IL46825A - Method and apparatus for producing animal feed grade phosphates from phosphoric acid and mineral calcium compounds - Google Patents

Description

METHOD AND APPARATUS FOR PRODUCING ANIMAL FEED GRADE PHOSPHATES FROM PHOSPHORIC ACID AND MINERAL CALCIUM COMPOUNDS This invention relates to a method and apparatus for producing calcium phosphates from phosphoric acid and mineral calcium compounds. More particularly the invention related to a method for manufacturing animal feed grade phosphates, mono-calcium phosphate (MCP) and di-calcium phosphate (DCP) or a mixture of these containing P and Ca in a specified ratio and in a form in which the phosphorus component can be readily absorbed by the animal organism. Such mineral feed additives must comply with certain standards of purity in respect of content of fluorine; arsenic and heavy metals.

To facilitate handling and use it is important that these feed grade phosphates are in the form of strong and free-flowing granulates of appropriate litre weight and grain size, and that they possess good storage properties.

The manufacture of such animal feed grade phosphates by reaction between phosphoric acid and fine-grained mineral calciiim compounds is prior "known.

It is further known to carry out the reaction by direct conversion between a purified, comparatively concentrated phosphoric acid and a purified calcium component which may, if necessary, be in the form of an aqueous suspension. During conversion the reaction mixture will first pass through a glutinous and sticky phase, after which it will harden as reaction continues. This gives consistency and handling problems which cause serious trouble during these stages of the reaction.

When the calcium component is present in the form of CaC03 considerable quantities of gas will be formed during the reaction, which further increases the consistency and handling problems in the apparatus . This is probably the reason why so many prior known processes for the manufacture of animal feed grade phosphates prescribe the use of other calcium compounds which cause less serious consistency and handling problems.

The reaction mixture is so glutinous and viscous that very powerful mixing is necessary during further processing. In the prior known granulating techniques there are difficulties in handling he? mixture without recycling considerable quantities of the reacted product.

The consistency problems also make it difficult to achieve homogenisation and proper mixing of the reaction mass. Local concentrations of acid resulting from inadequate mixing will in turn result in a product with inferior handling and storage properties.

Because of these special difficulties no one, as far as we know, has yet succeeded in developing a simple and technically reliable process for the manufacture of non-dusting granulated animal feed grade phosphates from phosphoric acid and CaC03. According to South African Patent No 66/7774, which relates to such a reaction process, the granulating stage is eliminated and the viscous and sticky reaction mixture is transferred instead to a slow-moving conveyor belt on which the reaction is completed during hardening of the reaction product. The solid mass is then crushed by means of rotary able quantities of reacted, recycled material, as much as up to 25 times the weight of the reaction mixture, during the granulating process. The reaction components are distributed in this recycled material. The use of cuch recycled material complicates the process and reduces the production capacity of the granulation stage. Close and careful control of the reaction conditions With respect to the water/acid ratio, reaction times and temperature is also difficult with this process .

The present invention provides a new and improved method for the production of granulated animal feed grade phosphates from phosphoric acid and an aqueous suspension of calcium carbonate .

It is the object of this invention to provide a simple and effective method which is suitable for modern industrial plants, and which can be employed for continuous and for direct production of free-flowing granulates in the form of MCP or DCP or a mixture of these, easy to handle and with good storage properties.

Another object of the invention is to provide a process whereby a viscous and sticky reaction mixture will not prevent the intensive mixing which is necessary to obtain a reaction and at the same time achieving a stable consistency which provides an efficient and controllable process, giving a solid granulated material with grain size and properties as desired.

A further object of the invention is to provide a process which can also be carried out with Ca-containing phosphoric acid, a starting material which when used in conventional animal feed grade phosphate processes causes the most difficult consistency problems.

