uater, 6 kg of cement 250, and a make-up amount of sand uas prepared and casted into marked moulds. Next, a mix uas made in which 1.8 kg of cement was replaced by the converted phosphogypsum uastes obtained as in example 1, and casted into marked moulds. After 28 days, all the samples uere tested. The average compression strength of the samples containing only cement uas 55.1 MPa and theit average absorba- bility uas 12.9 %, uhile the average compression strength of the samples uith 30 % by ueight of cement being replaced by converted phosphogypsum uastes was 51.6 MPa, uith theit average absorbability amounting to 11.85 %.

Example 5. 100 kg of unconverted phosphogypsum wastes containing

_2 0.47 % by weight of SO, groups - as determinated at a temperature of 20 °C by adding a solution of barium chromate in hydrochloric acid, and after further treatment by titration with sodium thiosul- phate in the presence of starch at 20 °C - were converted into a fluidised state in a fluidisation column and heated at an average heating rate of 0.1 deg/s and then the temperature was maintained at 15 °C for 7 minutes, followed by further heating at a rate of 0.12 deg/s and maintaing the temperature at 287 °C for 5 minutes.

A sample αf that product analysed by the above method showed the

-2 content of SO, groups at a level of 0.158 %. From the fluidisa- tion column 35 kg of converted phosphogypsum wastes were taken out.

The rest uas further heated at a heating rate αf 0.23 deg/s follou- ed by isothermal heating at a temperature of 337 °C for 10 minutes.

41.3 kg converted phosphogypsum uastes uere obtained, containing

_2 0.087 % by ueight of SO, groups determined as above.

Example 6. The converted phosphogypsum uastes from example 5,

_2 containing 0.158 % of SO, groups, in an amount of 6 kg uere mixed uith 4 kg of uater to prepare a mix suitable for pneumatic conveying and for filling empty spaces in mine excavations. Example 7. To a previously homogenized composition containing 5.5 kg of Alkyd Resin 65-60 of QILCHEFl, 1.5 kg of titanium uhite,

1.5 kg αf sunflouer oil and 0.92 kg of ethaπol, on constant stirring 0.5 kg of converted phosphogypsum uastes containing 0.87 %

_2 of SO, groups as obtained in example 5 uas added and then 0.5 kg of 10 % solution of cobalt naphthenate. The resultant paint uas used to coat a protective shield of a lathe and after 24 hours, a glossy layer, 85 μm in thickness, uas obtained. Within 7 days the coating reached its full performance qualities. Tests according to PN-76/C-81521, confirmed that the coat absorbability was 0.52 %. Example 8. A composition containing 1.75 kg of alkyd resin from example 7, 0.35 kg of titanium white, 0.2 kg of sunflower 0.68 kg xyleπe and 1 kg of aluminium hydrate, was prepared.

Subsequently 1.2 kg of the converted phosphogypsum waste containing

_2 0.87 % by weight of SO, groups from the example 5 was added. The uhole uas stirred to obtain a very thick, mat, incombusible alkyd paint, suitable also as a painter's putty. A surface was coated uith this paint and after 7 days the tested coating shoued a uater absorption of 0.84 % by weight.

Claims

Claims :

1. Process for converting phosphogypsum wastes into a form especially suitable for use as raw materials in the production of building materials and as filling additives to paints, enamels and synthetic resins, characterised in that the conversion is uasteless and proceeds by heating on continuous stirring from ambient temperature,

_2 at a heating rate of 0.05 - 0.33 deg/s until the content of SO, groups is reduced belou 0.23 % by ueight, as determinated by any knoun quantitative method.

2. Process according to Claim 1, characterised in that the heating process is carried out uith the temperature gradient 0.10 - 0.26 deg/s, uhile after successively reaching temperature ranges of 107 - 117 ┬░C, 277 - 287 ┬░C and possibly 327 - 337 ┬░C, isothermal heatings are performed ui hin these ranges for 5 - 12 minutes.

3. Process according to Claim 1 or 2 characterised in that the heating process is carried out in a fluidised bed.