the iron oxide used as starting material may be natural hematite ( fe 2 o 3 ). another alternative is to use the type of iron oxides which are obtained as by - products from acid regeneration processes . in order to obtain a product having the desired properties the particle size of the starting material should preferably not exceed 55 μm the reduction of the starting material is performed with hydrogen gas or a mixture of carbon and hydrogen gas . preferably the reduction is performed in a belt furnace at temperatures up to 1100 ° c . an important feature is that the reduction is performed in such a way that the resulting product is in the form of a powder or a slightly sintered cake which can easily be milled without any impact or with only slight impact on the particle shape and other properties . a process for the production of a sponge iron powder for metallurgical purposes is disclosed in the gb patent 704026 . this patent teaches that in order to obtain such a powder the particle size of the starting material should be below about 150 μm ( 100 mesh ) and not exceed that of the final product . furthermore it is disclosed that the starting material could be reduced with solid or gaseous reducing agent at such temperature that sintered sponge iron cakes are formed . after cooling the sintered cakes are comminuted to the desired size . by way of example it is specifically disclosed that a magnetite concentrate is reduced with charcoal at a temperature of 1000 ° c . another reduction process is disclosed in the u . s . pat . no . 5 , 713 , 982 . this process is preferably performed in such a way that the iron oxide powder is incrementally heated to approximately 1200 ° f . ( 649 ° c . ), then to approximately 1400 ° f . ( 760 ° c .) and finally to approximately 1500 ° f . ( 816 ° c .) in a reducing atmosphere . hydrogen is preferably used as reducing atmosphere . according to this known process , an iron powder , the particles of which are rounded , are obtained . this known powder is stated to be useful for injection moulding processes . according to the present invention it has been found that , in order to obtain a satisfactory dissolution rate and bioavailability , the iron particles obtained after the reduction should have an irregular shape . it has been found that the critical feature can be obtained by performing the reduction of the iron oxide powder at somewhat higher temperature and / or somewhat longer heating times than what is disclosed in the us patent . as an example a combination of carbon and hydrogen gas can be used as the reducing agents at a temperature of about 1000 ° c . the exact conditions for the preparation of iron powders having the irregularly shaped particles can be determined by the man skilled in the art . an important and critical feature of the powder is the porous and irregular shape and consequently the low apparent density , ad , which has been found to be preferably less than 2 g / cm 3 . furthermore , the pores of the powder should be open , facilitating the penetration of the gastric juice into the iron particles giving a sufficient high dissolution rate of the iron . a low - degree of open porosity is manifested in a value of particle density close to the value of the true density of iron , which is about 7 . 86 g / cm 3 . it has then be found that the relation between ad and pd should be less than 0 . 3 . as used herein the particle density , pd , is measured by using a pycometer apparatus , which allows liquid to flow into open pores of the iron particles in a container of definite volume under controlled conditions . the particle density is defined as the particle mass divided by the particle volume , including the inside closed pores . as the liquid fluid was 5 % of a 99 . 5 % ethanol solution used . the weight of the pycnometer , the pycnometer including the iron powder sample , and the pycnometer including the iron powder sample filled with the penetration fluid up to the definite volume were measured . as the definite volume of the pycnometer and the density of the penetrating fluid is known the particle density can then be calculated . the particle size of the iron powder particles is also a parameter influencing the dissolution rate . a too coarse particle size will negatively influence the dissolution rate and a too fine particle size of the iron powder increase the risk for dust explosions during handling . a sufficiently high dissolution rate is obtained when the average particle size is between 5 and 45 preferably between 5 and 25 μm . as method for assessing the bioavailability of the iron powder we have used a method including the steps of dissolving the 50 g iron powder in hydrochloric acid at ph 1 and 37 ° c . the amount of dissolved iron is measured after 30 minutes and is reported as dissolution rate in table 2 below . according to the present invention it has been found that the iron powder should have a dissolution rate in hydrochloric acid at 37 ° c . and ph 1 of at least 40 % by eight after 30 minutes . although described particularly with reference to food applications it is obvious that the iron powder according to the present invention can be used also within other fields where rapid dissolution is required . this example discloses a process for the preparation of an iron powder , which has high bioavailability as measured by the dissolution rate and which can be used as food or feed fortification . hematite with a grain size of 5 - 20 μm was mixed with coal having a grain size less than 1 mm . the amount of added coal was 12 % by weight . the obtained mixture was reduced in an furnace at about 1000 ° c . hydrogen gas was added at the end of the furnace and forced to flow in a direction counter the flow of the hematite material . the added amount of hydrogen gas was 610 m 3 / ton of reduced iron . the obtained cake of sponge iron was gently crushed in a standard milling device and sieved on a 325 mesh screen . fig1 is a micro photograph of the iron powder produced . the process according to example 1 was repeated but the reduction was performed with hydrogen only and the starting material used was an iron oxide which had been obtained as a by product from acid regeneration . fig2 is a micro photograph of the iron powder produced according to this example . a comparison between the new powders a and b according to the above examples 1 and 2 , respectively , and presently used iron powders for food fortification is disclosed in the following , wherein table 1 lists the type of powders and table 2 the properties of these powders . the contents of as , hg and pb in the powders according to the invention were all below the critical limits accepted in iron powders for food fortification .
the invention concerns an iron powder for fortifying foodstuff . the powder consists of a reduced iron powder having irregularly shaped particles , wherein the iron powder has a ratio ad : pd less than 0 . 3 , wherein ad is the apparent density in g / cm 3 , and pd is the particle density in g / cm 3 . the specific surface area of the powder particles is above 300 m 2 / kg as measured by the bet method and the average particle size is between 5 - 45 μm .