The reaction between phosphoric acid and calcium carbonate to form mono-calcium phosphate (MCP) or di-calcium phosphate (DCP) proceeds as follows; MCP: CaC03 + 2H3P04 ~> Ca(H2PC>4)2 + H20 + C02 DCP: CaC03 + H3P04 CaHP04 + C02 -»- H20 When using Ca-containing phosphoric acid, for example containing Ca from a preneutralising plant or Ca remaining in the digestion liquor from the Odda process, it is found that with the method according to the invention, Ca(H2P04)2 in the acid, even in a precipitated state, will be converted as follows: Ca(H2P04)2 + CaC03 — 2CaHP04 + C02 + H20 By using preheated reactants an aqueous suspension of finely ground calcium carbonate can be caused to react very rapidly with phosphoric acid with violent formation of gas and fcam. According to the invention such violent reaction between hot reactants is utilised in a very special manner, whereby there takes place an intensive mixing of the reactants without the aid of power-consuming mechanical mixers and where the reaction mixture shortly afterwards is discharged or expelled to the air. Most of the reaction takes place while the reaction mixture is suspended in the air in the form of separate drops of foam. It is during this stage that the sticky consistency normally occurs, and which has been described above as ctitical with regard to mixability, requirements of power, etc. When the brief period of suspension is over the reaction mixture has the form of a moist, nonsticky grain material.

The process according to the invention is particularly characterised by employing a preheated, aqueous suspension of fine-grained calcium carbonate which is brought to react with preheated phosphoric acid by introducing the reactants into a short,. ' ubulax:\reac ixiirt.∞ner^cd^rthe resulting gas evolution causes intensive mixing and agitating of the reactants, and where the gas further causes a high pressure which shootes or jettisons a foaming reaction mixture out of the open end of the tubular reaction zone, the reaction being almost completed while the developed drops of reaction mixture are in free suspension, whereafter they are collected in the form of moist, non-sticky particles.

Further special characteristics of the invention will appear below especially in connection with the accompanying patent claims.

The method and apparatus will now be described in conjunction with the accompanying drawings wherein Pig . 1 is a schematic and simplified flow diagram, and Fig. 2 a sectional view through a tube reactor suitable for carrying out the method. Fig. 3 is a crods sectional view through the same reactor along the line a - a in Fig. 2.

Fig. 1 shows a feed line 1 for calcium carbonate suspension and another feed line 2 for phosphoric acid. A pump 3 pumps the hot chalk suspension through the line 1 to a short tube reactor 5, the design and mode of operation of which will be described in more detail below.

Hot phosphoric acid is pumped through line 2 to the reactor 5 by means of a pump 6, and the two reactants are thoroughly mixed in the reactor and gas and foam are formed.

The reaction mixture is then expelled through the open end of the reactor into a chamber 7 where the stream disintegrates into particles which are freely suspended in gas until the reactio product falls down and is collected in a collecting devise 8. The upper end of the chamber is fitted with a gas outlet which is connected to a cyclone separator and an exhaust fan 9. When it falls to the bottom of the chamber 7 the animal feed phosphate is in the form of a moist, but free-flowing product. With suitable control and regulation of the reaction conditions is obtained a product which is easy to handle and which can contain all desired Ca/P ratios.

Because of the development of gas during the reaction between phosphoric acid and calcium carbonate the product will have a somewhat vesicular structure with gas-filled pores, which at times may be too light for direct mixing with the other components in some types of animal feed mixtures . However, the moist primary material obtained in the first stage of the process is also very suitable for further processing as well as for granulating with other mineral substances and tracers, which may be effectively mixed in the recycle stream prior to the mixing and collecting device 8. The invention therefore provides a method which makes further processing possible and enables the porous structure to be broken down, whereby a granulate with a higher litre weight can be produced. The intermediate product from the collecting device 8 is discharged via a disc feeder 10 which ensures free flow from a wide opening. The desired depth of the material above the disc feeder is achieved by means of a level indicator (not' shown ih the drawing) controlling the feeder ' s scraping off position. The chamber is further provided with instrumentation which enables a controlled flow of air to pass through the porous, hot and somewhat moist product near the outlet. The product is then compacted in a continuously operating single or multistep press 11, from which the material in the form of pressed flakes is led to a granulator 12, where it is granulated in its moist state. A high yield of a non-dusting fine granulate is thereby obtained, which is sent through a dryer 13 and then a screen 14, whereafter the free-flowing, non- adhering granulate is transported to storage.

