There has been known a thermally developable photothermographic imaging material which comprises a photocatalyst such as silver halide, a reducing agent and an organic silver salt reducible by the reducing agent and in which a latent image formed by exposure forms a black silver image by thermal development via oxidation-reduction (or redox) reaction between the organic silver salt and the reducing agent.
Organic silver salt particles, in general, are formed in a mixing vessel provided with a dynamic stirring means which conducts a stirring operation such as rotation. Formation methods of organic silver salt particles and preparation apparatuses thereof include, for example, a method using a closed mixing means, as disclosed in JP-A No. 2001-33907 (hereinafter, the term JP-A refers to an unexamined Japanese Patent Application Publication). JP-A No. 2003-43607 also discloses a preparation method of organic silver salt particles using a micro-mixer.
However, formation of particles in a mixing vessel incorporating a stirring means causes problems that produced nucleus particles are cycled and returned, whereby nucleation cannot be performed under a homogeneous condition.
In the method disclosed in JP-A No. 2001-33907, the closed mixing means is internally installed with a stirring means, making a complex structure and problems such as retention of fluid cannot be ignored microscopically. In a closed-type continuous mixing system, retention of fluid not only becomes a factor of perturbing particle sizes of an organic silver salt or the distribution thereof but it also results in increased fogging caused by retained particles being exposed to silver ions at relatively high concentration.
In the use of the micro-mixer disclosed in JP-A No. 2003-43607, static mixing is conducted without using an internal mixing means and mixing is performed in a laminar flow state. In this method, the reaction zone is so small that mixing needs to be conducted not only at relatively low solution-supplying rate but also using a solution at relatively low concentration, resulting in reduced productivity.