JavaCV detect captcha letters on a binary image

I'm new to JavaCv. My task is to find symbols on an image and generate pictures with single symbol on each other. Firstly, I have this picture: Then I do thresholding and get this: I'm trying to use cvFindContours an then draw a rectangle around each symbol, my code:

 String filename = "captcha.jpg";
    IplImage firstImage=cvLoadImage(filename);
    Mat src = imread(filename, CV_LOAD_IMAGE_GRAYSCALE);
    Mat dst = new Mat();
   threshold(src, dst, 200, 255, 0);
   imwrite("out.jpg", dst);

    IplImage iplImage=cvLoadImage("out.jpg",CV_8UC1);
    CvMemStorage memStorage=CvMemStorage.create();
    CvSeq contours=new CvSeq();
    cvFindContours(iplImage,memStorage,contours,Loader.sizeof(CvContour.class),CV_RETR_CCOMP,CV_CHAIN_APPROX_SIMPLE,cvPoint(0,0));
    CvSeq ptr;
    CvRect bb;
    for (ptr=contours;ptr!=null;ptr=ptr.h_next()){
        bb=cvBoundingRect(ptr);
        cvRectangle(firstImage , cvPoint( bb.x(), bb.y() ),
                cvPoint( bb.x() + bb.width(), bb.y() + bb.height()),
                CvScalar.BLACK, 2, 0, 0 );

    }
    cvSaveImage("result.jpg",firstImage);
}

I want to get output like this:, but really I get this:

Please, need your help.