The tube reactor 50 is shown in more detail in Fig. 2 and 3. It consists of three main sections, a mixing chamber 55, a pressure chamber 56 and a discharge tube 53.

Two separate feed lines 51 drid 52 direct the reactants tang- eriti lly into the mixing chamber 55 (Fig. 3) in such a way as to cause a powerful turbulence and mixing as the formation of gas increases in intensity. A high pressure is generated in the pressure chamber 56, which pressure expelIs and accelerates the flow of foaming mixture through the somewhat narrower outlet of the discharge tube 53. This tube is preferably detachable from the pressure chamber and may be given various constructional forms. The cross section and the total reactor volume selected may be such that there is optimum initial velocity of the reaction mixture in relation to the reactants* reactivity. The choice of materials in the reactor is not critical and the outlet may be of polished steel, Teflon or some other plastic or rubber materia , Plastic and rubber materials are preferred if the components in the mixture have a tendency to deposit layers in the inside of the reactor.

At the rear, closed end of the reactor a rod 54 is arranged, which can be used to keep the reactor open if it becomes clogged as a result of an unexpected shut-down of the system, etc.

Examples: 1 (MCP) 55,1 kg/h of precipitated calcium carbonate, ground to an average grain size of 15 - 20^ and with a maximum particle size of about SO - -/was used in the form of a 60% + o suspension in water. The temperature was 90-3 C, and the suspension was fed continuously to the reactor together with 127.5 kg of phosphoric acid per hour at 116±2°C. The acid contained 82.8% H3P04. 184 kg h of a product containing 21.8% H20 was obtained prior to the granulating process. The carbon dioxide content was 0.6% and the material was well suited for transport and handling, and eventual granulation. It was easy to dry the product continuously to a water content of below 1.4%.

After drying the particles were strong and dense. The P content in the dried granulate was 22.6%. The weight radio of Ca/P was 0.7.1. 2 (MCP) Calcium carbonate of the same fineness as that used in Example 1 was reacted with a less concentrated phosphoric acid, which resulted in a more aqueous reaction product. 141.8 kg of 74.5% phosphoric acid and 88.8 kg 64.5% calcium carbonate suspension per hour were fed to the reactor which was operated at a pressure of 3.2 ato. This resulted in a very fine-grained reactor product having properties which made it suitable for further handling and granulation. 3. (MCP) Calcium carbonate of the same origin as in Example 1, but with an average grain size of about 50 , was processed in the same pilot plant. The concentration and quantity of the suspension were the same as in Example 1, and the temperature was 87°C. The acid content was the same as in Example 1, and the temperature was 117al°C. The drying process was time-consuming and the product was incompletely dried. The product was sticky and corrosive and contained 3.3% CO . The sticki- 2 ness proved to be a nuisance during a subsequent granulation process . 4 (DCP) The following example and Example 7 show how the DCP was produced. A product with low P-aaaiysis w¾ts produced in the same apparatus as used in the aforementioned examples, by reacting an increased quantity of carbonate with phosphoric acid.

Precipitated calcium carbonate ground to 98% under 20/j*--, corresponding to an average grain size of 4 - 5/*-, was used in a 59% suspension in water. The suspension was heated to 65°C only, and 177 kg h was fed to the reactor. 110 kg of an 87.6% phosphoric acid at a temperature of 113°C was fed to the reactor.

The reaction product contained 33% 1^0 and 1.8% C02, and was suitably plastic for granulation. With a weight ratio of Ca/P = 1.4 the analysis of the dried product showed 18.8% P and 26.5% Ca. 5 (MCP) Precipitated calcium carbonate of practically the same fineness as that used in Example 1 was prepared as a 42.6% suspension in water, and a quantity of 54 kg/h was fed to the reactor at 92°t!. In order to produce an animal feed grade phosphate containing about 24% P this quantity of chalk was reacted with an F-free, Ca-containing phosphoric acid at 128°C at the rate of 128.6 kg h. The P-content of the acid was 25.9% and the weight ratio of Ca/P was 0.43, so that the product's Ca/P would be close to 0.70.

Reaction velocity was satisfactory, and the plasticity and the water content of the reaction product made it well suited for granulation (with 18.5% H20) . The product contained 0.6% CO-, as a result of non-converted CaC0_. 6 ( CP) The same grade of calcium carbonate as used in Example 5, but in a less aqueous suspension, 60% CaCO^, was reacted with the same acid and under the same conditions and temperatures. Reaction was slow and incomplete. The product was unsuitable for granulation and was found to contain 2.5% C02. There was a considerable degree of delayed reaction when the product was further processed. 7 (DCP) Calcium carbonate of an average grain size of 10 - but in a 46,8% suspension fed at the rate of 159.6 kg/h was advantageously reacted at 98°C with 129.1 kg of Ca-containing phosphoric acid with Ca/P = 0.322 and a total of 25.8% P. The temperature of the acid was 132°C. The more time-consuming reaction did not prevent the further processing of the reaction product with about 35% H20. With ί' ¾ΜΦ ratio 1*22 there was no acid residue giving trouble, i spifce of a non-reacted quantity of carbonate^ giving a C02 analysis -bf 1.7%.

The greater quantity of water removed during drying resulted in the product having a weight of 750 g per litre when all particles under 0.1 mm and over 1.5 mm were removed.

Particle strength was also somewhat weak compared with a product with a lower Ca/P ratio. A product sample with 3% Η,,Ο (determined by drying for 3 hours at 105°C) had a P analysis « 20.1%. 8 (Storage tests) Various qualities of the animal feed grade phosphates were stored for 12 weeks under fluctuating climatic conditions. The bags were placed in hydraulic presses under a pressure of about 1500 kg. The calcium phosphates had a Ca/P weight ratio of 0.7 - 1.3. There were no particles over 1.5 mm. Those below 0.1 mm accounted for 1 - 23% and the water content, determined by drying for 3 hours at 105°C, was 0.4 - 4.9.

Mixtures containing up to 33% finely crushed limestone and up to 6% powdered MgO were also prepared from the same products. Only one showed signs of caking after 12 week's storage. This was a deliberately poorly dried and dusty MCP, mixed with limestone (18% dust and 4% water) .

Mixtures containing MgO showed no signs of caking, neither did MCP alone, even with 23% dust and 5% H20.

All percentages given in the above examples relates to weight per cent.

It is important that the calcium carbonate is sufficiently fine-grained. Tests have been made with various grain sizes, and a grain size preferably of < 50^ was found to be necessary. The preferred average grain size ie in the range ^20^.. A very finely ground quality with 98 wt.% of grain size <20t/„and an average grain sise of 4 - Sju also resulted in a completely satisfactory end product. This is a much more finely grained material than is used in other prior known commercial processes, where in order to prevent consistency problems coarser particles are used to prevent too rapid a reaction. Increased quantities of water result in more rapid reaction. However, it is not possible to make use of this advantage in order to use much coarser calcium carbonate, because the carbonate will not be sufficiently reactive.

Mineral as well as precipitated calcium carbonate may be used. The latter may be calcium carbonate converted from calcium nitrate Ca( 03>2 from the Odda process with NH^ and C02.

As already mentioned, the process is also flexible, since MCP or DCP or other "more average" qualities can be manufactured. The flexibility of the process in respect of raw materials has already been described. Both Ca-free and Ca-containing phosphoric acid have proved to be suitable, the quantity of CaC03 being correspondingly reduced in relation to the amount of calcium contained in the phosphoric acid.

In both cases the Ca/P ratios in the product may be selected as desired.

Commercially available types of phosphoric acid meeting the previously mentioned requirements in respect of purity will be comparatively concentrated. Most of the tests have been carried out with two qualities of phosphoric acid, pure phosphoric acid containing from 74 - 88 wt.% of H3P04 and Ca-containing acid produced from the Odda process digestion liquor. Neither type was diluted and both reacted satisfactorily.

More aqueous types of acid may also be used provided they otherwise satisfy the requirements as regards quality. Adjustment of the water content of the chalk suspension, and adjustment of other process variables ensure the desired reaction velocity and consistency if the process described is adhered to.

Ca-containing acid, which gives rise to the most difficult consistency problems when used in the conventional manufacture of animal feed grade phosphates, is surprisingly easy to employ in the new process .

However, it is important that the Ca/P weight ratio is not too high, and according to the invention it has been found best to employ a Ca p radio 0.45. The phosphoric acid and the chalk suspension are preheated before being introduced into the reaction zone. We have used temperatures of 65 -100°C in the case of the chalk suspension and 113 - 132°C in the case of phosphoric acid. This meets the requirements with regard to degree of reaction and product consistensy with calcium carbonate of reasonable grain fineness. If the calcium carbonate is very finely ground the temperatures may be correspondingly reduced so that the desired reactivity is always easy to achieve.

Claims (12)

WE CLAIM;

1. Method of producing animal feed grade phosphates such as mono-calcium phosphate, di-calcium phosphate or mixtures of same by direct reaction between phosphoric acid and Ca-containing materials, characterized in that a finely ground, preheated calcium carbonate suspension is brought to react with a suitable quantity of preheated phosphoric acid by introducing the reactants into a short, tubular, reaction zone, open at one end, whereby development of gad and intensive ittixirtg of the reactants in the reaction έοήβ take place, the gas formed expelling or ejecting the foaming Reaction mixture from the tube-like reaction zone, the reaction being substantially completed while the individual particles in the mixture are freely suspended in air or gas, whereafter the reacted mixture is collected in the form of a free-flowing -granulate having improved handling properties.

2. Method of producing animal feed grade phosphates in accordance with Claim 1, characterized in that the product is further processed by compressing in one or more stages, whereafter the moist compressed flakes thus obtained are disintegrated to particles in a granulation step.

3. Method of producing animal feed grade phosphates in accordance with Claims 1 and 2, characterized in that the calcium carbonate suspension transferred to the reaction zone is in the form of an aqueous suspension in which at least 97 wt.% of the calcium carbonate has a grain' size ^50/-r.

4. Method in accordance with Claim 3 characterized in that the average grain size of the calcium carbonate is .20 .

5. Method in accordance with any of the Claims 1 - 4, characterized in that the calcium carbonate suspension which is transferred to the Reaction zone is preheated to a temperature in the range 60 - 10C°C.

6. Method in accordance with any of the Claims 1 - 5, characterized in that the phosphoric acid which is transferred to the reaction zone is preheated to a temperature in the range 113 - 132°C.

7. Method in accordance with Claim 6, characterized in that a pure, comparatively concentrated phosphoric acid containing from 74 - 88 wt.% of Η^Ρ04 is used.

8. Process in accordance with Claim 6, characterized in that Ca-containing phosphoric acid is used.

9. Process in accordance with Claim 8 characterized in that the Ca/P ratio of the Ca-containing phosphoric acid does not exceed 0.45.

10. Apparatus for carrying out the process in accordance with Claims 1-9 comprising a reaction chamber connected with separate feed lines for calcium carbonate suspension and phosphoric acid respectively, and means for collecting and further processing the resultant reaction product, characterized in that the reaction chamber has the form of a tube reactor (5) with an open end section which forms a discharge means (53), which means is arranged for cooperation with a suspension chamber (7) and a collecting device (8) arranged at the lower end of the chamber.

11. Apparatus in accordance with Claim 10 character-ized in that the collecting device (8) via a discharging device (10) is connected with a press (11) and a granulating device (12) .

12. Apparatus in accordance with claim 10 or 11 characterized in that the tube reactor (5) comprises three sections arranged one after the other, a mixing chamber (55), a reaction or pressure chamber (56) and a discharger tube (53) and where the mixing chamber is connected with tangent-ially arranged reactant feed lines (52, 57) . Attorney for Applicants